Extrasolar Planets (original) (raw)

The Search for the Extrasolar Planets: A Brief History of the Search, the Findings and the Future Implications

Table of Contents

  1. PREFACE
  2. SECTION 1: INTRODUCTION
  3. SECTION 2: BARNARD'S STAR AND VAN DE KAMP'S PLANETS: THE BEGINNING
  4. SECTION 3: A GENERAL DESCRIPTION OF THE EXTRASOLAR PLANETARY WORK RECENTLY IN PROGRESS
  5. SECTION 4: FUTURE CONSIDERATIONS
  6. SECTION 5: CONCLUSIONS
  7. APPENDIX

PREFACE

This web page is an attempt to provide a review of humankind's quest for the discovery of planets outside our Solar System. In addition, a series of major web sites dealing with the search for extrasolar planets are listed. They are as follows:

In addition to the above sites, NASA's Origins Program is attempting to answer an important question (among others), Are there worlds like the Earth around nearby stars? If so, are they habitable, and is life as we know it present there?

SECTION 1: INTRODUCTION

Our solar system exists. This is an irrefutable fact. Because of our solar systems existence, the question that begets itself time and time again is whether or not planets exist around stars other than our own. Based on the size of our universe and the laws of probability, the odds are excellent that our solar system is not unique in the universe. To better appreciate the odds, it is useful to consider the size of the universe . At the present time it is estimated that 50 billion galaxies , the largest of which contain thousands of billions of stars, are visible to modern telescopes including theHubble Space Telescope . (1) . The notion of planetary worlds orbiting stars other than our Sun is not new. Throughout history, beginning with the Greeks, humans have been pondering over the possibility of other solar systems beyond our own. From mere speculation to the astronomical amassing of data, as in the case of Barnard's Star, and others, our speculations and searches for these extrasolar planets have spanned well over 2,000 years. Although it is beyond the scope of this paper to provide a chronology of all these speculations, an excellent book published in 1996 provides the background for this interesting history. (2)

In order to determine the existence of extrasolar planets it is important to consider one minor and one major premise. The minor premise, and to a large degree a philosophical one, is to consider the existence of additional solar systems based on the probability factor. As stated above, with the number of galaxies and the number of stars contained within each of the galaxies, the probability of another solar system existing is excellent. The speculation based on probabilities is not new. Two very important books on the search for extraterrestrial life, written in the 1960's attest to this thesis.

"With 10 to the 11th stars in our galaxy and 10 to the 9th other galaxies, there are at least 10 to the 20th stars in the universe. Most of them may be accompanied by solar systems. If there are 10 to the 20th solar systems in the universe, and the universe is 10 to the 10th years old -- and if, further, solar systems have formed roughly uniformly in time -- then one solar system is formed every 10 to the negative 10 yr = 3 x 10 to the negative 3 seconds. On the average, a million solar systems are formed in the universe each hour." (3)

"The implication is that solar systems are common, but the argument will be greatly strengthened if there is real agreement on how our solar system came about. The space exploration of the next decade should enable us to narrow down the theories to a great extent. We will have samples of the Moon and direct knowledge as to the nature of its interior. We will learn the precise compositions of other planets and their atmospheres to compare with those of our Earth. However, study of our own solar system is not the only way to learn if it is unique. Another approach is to search for clues among the other stars of our galaxy. Such observations, carried out originally without reference to the question of whether or not there are planets elsewhere, led to surprising discoveries..." (4)

The first of these quotes can be found in a monograph co-authored by Carl Sagan who needs no introduction, while the second was quoted by Walter Sullivan, who at that time was Science Editor of The New York Times . These two monographs, although concentrating on the possibilities of intelligent life, needed to make a strong statement regarding the probability of the existence of extrasolar planets. If indeed the probability of intelligent life is a strong possibility, then the existence of extrasolar planets is a definite.

On the other hand, the major premise, and certainly the most important, is to ascertain the existence of extrasolar planets by direct astronomical observations. Speculation is easy, scientific endeavors are not. During the past several years the astronomical techniques used for observations have become more and more sophisticated leading to precise indirect methods of detecting planetary bodies orbiting stars other than our Sun. Although the evidence is compelling for the existence of extrasolar bodies, there has been no direct observation of an extrasolar planet; i.e., a viewing of a planetary body via a telescope and/or a photograph. A number of these astronomical techniques are discussed in Section 2 and 3 of this paper.

It is, therefore, the purpose of this paper to consider a brief history on the work that has been done on planetary bodies outside of our solar system during the past decade or so, beginning with the controversy surrounding the search for companions around Barnard's Star. New astronomical methodologies and future programs and missions will also be considered. Because the discoveries and/or confirmations of said planets have been ascertained within the last several years, the main thrust of this writing will be confined to information accrued during the 1990's.

1. Gribbin, John. Companion to the Cosmos . London: Weidenfed & Nicolson, 1996. Pgs. 156-157.

2. Anonymous. Book Review: Mammana, Dennis L., and Donald W. McCarthy, Jr. Other Suns. Other Worlds?: The Search for Extrasolar Planetary Systems . New York: St. Martin's Press, 1996. 225p. Astronomy 24(11):100, November 1996.

3. Shklovskii, I.S. and Carl Sagan. Intelligent Life in the Universe . New York: Dell Publishing, 1966. 509p. Pg. 130.

4. Sullivan, Walter. We Are Not Alone: The Search for Intelligent Life on Other Worlds. New York: McGraw-Hill, 1964. 325p. Pg. 43.

SECTION 2: BARNARD'S STAR AND VAN DE KAMP'S PLANETS: THE BEGINNING

In the September 15, 1916 issue of The Astronomical Journal (1) and the September 7, 1916 issue of Nature , (2) an article appeared that dealt with the discovery of a rather small, insignificant star that demonstrated a large proper motion. The purpose of the article was to alert the astronomical world that indeed, E.E. Barnard detected a unique find, that is a star with a proper motion larger than any star that had been studied previously. The large proper motion was calculated by Barnard to be approximately 10.3 arcseconds per year. (3) The proper motion is defined as "the apparent angular motion per year of a star on the celestial sphere, i.e., in a direction perpendicular to the line of sight" . (4) Proper motion is attributed to two basic premises; the star can move of its own accord and the star's galaxy can also move. Because of the vast distances between us and stars, the stars appear to be glued in the night sky and that the only apparent movement of stars that we see is due to the rotation of the Earth about its axis. If you look at a constellation, such as Orion this year, you will note that it will look exactly the same next year, and the year after that, and so on. It takes thousands of years to see that the stars have moved by seeing a change in the position of the stars with respect to one another. For example, the stars in the Big Dipper did not have the same positions 2,000 years ago, and hence, did not look like the Big Dipper.

Because of the large proper motion of Barnard's Star, astronomers felt the need to determine additional information regarding this amazing star. It was found to be a red dwarf, and its distance from the Earth to be 1.82 parsecs (5.95 light years); practically a next door neighbor in astronomical terms. Not only was the distance determined but the star was found to be moving closer to us. By 11,800 AD it will be as close as 1.16 parsecs (3.8 light years) from us. (5) Because Barnard's Star is a red dwarf (common in our galaxy), its close proximity to us, and its large proper motion, this star was a prime candidate for further study regarding the search for extrasolar planets. It should be noted, however, that it was the large proper motion of the star that encouraged astronomers to determine its other physical parameters.

Before the story of Barnard's Star continues it would be best to consider a few astronomical terms that will play a large role in the supposed confirmation of planets revolving around the star. These terms are astrometry and perturbation. Astrometry is that part of astronomy that measures the proper motion of stars as a function of time. In the case of Barnard's Star, photographic plates were examined over many decades in order to determine its motion. Perturbation, on the other hand, is a way of describing any abnormalities in the stars motion. Does the star move in a straight line or does it exhibit some wobble (or sinusoidal) motion? If so, this wobble could be explained by gravitational forces between the star and an unseen body or bodies revolving around the star.

With the above in mind, the story of this star continues with the photographic plates of Peter van de Kamp. Working at the Sproul Observatory of Swarthmore College, van de Kamp devoted most of his life over 2,000 plates of Barnard's Star that he and his students had taken from 1938 through 1962. According to van de Kamp, a wobble in the movement of Barnard's Star was detected which he determined was the result of a body revolving around the star. The body, according to van de Kamp, was 1.6 times the mass of Jupiter and that its rate of revolution was 24 years. He also suggested that the orbit was elongated. (6)

Over the years Peter van de Kamp published a series of papers refining his initial results of the companion planet revolving around Barnard's Star. In the March 1969 issue of the Astronomical Journal (7) he reconsidered a number of physical parameters regarding the companion. From photographic plates from 1916-1919 and a large number of plates from 1938-1967 he determined that the planet (van de Kamp sometimes referred to it as "his" planet) revolved around Barnard's Star every 25 years and that it was extremely large, i.e., 1.7 times the mass of Jupiter. In August of the same year another article was published by van de Kamp in the Astronomical Journal (8) stating that while reconsidering the photographic plates in his possession, there were not one but two planets revolving around Barnard's Star. Not only were there two planets but instead of an elongated orbit, the orbits of both companions were found to be circular. In addition, by reconsidering his calculations, he ascertained that one of the planets revolved around the star every 26 years while the other 12 years. Likewise, the previous mass he had given the first companion at 1.7 times the mass of Jupiter was incorrect. At this point in time the companions seemed to have masses 1.1 and 0.8 the mass of Jupiter. In 1975, van de Kamp published yet another article (9) analyzing astrometric data from 1950 to 1974. According to this paper the masses of the two planets were determined to be 0.4 Jupiter masses and 1.0 Jupiter masses. In addition, the number of years that the planets revolved around the star were recalculated to 22 years and 11.5 years. The 0.4 Jovian mass planet was determined to be the body that revolved around the Star every 22 years while the 1.0 Jovian mass planet was thought to revolve around Barnard's Star every 11.5 years.

However, van de Kamp did not stop there. He continued to amass more photographic plates on Barnard's Star. In 1982, with measurements of photographic plates from 1938 through 1981, he published yet another paper with slightly different results of the planetary companions that had been published previously. In his 1982 paper (10) , he reconfirmed the fact that both orbits were circular. However, he reconsidered their periods of revolution around Barnard's Star as well as the planets' masses. In re-examining his data, he reconciled the planets revolutions around the star to be 12 and 20 years with masses of 0.7 and 0.5 the size of Jupiter. According to van de Kamp the reason for the changes in the physical parameters of the two planets had to do in part with the reference stars he was using when examining the photographic plates. In additon, he now had more years of photographic plates to work with than what he had when publishing his earlier papers.

While van de Kamp was honing his measurements on the two companions revolving around Barnard's Star, a few papers in 1973 were published that questioned his claims either directly or indirectly. Astrometrists Gatewood and Eichhorn (11) examined photographic plates taken with a 20-inch refractor at the Van Vleck Observatory located at Wesleyan University and the 30-inch Thaw refractor of the Allegheny Observatory in Pittsburg. They were unable to detect any wobble in the proper motion of Barnard's Star. In short, if there was no wobble, then no planetary companions existed. In the same year, John L. Hershey, while working at the Sproul Observatory (12) , analyzed the same photographic plates as van de Kamp. He made a systematic study of not only Barnard's Star but a number of other stars found on the plates. A total of twelve stars were considered in the study. It should be noted that these plates were a result of the use of the telescope at the Sproul Observatory. To Hershey's amazement, not only did he detect a wobble in the proper motion of Barnard's Star, but wobbles in all the stars compiled in the study. A number of inferences could be made with these results. One possibility is that all the stars had planetary companions, or on the other hand, there were problems with the Sproul telescope that had been used to photograph the stars in question. Indeed, if it were the latter case, then van de Kamp, who based his conclusions on the photographs was using incorrect data, as was Hershey. It was found that there seemed to be a large discrepancy in the results of star movement on the plates in 1949 through 1956 and then again in 1957. In 1949 a new cell for the 24 inch lens was installed and in 1957 the objective lens was adjusted. (13) Could the readjustment of the telescope be the reason for the big jump in star positions?

Years later, Robert Harrington, using a 61-inch reflector of the United States Observatory at Flagstaff, compiled over 400 plates of Barnard's Star. Unfortunately, in studying these plates Harrington could not detect a wobble. Laurence Fredrick, working with a 26-inch refractor at the McCormick Observatory of the University of Virginia, also recorded no detectable wobble. Neither Harrington or Fredrick are willing to toally discount the existence of planetary bodies, but on the other hand, are extremely pessimistic. (14) Harrington, however, is quick to add that in 1977 there was some wobble detected in the North-South direction. 15 In 1985, a few years after van de Kamp published his definitive and last paper on the subject 10 , a Govert Schilling from Utrecht, The Netherlands 16 published the account of an interview with van de Kamp. In essence van de Kamp admonished his critics by stating this his study of Barnard's Star was longer (over 40 years), the number of photographic plates he studied was larger (tens of thousands of plates), he did everything he could to eliminate errors, and yes, he is still under the opinion from his observations that there are indeed two planetary companions revolving around Barnard's Star and that the masses of these planets are 0.7 and 0.5 times Jupiter's mass. To his critics van de Kamp suggests that they (present researchers) spend the same amount of time with the same amount of plates and then, after that, he (van de Kamp) would be happy to talk with them. (Peter van de Kamp died in 1995. I do not know whether he and his critics ever got together to discuss his life's work.)

As a footnote to the above account, it should be noted that Gatewood, using astronometric techniques is trying to ascertain what planetary bodies would be most unlikely to be revolving around Barnard's Star. In a 1995 paper Gatewood suggested that brown dwarfs (more massive than Jupiter by greater than 10 masses, but not massive enough to glow as a star) could not exist around Barnard's Star. In addition, he feels that planets having a mass smaller than Jupiter's may possibly be present. 17

Although work on Barnard's Star has spanned well over a half century, the jury still seems to be out. No definitive confirmation by the astronomical community as a whole has been established. Only time will tell as to whether a planetary body or bodies orbit Barnard's Star as suggested by van de Kamp.

1. Barnard, E.E. "A small star with large proper-motion." The Astronomical Journal 24(3):181, September 15, 1916.

2. Barnard, E.E. "Faint star with large proper motion." Nature 98:22, September 7, 1916.

3. Croswell, Ken. "Does Barnard's Star have planets." Astronomy 16(3):6-17, March 1988. Pg. 13.

4. Illingworth, Valerie, Ed. The Facts on File Dictionary of Astronomy , 3rd ed. New York: Facts on File, Inc., 1994. Pg. 354.

5. Harrington, Robert S. "Barnard's Star: A status report on an intriguing neighbor." Mercury 16:77-79 & 87, May/June 1987. Pg. 77.

6. Croswell, Ken. "Does Barnard's Star have planets." Astronomy 16(3):6-17, March 1988. Pg. 13.

7. van de Kamp, P. "Parallax, proper motion acceleration, and orbital motion of Barnard's Star." Astronomical Journal 74(2):238-240, March 1969.

8. van de Kamp, P. "Alternate dynamical analysis of Barnard's Star." Astronomical Journal 74(6):757-9, August 1969.

9. van de Kamp, P. "Astrometric study of Barnard's Star from plates taken with the Sproul 61-cm refractor." Astronomical Journal 80(8):658-61, August 1975.

10. van de Kamp, P. "The planetary system of Barnard's Star." Vistas in Astronomy 26:141-157, 1982.

11. Gatewood, George, and Heinrich Eichhorn. "An unsuccessful search for a planetary companion of Barnard's Star (BD + 4 degree3561)." Astronomical Journal 78(8):769-776, October 1973.

12. Hershey, John L. "Astrometric analysis of the field of AC +65degrees6955 from plates taken with the Sproul 24-inch refractor." Astronomical Journal 78(5):421-425, June 1973.

13. Croswell, Ken. "Does Barnard's Star have Planets." Astronomy 16(3):6-17, March 1988. Pgs. 15-17. 14. Ibid

15. Ibid

16. Schilling. Govert. "Peter van de Kamp and his "lovely Barnard's Star"." Astronomy 13:26 & 28, December 1985.

17. Gatewood, George D. "A study of the astrometric motion of Barnard's Star." Astrophysics and Space Science 223(1-2):91-101, 1995.

SECTION 3: A GENERAL DESCRIPTION OF THE EXTRASOLAR PLANETARY WORK RECENTLY IN PROGRESS

Within the last several years a great deal of study has been focused on the search for extrasolar
planets, with a number of confirmations. The basic techniques, according to J. Schneider, used in the search for extrasolar worlds are astrometric detection , direct imaging , radial velocity, ground based photometry, and occultation. (1) Of the various techniques used in determining the existence of a planetary body, the measurement of the radial velocity of a star, using doppler shift methods , has been used to a greater degree than any of the aforementioned. This can readily be seen in examining the Appendix, which can be found at the end of this paper. It may be interesting to note that Bruce T.E. Campbell has developed a rather sensitive technique for measuring velocity perturbations using doppler shifts. He compares the spectrum of stars by using a reference spectrum involving hydrogen fluoride. (2)

Astrometry, which was mentioned in Section 2, is used to determine theproper motion of a star, using other stars as reference points. If a body is revolving around a star then the body will affect the circular motion of the star. As one measures the stars linear motion, it will be found that the motion is not in a straight line, but rather in a wobbly line due to the presence of a planet or planets revolving around the star. This situation is similar to observing a person spinning a shotput around his or her body. The person shows a wobbling type motion due to the heavy load that is being rotated about his or her body. The person represents the star while the shotput represents a planet. In addition, the person can move from point A to point B while the shotput is revolving. Therefore, there are two motions, the wobbly motion caused by the rotation of the shotput, and the linear motion caused by the movement of the person. (3)If the person was not rotating a shotput and simply walking, then the only motion observed would be the distance from point A to point B, and no wobble in the motion would occur. This, of course, presupposes that the person is sober. In like manner, if one were to observe the proper motion of a star without any planets revolving about it, then the distance the star moves from point A to point B would not reflect a wobbling motion, the motion would be in a straight line.

Radial velocity, which is measured by the doppler effect (lines in the stars spectrum) takes into account the line-of-sight velocity; i.e. the velocity of which a star is moving towards or away from us. If the light from the star is moving towards us then the spectrum of the star will be shifted to the blue portion of the spectrum (blueshift) and the velocity would be negative. If, on the other hand, the star is moving away from us then the spectrum of the star will be shifted to the red portion of the spectrum (redshift) and the velocity will be positive. By observing the spectral shift one can determine the rate at which the star is moving.

How does this relate to the detection of planetary bodies revolving around stars? If a planet or planets revolve around a star then the motion of the star will be affected. According to Alan Boss, writing in Physics Today (4) , if a star is orbiting around the center mass of a system, then it suggests a planetary body or bodies revolving about it. More to the point, if the above is true then there would be a periodic shift in the doppler velocity. This is the star's spectrum would exhibit a shift to the red and then to the blue, and then to the red, periodically. In other words, the effect of planetary bodies around a star will affect the radial velocity of the star so that it would be moving towards us and then away from us, and continue to repeat that pattern. It should be noted that the perturbation or doppler velocity shift is very small, and therefore, extremely difficult to detect.

Many of the ensuing discoveries rely heavily upon radial velocity techniques. Highly specialized spectrographs, that can detect tiny doppler-induced wavelength shifts in a star's spectra are used to calculate the radial velocities. However, it will be of no surprise to note that the planets that were discovered using this technique are large and/or are in tight orbits, because this technique disposes itself to that type of finding. (5)

The direct imaging method is based on the fact that planets reflect the stars' light. Planets do not give off any light of their own. For example, the various planets we see in the night sky are a result of the sun's light reflecting from them. Likewise, planets around other suns would also reflect the light of their suns. This method is used in order to determine reflected light from an extrasolar planet. It is obvious that only extremely large planets may be detected using this method. The major problem with this technique is that the star is much brighter than the planet it illuminates, and can tend to obfuscate it. (6)

Photometry can be used to detect a change in the brightness of a star, as in the case when a planet occults a star. On Earth we can observe this during a solar eclipse. That is, our Earth occults our Sun during an eclipse. As an extrasolar planet revolves about its star, it will pass between its star and the line of sight as seen from the Earth. A change in the brightness of the star due to this transit would then suggest a planetary body. (7)

What follows is a series of reports that deal with the history and ongoing research in the search for extrasolar planets. These reports are not to suggest that every researched star listed have planets orbiting them. Many of the planetary bodies detected by astronomers and their co-workers have not been confirmed or established as fact in the astronomical community at large.

For the sake of clarity, the following section is arranged alphabetically by the name of the star in question. At the present time no specific names have been given to the planetary bodies. However, some of the newly discovered planets are named after the name of the star, and are set apart from the star name by an additional letter. For example, one of the stars in question is Peg 51. The planetary companion discovered around it has been labeled Peg 51 B.

In addition to the ensuing reports, the Appendix, which can be found at the end of this paper, provides in tabular format a summary of the physical parameters of the stars and their supposed planetary bodies. If more than one planetary body has been detected, then the star, in a number of cases, is listed as many times as there are companions, with the planet closest to its star listed first. From this table it is clear that many of the findings have not yet been confirmed. In fact, in a few cases retractions were made by the researcher. On further examination of the Appendix it is also clear that the majority of detected planets are of Jupiter mass or larger, most have circular orbits, most have been detected by radial velocity measurements using doppler shifts, the majority of them are less than 1 astronomical unit from their star, and except for the pulsars, the majority of the planets discovered are orbiting stars that are similiar to our Sun.

1. Schneider, Jean and Laurance R. Doyle. "Ground-Based detection of terrestrial extrasolar planets by photometry: the case for CM Draconis." Earth, Moon and Planets 71(1-2):153-173, 1995. Pg 154-155.

2. Black, D.C. "Worlds around other stars." Scientific American264(1):76-82, January, 1991. Pg. 79.

3. NOVA: Hunt for Alien Worlds . Presented on PBS (Channel 8):8:00 PM - 9:00 PM, February 16, 1997.

4. Boss, Alan P. "Extrasolar planets." Physics Today 49(9):32-8, 1996. Pg. 33.

5. MacRobert, Alan M., and Joshua Roth. "The planet of 51 Pegasi." Sky and Telescope 91(1):38-40, January 1996.

6. Black, D.C. "Worlds around other stars." Scientific American264(1):76-82, January 1991. Pg. 78.

7. Schneider, Jean and Laurance R. Doyle. "Ground-based detection of terrestrial extrasolar planets by photometry: the case for CM Draconis." Earth, Moon and Planets 71:153-173, 1995. Pg. 155.

BETA PICTORIS

Beta Pictoris is a star belonging to A5V spectral class, it has a visual magnitude of 3.85, and is approximately 17.17 parsecs (58.68 light years) from Earth. In addition, it has a dust ring revolving about it. The dust ring, according to Fridman and Gor'kavyi has a radius of 100 to 500 astronomical units. (1) However, Roques, et al, report (2) that, in 1987, Smith and Terrile used coronographic images to find that the radius of the disk measures over 1,000 AU's. Smith and Terrile (3) actually suggest that the disc extends more than 1,100 AU's from the star. They go on to say that after examining over 100 nearby stars, no other circumstellar disk is comparable in visible optical properties as those of Beta Pictoris. In any event, the presence of a dust ring around the star and the observations of absorption spectra of comets revolving around the star are leading astronomers to conclude that a planetary body exists around Beta Pictoris.

In so far as the dust ring is concerned, researchers have observed a void in the dust ring; i.e., a depletion of dust. This finding encourages the idea is that a planet is making a path in the dust as it orbits the star. Levison and his team suggest that this void in the ring is between 10 and 30 astronomical units from Beta Pictoris, and further suggest that not one but two planets are causing the dust depletion zone. (4)

Levison is not the only researcher to suggest that a planet is revolving around Beta Pictoris. Writing in Icarus , Roques (5) and Lazzaro (6) present evidence that suggests that the planetary body hypothesis holds true. Their premise is based on numerical, and analytical hypothetical models, respectively. They suggest that the planet is 20 astronomical units from Beta Pictoris, has an orbital eccentricity of 0.01; a nearly circular orbit, and a mass five times that of the Earth. In addition to the models created by Roques and Lazzaro, other researchers (7) present a model based on an analysis of physical processes that occur in disks. According to the model, the astronomers are confident that indeed a planet can create the type of path that has been suggested in the disk (dust void). As they state, "Now, we can confirm that, in spite of the collisional destruction process, a planet can effectively create asymmetry of about 10 or 20% in the dust disk of B Pictoris." (8)

As with the dust disk models, the comet evidence is compelling. Because of the short-lived absorptions that have been found while monitoring Beta Pictoris, scientists conclude that they may be caused by comets revolving in eccentric orbits around the star. Based on a model using our own Solar System, they feel that the comets orbits may be due to the presence of two or more planets revolving around Beta-Pictoris. (9)

It should be noted, however, that although both the dust disk model and the short-lived absorption spectra of the star are compelling evidence of a planet or planets revolving about Beta-Pictoris, the existence of said planets have not been confirmed.

1. Fridman, A.M. and N.N. Gor'Kavyi. "On the possibility of detection of massive planets in protoplanetary disks." Astronomical and Astrophysical Transactions 5(3):249-51, 1994.

2. Roques, Francois, et al. "Is there a planet around beta Pictoris? Perturbations of a planet on a circumstellar dust disk. 1. The numerical model." Icarus 108(1):37-58, 1994. Pg. 38.

3. Smith, B.A. and R.J. Terrile. "The Beta Pictoris disk: recent optical observations." Bulletin of the American Astronomical Society . 19(3):829, 1987.

4. Anonymous. Sky and Telescope 89(2):10-11, February 1995.

5. Roques, Francois, et al. "Is there a planet around beta Pictoris? Perturbations of a planet on a circumstellar dust disk. 1. The numerical model." Icarus 108(1):37-58, March 1994.

6. Lazzaro, D., et al. "Is there a planet around beta Pictoris? Perturbations of a planet on a circumstellar dust disk. 2. The analytical model." Icarus 108(1):59-80, March 1994.

7. Lecavelier des Etangs, A., et al. "Perturbations of a planet on the B Pictoris circumstellar dust disk. 3. Time scale of collisional destruction versus resonance time scale." Icarus 123(1):168-179, September 1, 1996.

8. Ibid Pg. 178.

9. Anonymous. "More evidence for extrasolar planets." Sky and Telescope 89(2):10-11, February 1995.

CM-DRACONIS

Although no planetary bodies have been detected orbiting CM-Draconis , it is worth considering, because of the research program focused on this star and the use of the astronomical technique known as occultation. At the present time very few star systems that are being studied make use of photometry in order to detect occultations. In short, Doyle (1) and Schneider (2) feel that because this star is a small eclipsing binary , not only will they be able to ascertain the existence of terrestrial planets, but Jupiter-mass planets as well. By studying CM Draconis, which is a small eclipsing binary, the use of photometry can be improved. In essence, the researchers feel that it will be easy to detect an occultation of the two stars by a planetary body, whether that body is small or large. When a body orbiting the star passes in front of it in relation to our line of sight there will be a change in the brightness of the star. Because there are two stars it is felt that the other star will also be occulted by a planet, thereby exhibiting a change in brightness as well. Because these two stars are very close to one another (about 15.4 CM Draconis radii) one occultation should follow the other, therefore suggesting some type of planetary orb.

According to Schneider and Doyle, using photometric techniques should allow one to detect large Jupiter-mass planets as well as those that are similiar in size to the Earth. "Terrestrial-sized (Earth-Neptune-radii) extrasolar planets may be detectable in the CM Draconis system and several others using a ground-based network of 1-meter-class telescopes performing CCD photometry over several months. The small size of the system, and the orbital plane being edge-on, enhance both the probability as well as the sensitivity of detection." (3) These detections are based on the transits of short-period planets. This work is currently in progress. No planets have been detected as of this writing.

1. Doyle, Laurance R., et al. "Ground-based detectability of terrestrial and Jovian extrasolar planets: Observations of CM Draconis at Lick Observatory." Journal of Geophysical Research 101(No. E6):14823-14829, June 25, 1996.

2. Schneider, Jean, and Laurance R. Doyle. "Ground-based detection of terrestrial extrasolar planets by photometry: The case for CM Draconis." Earth, Moon and Planets 71(1-2):153-173, 1995.

3. Ibid Pg. 170.

16 CYGNI B

16 Cygni B is one of two stars in a wide binary system, which is known as 16 Cygni AB. Because the planet detected has been found orbiting 16 Cygni B, the thrust of this report will be concentrated on this star.

By means of radial velocity observations, researchers suggest that a planet orbits around the star 16 Cygni B, a star similiar to our Sun. According to measurements by Cochran and others (1) the planet revolves around the star every 800.8 days, with a velocity amplitude of 43.9 m/s, and an extremely large eccentricity of 0.63. It is felt that this is the largest eccentricity of any of the extrasolar planets discovered up to this point in time.

According to Mazeh and others (2) , the reason for the high eccentricity is due to the binary system, Cygni AB. They suggest that the distance between the two stars is approximately 1,100 AU. By means of numerical simulations they feel that at one time the planet in question had a small eccentricity of about 0.15, and that over a ten million year period the eccentricity changed due to the tidal forces of 16 Cygni A. They also go on to state that in a private communication with Marcy it is possible that other stars mentioned in this report are also part of a wide binary system. These stars are Rho 1 Canceri, Upsilon Andromedae, and Tau Bootis. The planets detected orbiting these stars, however, show a low eccentricity, which the researchers feel is due to a small orbital period, unlike the orbital period of the planet orbiting 16 Cygni B.

In addition, Cochran and his colleagues further suggest that the planet's mass is about five times the mass of Jupiter. Astronomers feel that this planet could, indeed, be an extremely low mass brown dwarf. (1) The planet lies approximately 1.8 AU from 16 Cygni B. (3)

1. Cochran, W.D., et al. Astrophysical Journal (in press ).

2. Mazeh, Tsevi, Yuval Krymolowski, and Gady Rosenfeld. "The high eccentricity of the planet orbiting 16 Cygni B." Astrophysical Journal 477(2, Pt. 2):L103-L106, March 10, 1997.

3. Extrasolar Planet Search (3)

EPSILON ERIDANI (HD22049)

Epsilon Eridani is considered to be a star very similiar to our sun. It is a main dwarf sequence star with a temperature of approximately 5,000 K. It has been found to be 3.23 parsecs (10.7 light years) from the Earth. Van de Kamp uncovered information that indicates a wobble in the star's proper motion. Because of its similarity to our sun, its close proximity to us, and the strometric information compiled by van de Kamp, this star has been examined for possible planetary bodies.

Based on the research, two speculative conclusions arise. One is that Epsilon Eridani's companion is nothing more than a dwarf star making this a binary star system. The other train of thought is that based on astrometric measurements, radial velocity measurements, and the existence of a dust cloud, a planetary body exists. This planetary body has been calculated to have a mass twice that of Jupiter, an orbital period of approximately 5 years, and a distance of 5 AU from Epsilon Eridani. (1) At the present time no definitive conclusions can be drawn as to whether there is indeed a planet revolving around this star.

1. Lawton, A.T., and P. Wright. "The search for companions to epsilon Eridani."Journal of the British Interplanetary Society . 43(2):556-8, December, 1990.

GAMMA CEPHEI

The possibility of a planet orbiting this star is rather speculative, unconfirmed, and probably is non-existent. In fact, the original team that suggested a planet recanted their findings in 1992. Since that retraction no information has been forthcoming. Gamma Cephei, a star having a diameter of six times that of our Sun, and producing a brightness equivalent to 11.5 times that of our Sun, has been studied for the possibility of a companion using radial velocity measurements. It lies approximately 15.4 parsecs (51 light years) from our Solar System. Lawton and Wright, citing Campbell, Walker and Yang's brief discussion in Astrophysics of Brown Dwarfs, 1986 state that Campbell and associates feel they have discovered a planet orbiting Gamma Cephei with a mass of 1.5 to 2.0 times the mass of Jupiter lying at a distance from the star at approximately 4.5-5.0 astronomical units; similiar to Jupiters distance from our Sun. (1) In 1992, Walker and his colleagues (2) retracted their previous statement. After 11 years of radial velocity measurements, they conclude that what they were describing previously was the star's period of rotation.

1. Lawton, A.T., and P. Wright. "A planetary system for Gamma Cephei?" Journal of the British Interplanetary Society 47(7):335-336, July 1, 1989.

2. Walker, G.A.H., et al. "Gamma Cephei: rotation or planetary companion?" Astrophysical Journal 396(2, Pt. 2):L91-4, September 10, 1992.

HD3346 (HR152)

In 1996, Noyes, et al. (1) discussed the possibility of two planets revolving about HD3346. Their evidence was based on short-term radial velocity variations. When measuring the radial velocity of the star they noted that the velocity variations were larger than would be expected. They attributed these large variations to the possibility of two planetary bodies revolving about the star. They further suggest that the masses of the two planets are approximately sixty times the mass of Jupiter and approximately ten times Jupiter's mass respectively.

1. Noyes, R., et al. "HD 3346." International Astronomical Union Circular . Issue 6316, February 16, 1996:1p.

HD 114762

This star, found in Coma Bernices, is approximately 27.27 parsecs (90 light years) from Earth. Because of periodic variations in its proper motion, this star has become yet another candidate for the existence of a planetary body or bodies revolving about it. (1)

In the early 1990's two papers were published discussing research on this star and its possible companion. The first of these was involved in the search for eclipses of the star caused by a companion, while the second paper concentrated on radial velocity measurements.

Robinson and his colleagues (2) , who worked on the eclipse problem, felt it was necessary to tabulate eclipse information in order to ascertain the inclination of the stars orbit to the line of sight. The inclination needed to be ascertained in order to identify the companion. The researchers go on to speculate that if the inclination of the orbit is head on from our line of sight then they may be dealing with a companion with a size greater than the mass of thirteen Jupiters. Based on the above speculation the companion could be a brown dwarf or another star. Their research on the eclipse suggests that because the inclination of the orbit was found to be 89 degrees (an inclination far from 90 degrees is needed), their results on the mass of the companion is inconclusive. On the other hand, they do feel that using the occultation method is a way of determining information about a companion, whether it be a star or a planet.

Cochran and his colleagues (3) on the other hand provided research results using high-precision radial velocity. Again the results of their research are inconclusive, because they cannot ascertain whether the companion is a star or a planet. According to Cochran and his group "the determination of the mass of the companion object thus depends on two quantities which remain unknown: the stellar equatorial velocity and the relative orientation of the orbital axis and the rotational axis". (4) They feel that it is imperative to continue to study the star in order to ascertain the star's radial velocity.

In 1995, Alan Hale writing in Publications of the Astronomical Society of the Pacific (5) , continued the study of the star based on the previous results of the early 1990 papers discussed above. Based on measurements taken from spectra at Kitt Peak , it was found that the equatorial inclination of the companion is low, which would also suggest that its orbital inclination as well would be low. Based on the evidence Hale suggests that the companion is probably a low mass M star. He further goes on to say, as Cochran suggested, that the stars radial velocity needs to be ascertained.

At present neither the stars radial velocity has been ascertained nor has there been any confirmation regarding the companion.

1. Anonymous. "Extrasolar planets: A definite "maybe." Sky and Telescope 83(8):8, January 1992.

2. Robinson, Edward L., et al. "A search for eclipses of HD 114762 by a low-mass companion." Astronomical Journal 99(2):672-674, February 1990.

3. Cochran, William D., Hatzes, Artie P., and Terry J. Hancock. "Constraints on the companion object to HD 114762." Astronomical Journal 380(1, Pt 2):L35-L38, October 10, 1991.

4. Ibid Pg. L38.

5. Hale, Alan. "On the nature of the companion to HD 114762." Publications of the Astronomical Society of the Pacific 107(707):22-26, January 1, 1995.

LALANDE 21185

Using photoelectric astrometry Lalande 21185 was studied over a four-year time period for perturbations in the proper motion of the star. Fifty-four observations were made from 1988 through 1991. In 1992 Gatewood and others writing in the Astronomical Journal (1) stated that they were not able to detect any significant perturbation in the star's proper motion. Lalande 21185 was studied because it is very much like our own sun, is relatively close to us (approximately 2.5 parsecs or 8.25 light years), and has a fast proper motion (4.78 arcseconds/year).

Gatewood, in examining 50 years of radial velocity data of Lalande 21185, as well as using a more sophisticated set of observations, contend that, indeed the velocity of the star has been changing over time, thereby suggesting a planetary companion. Gatewood suggests that the companion is between 0.5 to twice the size of Jupiter, revolves around Lalande 21185 between 35 and 50 years, and is approximately 9.5 astronomical units from its sun. (2)

A month later writing in Astronomy , Gatewood revised his conclusion to include not one but possibly two planetary companions revolving about Lalande 21185. Gatewood suggests that the second body may be less massive than Jupiter, and orbiting the star at a distance of approximately 3.5 astronomical units. (3)

At the present time no confirmations have been made on either of the two planets.

1. Gatewood, George, et al. "Multichannel astrometric photometer and photographic astrometric studies in the regions of Lalande 21185, BD 56degrees2966, and HR 4784." Astronomical Journal 104(3):1237-1247.

2. Anonymous. "A nearby extrasolar planet?" Astronomy 24(8):22, August 1996.

3. Anonymous. "Extrasolar planet update." Astronomy 24(9):26 & 28, September, 1996.

Peg51

Mayor and Queloz, using radial velocity measurements, announced to the world in 1995 that they indeed had discovered an extremely large planet (Peg 51 B) orbiting the star Peg 51 , which is a star similiar to our sun; spectral type G2.3. Peg 51 is considered to be approximately 13.7 parsecs in distance from the Earth (45 light years). Their official announcements were made in the October 25, 1995 International Astronomical Union Circular (1) as well as a more formal article that appeared in the November 23, 1995 issue of Nature . (2) The radial velocity measurements were obtained by concentrating their efforts on the doppler shifts of the star over a one year period (1994-1995). The ELODIE spectrograph of the Haute-Provence Observatory in France was used in the study. Although the radial velocity measurements could indicate other possibilities (spot rotation and pulsation), Mayor and Queloz are confident that these alternative possibilities can be ruled out.

By compiling the data, they determined that the planetary body has a minimum mass of half the size of Jupiter, with a mass no more than twice Jupiter's. Lin and others (3) suggest that the mass is probably that of one Jupiter. Other information gathered by Mayor and Queloz (2) suggest that the planet lies about 0.05 astronomical units from Peg 51, with a temperature of 1,300 K, and an orbit having an eccentricity of approximately 0.09, indicating a circular orbit. In addition, the orbital period was calculated to be 4.23 days. It should be noted that Mercury lies between 0.3 and 0.4 astronomical units from the sun, making Peg 51 B much closer to Peg 51 than Mercury is to our Sun. In addition, Mayor and Queloz speculate from their data that perhaps a second low mass planet is revolving about Peg 51 much further out in space than Peg 51 B. They base this speculation on Peg 51's long period perturbation.

One of the burning questions (no pun intended) that begets itself from the above parameters is how could a planet as large as Jupiter form so close to its sun? Based upon current knowledge this does not seem possible. Lin and others (3) feel that Peg 51 B did not form at its current location. Rather, it formed from an amassing of solids and gases at about 5 astronomical units from Peg 51. They feel that it began to approach the star, stopping at its present location due to the result of tidal interactions (inward and outward forces on the planets orbit).

Another point to take into account regarding the above parameters is whether indeed Mayor and Queloz had discovered a planet or another type of body. According to the discoverers they speculate that due to the high temperature of the planet this body could possibly be a low mass brown dwarf. Guillot and others (4) also speculate that Peg 51 B may indeed be a tidally stripped brown dwarf or other star. However, it should be realized that they also consider within the realm of possibility that Peg 51 B may be a giant terrestrial planet.

Marcy and Butler has confirmed Mayor and Queloz's findings. (5) As a footnote, Mayor and Queloz recognize and thank other teams of researchers for confirming their results that indeed a planetary body is orbiting Peg 51. They write at the end of their article the following: "After the announcement of this discovery at a meeting held in Florence, independent confirmations of the 4.2 day period radial-velocity variation were obtained in mid-October by a team at Lick Observatory, as well as by a joint team from the High Altitude Observatory and the Harvard-Smithsonian Center for Astrophysics. We are deeply grateful to G. Marcy, P. Butler, R. Noyes, T. Kennelly and T. Brown for having immediately communicated their results to us." (2)

In addition to the discovery of the planet orbiting Peg 51, Gehman and others (6) suggest that it is very possible for an Earth-like planet to orbit Peg 51. In fact, they extend their premise beyond Peg 51 to further suggest that stars such as Rho 1 Canceri, 47 Ursae Majoris, and 70 Virginis (to some degree) are also strong candidates to have at least one Earth-like planet as part of their solar system family. Further information on Rho 1 Canceri, 47 Ursae Majoris, and 70 Virginis can be found elsewhere in this Section.

Gehman and his colleagues feel that in order for a habitable planet to orbit a star, the orbit of the planet must be dynamically stable, and the planet's temperature must be suitable for liquid water. They base the orbital stability of these systems by applying the circular form of the restricted three-body formulation, and noting the temperature of the planet by obtaining probable values of the incident stellar radiation flux, and the atmospheric and surface properties of the planet. Venus was used as a reference point for the probability study. By applying these two techniques mentioned above, Gehman and associates feel that all four of these star systems could possibly have Earth-like planets, with the possibility of habitability. The results found in 70 Virginis's system is not quite as strong as in the other three. It should be noted that this work was based totally on the fact that a planet had been discovered around each of the four stars under discussion, and that the physical characteristics of these planets were used to determine the probability of other planets in the system; in this case, Earth-like planets.

1. Mayor, M., et al. "51 Pegasi." International Astronomical Union Circular no. 6251:1p.

2. Mayor, M., and D. Queloz. "A Jupiter-mass companion to a solar-type star". Nature 378(6555):355-9, November 23, 1995.

3. Lin, D.N.C., Bodenheimer, P., and D.C. Richardson. "Orbital migration of the planetary companion of 51 Pegasi to its present location." Nature 380(6575):606-7, April 18, 1996.

4. Guillot, T., et al. "Giant planets at small orbital distances." Astrophysical Journal459(1, Pt. 2):L35-8, March 1, 1996.

5. Glanz, James. "Is first extrasolar planet a lost world?" Science 275(5304):1257-1258, February 28, 1997.

6. Gehman, Curtis, Fred C. Adams, and Gregory Laughlin. "The prospects for Earth-like planets within known extrasolar planetary systems." Publications of the Astonomical Society of the Pacific 108(729):1018-1023, November 1996.

POSSIBLE PLANETARY BODIES AROUND PULSARS

Because the following four stars are pulsars, and are rather different than any of the other stars mentioned, it would be best to consider some general information about pulsars, and how it may be possible for planets to have evolved about them.

The discovery of pulsars portrays a rather recent history. Pulsars, by means of a radio telescope, were first identified as such in 1967 by Jocelyn Bell Burnell. Pulsars, which develop from a supernova, are considered to be neutron stars that spin (or pulse); pulsating stars emitting radio waves. These pulses are due to the fact that the magnetic and spin axes of pulsars are not aligned. (1) As of this writing over 650 pulsars are known to exist within our galaxy. (2) Millisecond pulsars is the name given to those pulsars that spin incredibly fast; i.e. faster than 0.01 seconds. Many of these millisecond pulsars are part of a binary star system . (3) This fact is important to keep in mind because in the ensuing entries the companion to a pulsar may be a star as part of the binary system or may be a planet constituting a tertiary system.

There are two main theories regarding the formation of planetary bodies around pulsars. One, which is known as the Salamander Scenario, (4) is that the planet existed around the star before it went supernova and somehow survived, while the other theory known as the Memnonides Scenario (5) and which seems to be more plausible, is that the planet formed after the star went supernova and became a pulsar. The Memnonides Scenario would also imply that some type of a dust disk formed prior to planet formation.

Livio, and others (6) consider a different type of model. They suggest that a pulsar could form from the collision of two white dwarfs and that the planets formed from the debris, which was produced as a result of the collision.

Stevens, and others (7) on the other hand, speculate on planet formation from a binary star system in which one of the stars is a neutron star (pulsar). They argue that the companion star to the pulsar expands and goes over its Roche limit. They feel that due to the disruption of the companion star, a large disc develops around the pulsar, eventually leading to the formation of planets.

In any event, no matter what theory one wishes to consider, it is important that we have a sound footing in the fact that the possibility exists that planets could have formed around a pulsar. If astronomers feel that it is virtually impossible for planets to have formed around a neutron star then the basis of the planetary discoveries that ensue will seem superfluous.

1. Phillips, J.A., and S.E. Thorsett. "Planets around pulsars: a review." Astrophysics and Space Science 212(1-2):91-106, February, 1994.

2. Gribbin, John. Companion to the Cosmos. London: Weidenfeld & Nicolson, 1996. 504 p. Pgs. 325-329.

3. Rankin, Joanna M. Pulsars, Observed Properties. In: The Astronomy and Astrophysics Encyclopedia . New York: Van Nostrand Reinhold, 1991. Pp.567-568.

4. Phillips, J.A. and S.E. Thorsett. "Planets around pulsars: a review." Astrophysics and Space Science 212(1-2):91-106, February, 1994. Pg. 100.

5. Ibid . Pg 100-101.

6. Livio, M., J.E. Pringle, and R.A. Saffer. "Planets around massive white dwarfs." Monthly Notices of the Royal Astronomical Society 257(1):15p-16p, July 1, 1992.

7. Stevens, I.R., M.J. Rees, and P. Podsiadlowski. "Neutron stars and planet-mass companions." Monthly Notices of the Royal Astronomical Society . 254(3):19p-22p, February 1, 1992.

PSR0329+54

In 1994, Dagkesamanky and others (1) suggested, by means of pulsar timing techniques and over a 25 year period, that a planetary body may be orbiting an extremely bright pulsar known as PSR 0329+54 . They suggest that the planet revolves about the pulsar every 6,140 days. In addition, they continue to speculate that a second planet with an orbital period of 1,110 days may also exist. No information since their original announcement in 1994 has been forthcoming. In all practicality these discoveries have not been confirmed.

1. Dagkesamansky, R.D., Shitov, Y.P., and T.V. Shabanova. "PSR 0329+54." International Astronomical Union Circular no. 5930:1p., February 7, 1994.

PSR 1257+12

One week before Bailes and Lyne published their retraction of the existence of a planetary body revolving about PSR 1829-10, (1) Wolszczan and Frail (2) were announcing the fact they had discovered a planetary system around PSR 1257+12 , a millisecond pulsar about 500 parsecs from our Solar System (1,630 light years). In Wolszczan's 1992 paper, they suggest that two, and possibly three, planets may be part of the planetary system. Using a radiotelescope and recording timing measurements from the pulsar, they feel that these two planets have a mass of 2.8 times, and 3.4 times the mass of the Earth. The planet having the 2.8 mass is considered to be 0.47 astronomical units from the pulsar with a nearly circular orbit. The planet has been estimated to revolve about the pulsar in about 98.2 days. The 3.4 mass planet is considered to be 0.36 astronomical units from the pulsar with a nearly circular orbit. This planet has been estimated to revolve about the pulsar in about 66.6 days. The perturbations of the pulse arrival times of this pulsar were independently confirmed by Sallmen and Foster, (3) thereby offering more credence to the existence of the planets mentioned above. Sallmen and Foster felt that this independent confirmation was necessary due to the problems with PSR 1829-10.

Another paper that attempts to verify the existence of planetary systems, deals with the formation of the system around a pulsar. Chakrabarti and Swamy (4) argue that due to the way a planetary system was formed around a pulsar, a series of planetismals should also be present. These planetismals can be verified from spectroscopic analysis of OH, CN and C2. This type of experiment can be used to suggest the possibility of planetary systems around pulsars in general. In addition, they also consider the fact that a third planet may also be part of the system due to the angular momentum and the mass of the cloud circling the pulsar. They go on to say that the planet is approximately 1.1 astronomical units from the pulsar with an orbital period of about a year.

Bisnovatyi-Kogan (5) also supports the fact that this pulsar has more than two planets. In fact according to the above researcher a whole family of planets may exist around PSR 1257+12. He bases his statement that due to the thickness of the disk (the disk must exist because of the fact that planets have already been detected), the size of the disk would be large enough to create outer planets in this system.

Since Wolszcan's initial report suggesting at least two planetary bodies orbiting the pulsar, a number of papers confirming these findings were published. In 1993 (6) and then again in 1994 (7) S.J. Peale, studying the variations (perturbations) in the times of arrival of the pulses produced by PSR 1257+12 suggest that the two planet hypothesis offers the best conclusion of the perturbations. In addition, Wolszcan, himself, (8) also reports confirmation of three planets by measuring the gravitational perturbations of the planets orbits over a three year period by means of timing observations using the Arecibo (305 meter) radiotelescope. A good summary of the planetary system located around PSR 1257+12 can be found in a paper presented by Wolszczan (9) and published as part of the Astronomical Society of the Pacific Conference.

Wolszczan reports that three planets are orbiting the pulsar. The latest parameters of the planetary system are as follows: Planet 1 has a mass of 2.8 times that of Earth, is 0.47 AU away from its star, an orbital period of 98.22 days, and an orbital eccentricity of 0.0264. Planet 2 has a mass of 3.4 times that of the Earth, is 0.36 AU away from its star, an orbital period of 66.54 days, and an orbital eccentricity of 0.0182. Planet 3 has a mass of 0.015 times that of the Earth, is 0.19 AU from its star, an orbital period of 25.34 days, and an orbital eccentricity of 0.0. It should be noted that there have been discrepencies in the parameters in much of the preceeding literature. At the present time it would be best to consider the above parameters to be the definitive word. With more experimentation and confirmation, it is likely that these numbers may change.

An interesting side line regarding the findings of this newly discovered planetary system are the similarities it shows to our own Solar System when the masses and orbital radii of the three newly discovered planets are compared to our three innermost planets. The planet with the smallest mass of the three is located closest to the pulsar. Mercury has the smallest mass when compared with Venus and the Earth and is closest to the Sun. Venus and Earth masses are nearly identical, with Venus being somewhat less massive than Earth. The outer two planets of the pulsar system are also similiar in mass having a distance ratio from the pulsar similiar to the distance ratio for Venus and the Earth. On the basis of this information it would be foolish to base these similarities on all solar systems discovered. Nevertheless, the fact that these planetary systems, in all likelihood, evolved differently (protoplanetary solar accretion disk in the case of our Solar System vs. debris from binary companion in the case of PSR 1257+12's solar system), the similarities are worth noting. (10)

Although the majority of the astronomical community feels confident of this three planetary system, Gil and his associates (11) suggest alternative conclusions for the perturbations of the pulses. Although they do not rule out that the perturbations (tugging of the star back and forth) are due to planetary masses, they do suggest that the "combination of effects involving precession and/or nutation of the spin axis and/or migration of the magnetic axis around the spin axis is responsible for the observed residuals in PSR 1257+12." (11) Basically, what Gil was alluding to was, like the Earth, the pulsar wobbles as it spins thereby causing a precession. On Earth this precession is evidenced by the change in date and time for when a season commences. That is, the date and time varies from year to year. Therefore the perturbations of the timing data may be caused by precession and not by planetary bodies.

It seems safe to say, with the exception of Gil, that the discovery of three planets around the pulsar PSR 1257+12 has been confirmed by the astronomical community at large.

ADDENDUM TO THE ABOVE PARAGRAPHS REGARDING STATEMENTS MADE BY GILL AND HIS ASSOCIATES

This paper was written in May of 1997. Many changes have occured since then. On April 4, 2001, I received an e-mail from Axel Jessner, one of Gil's colleagues. This e-mail states that precession is no longer a good alternative. I wish to quote the statement sent to me by Axel Jessner regarding this issue.

I like to point out, that these ideas represented an alternative, just in case Malhotra's confirmation using secular variations should fail or be inconclusive. They were published before enough data were available for such an analysis. But because of the really striking verification of Wolszcan's interpretation by Malhotra, I do not think that precession is a good alternative to explain the TOA's of PSR 1257+12. I am certain that this view is also shared by my colleague Janusz Gil.

1. Lyne, A.G., and M. Bailes. "No planet orbiting PSR 1829-10." Nature355(6357):213, January 16, 1992.

2. Wolszczan, A., and D.A. Frail. "A planetary system around the millisecond pulsar PSR 1257+12." Nature 355(6356):145-7, January 9, 1992.

3. Sallmen, S., and R. Foster. "Pulsar's double period confirmed." Nature 358(6381):24-5, July 2, 1992.

4. Chakrabarti, S.K., and K.S. Krishna Swamy. "Is there a comet cloud around PSR 1257+12?" Astronomy and Astrophysics 263(1-2):L1-2, September, 1992.

5. Bisnovatyi-Kogan, G.S. "Planetary system around the pulsar PSR 1257+12." Astronomy and Astrophysics 275(1):161-2, August, 1993.

6. Peale, S.J. "On the verification of the planetary system around PSR 1257+12." Astronomical Journal 105(4):1562-70, April, 1993.

7. Peale, S.J. "On the detection of mutual perturbations as proof of planets around PSR 1257+12." Astrophysics and Space Science 212(1-2):77-89, February, 1994.

8. Wolszczan, A. "Confirmation of Earth-mass planets orbiting the millisecond pulsar PSR B1257+12." Science 264(5158):538-42, April 22, 1994.

9. Wolszczan, A. "Pulsar Planets." In: A.S. Fruchter, M. Tavani, and D.C. Backer (Eds.), Millisecond pulsars: a decade of surprise (pp.377-386). San Francisco: Astronomical Society of the Pacific, 1995. (Astronomical Society of the Pacific Conference Series; 72).

10. Mazeh, Tsevi, and Itzhak Goldman. "Similarities between the inner solar system and the planetary system of PSR B1257+12." Publications of the Astronomical Society of the Pacific 107(709):250, March 1995.

11. Gil, J.A., and A. Jessner. "Are there really planets around PSR 1257+12?" In: J.A. Phillips, J.E. Thorsett, and S.R. Kulkarni (Eds.), Planets around Pulsars (pp.71-79). San Francisco: Astronomical Society of the Pacific, 1993. (Astronomical Society of the Pacific; 36).

PSR1620-26 (PSR B1620-26)

During the past several years (1993-present) a series of papers have been published suggesting that a planetary companion may be orbiting the binary pulsar PSR 1620-26 , which is located in the globular cluster Messier 4. What is being considered then is a triple system composed of the binary and a companion. The first series of papers discussing this possibility came out in 1993. Backer (1) , Thorsett (2) , and Sigurdsson (3) all seem to agree that because of the anomalous spin period second derivative of PSR 1620-26, suggesting an additional source of acceleration, a third companion must exist. The nature of this third companion, however, differs from one researcher's findings to another.

Backer and others interpret the data to suggest that the third companion has a mass of 5-10 Jupiters and a circular orbital period of 100-120 years. Thorsett and others suggest either a planet ten astronomical units from the binary system or a star approximately fifty astronomical units from the binary system would reflect the data. Thorsett goes on to consider that much work still needs to be done, and that observations may need to be carried out during a ten-year period. Sigurdsson interprets the data to reveal a somewhat eccentric orbit for the third companion orbiting approximately seven astronomical units from the binary system. Sigurdsson feels that additional monitoring of the binary is necessary in order to ascertain whether the spin period second derivative suggests the existence of a planetary companion.

Rasio (4) reporting in 1994, seems to be more conservative in his findings than what had been mentioned previously. Considering the large eccentricity of the binary system, Rasio contends that a third member of this triple system could not produce this type of eccentricity. He goes on to suggest that a few more years of timing data will be necessary in order to ascertain whether the third companion in question is a planetary body or a star.

In 1994, the Astronomical Society of the Pacific, devoted their entire conference to millisecond pulsars. It was at this conference, that was held January 3-7 in Aspen, Colorado, that PSR 1620-26 was discussed by Backer and Thorsett (5) , Sigurdsson (6) , and Michel (7) . The thrust of these talks once again centered on how the anomalous spin period second derivative could be interpreted. Backer and Thorsett feel that the best interpretation to make is that there is a third body within the system with a limiting mass of between 0.02 and 0.05 that of our sun, thereby suggesting a Jupiter type of body. Sigurdsson suggests two possible scenarios. Either the results indicate a planetary body similiar to the mass of Jupiter with an orbital distance greater than ten astronomical units, and an eccentricity of 0.3-0.5 or a star in a highly eccentric orbit with an orbital distance of approximately fifty astronomical units.

Michel's talk centered on the orbital elements for the companion by means of differential equations. He suggests that a longer period of time is necessary in order to ascertain these orbital elements. In addition, he feels that the companion in question could have any mass. It seems clear from the published articles as well as the published proceedings of the conference that no confirmation has been made regarding the third companion. More work will need to be done in order to determine whether the companion is a star, a brown dwarf, or a planet.

1. Backer, D.C., R.S. Foster, and S. Sallmen. "A second companion of the millisecond pulsar 1620-26." Nature 365(6449):817-18, October 23, 1993.

2. Thorsett, S.E., Z. Arzoumanian, and J.H. Taylor. "PSR B1620-26: a binary radio pulsar with a planetary companion?" Astrophysical Journal 412(1, pt. 2):L33-6, July 20, 1993.

3. Sigurdsson, S. "Genesis of a planet in Messier 4." Astrophysical Journal 415(1, pt. 2):L43-6, September 20, 1993.

4. Rasio, Frederic A. "Is there a planet in the PSR 1620-26 triple system?"Astrophysical Journal 427(2, pt.2):L107-10, June 1, 1994.

5. Backer, D.C., and S.E. Thorsett. "PSR 1620-26--A triple system?" In: A.S. Fruchter, M. Tavani, and D.C. Backer (Eds.), Millisecond pulsars: a decade of surprise (pp. 387-390). San Francisco: Astronomical Society of the Pacific, 1995. (Astronomical Society of the Pacific Conference Series; 72).

6. Sigurdsson, Steinn. "The companion of M4A: a planet or a star?" In: A.S. Fruchter, M. Tavani, and D.C. Backer (Eds.), Millisecond pulsars: a decade of surprise (pp.429-431). San Francisco: Astronomical Society of the Pacific, 1995. (Astronomical Society of the Pacific Conference Series; 72).

7. Michel, F. Curtis. "Orbital elements of PSR 1620-26." In: A.S. Fruchter, M. Tavani, and D.C. Backer (Eds.), Millisecond pulsars: a decade of surprise (pp.421-423). San Francisco: Astronomical Society of the Pacific, 1995. (Astronomical Society of the Pacific Conference Series; 72).

PSR1829-10

Based on the Doppler shift of light emanating from the pulsar PSR 1829-10, Bailes and others (1) suggest that a planet has been found orbiting it. The planet has been calculated to be in an almost circular orbit with an eccentricity of 0.1, and having a mass ten times that of the Earth. The time it takes the planet to make one revolution about the pulsar is about six months. The planet is considered to be 0.7 astronomical units from the star. The pulsar itself, has been calculated to be approximately 10 kpc (about 30,000 light years) from our Sun.

One of the major aspects of this finding is how could a planet form around a pulsar. Because of the importance of this aspect several papers have been published following the announcement by Bailes. Both Lin (2) and Nakamura (3) concede that there is no way the planet could have been present before the star exhibited a supernova and became a pulsar. Therefore, they both suggest that a possible solution is that the planet formed as a result of the supernova. Lin suggests that during the supernova, large amounts of star material were thrown out into space. Some of this material found its way back towards the star. These particles coagulated into plantesimals which in turn accreted into a planetary body over a period of about one million years. Nakamura, on the other hand, considers that this was initially a binary system with a neutron star and a companion with mass of approximately 1.5 times that of the Sun. This companion formed a disk around the neutron star. When the star went supernova the companion star escaped, and the planet was formed from the remnants of the disk.

Podsiadlowski, and others (4) consider two possible explanations for the formation of the planet, which are somewhat similiar to those mentioned above. One possibility is that PSR 1829-10 formed from the merger of two white dwarfs. From this merger some material was left behind thereby forming the planet. The other scenario is that there was a collision between a neutron star and a solar type star that had planets around it. Because of this collision the inner planets of the solar type star, which orginally exhibited an eccentric orbit, became more circular by drag forces.

In considering the above scenarios, the most important point to keep in mind is that the possibility of a planet forming around a pulsar is good, thereby adding more credence to the fact that a planet can possibly exist around PSR 1829-10.

Unfortunately the best laid plans of mice and men do not always come to fruition. In 1992 Lyne and Bailes (5) announced to the world that they were retracting their initial conclusion that a planet exists around PSR 1829-10. They attributed the retraction to the fact that there was an error in the pulsar's original position, thereby creating faulty results in the processing of the information. As of this writing no further information has been forthcoming on the existence of a planetary body revolving about PSR 1829-10.

1. Bailes, M., A.G. Lyne, and S.L. Shemar. "A planet orbiting the neutron star PSR 1829-10." Nature 352(6333):311-13, July 25, 1991.

2. Lin, N.C., S.E. Woosley, and P.H. Bodenheimer. "Formation of a planet orbiting pulsar 1829-10 from the debris of a supernova explosion." Nature 353(6347):827-9, October 31, 1991.

3. Nakamura, T., and T. Piran. "The origin of the planet around PSR 1829-10." Astrophysical Journal 382(2, pt. 2):L81-4, December 1, 1991.

4. Podsiadlowsky, Ph., J.E. Pringle, and M.J. Rees. "The origin of the planet orbiting PSR1829-10." Nature 352(6338):783-4, August 29, 1991.

5. Lyne, A.G., and M. Bailes. "No planet orbiting PSR 1829-10." Nature 355(6357):213, January 16, 1992.

Rho 1 Canceri (HR 3522)

By measuring the doppler shifts in the spectrum of Rho 1 Canceri, a star similiar to our sun; spectral type G8, Marcy and Butler (1) announced that a planetary companion is indeed revolving about the star. The doppler shifts suggested a wobble in the star as seen from the Earth. Therefore, a gravitation pull on the star seems to exist. As a result of this observation, the conclusion is that there may be a planet exerting the gravitational pull. It was also determined by the wobble's characteristics that the planet lies approximately 18 million kilometers (0.11 AU) from the star, and has a mass similiar to that of Jupiter (i.e., 87% Jupiter's mass).

Baliunas, et al, writing in the Astrophysical Journal , (2) verify Marcy and Butler's claim by suggesting that the possibility of a planet revolving about Rho 1 Cancri cannot be ruled out. Their statement was primarily based on analyzing the spectra of the star. The researchers go on to suggest that the planet is probably orbiting the star at 0.1117 astronomical units.

Butler, Marcy, and associates (3) writing in the Astrophysical Journal state that after amassing 41 measured doppler velocities of the star, they infer that a companion does exist. The companion has been calculated to revolve about its star every 14.65 days with an orbital eccentricity of 0.05. In addition, its mass is 0.84 times the mass of Jupiter, and it lies approximately 0.11 astronomical units from its star. The planetary body has also been calculated to have a radius of 1.2 that of Jupiter, and a derived temperature of approximately 700 K. They also go on to suggest that because of the velocity perturbations measured, a second companion may be in the offing. They feel that this second companion has a mass of five Jupiters, and an orbital period of eight years. In addition, the planetary companion is considered to have a minimum mass of 0.84 the mass of Jupiter, with a distance from the star at 0.11 AU, an orbital period of 14.7 days with an orbital eccentricity of 0.05. This planet seems to be acknowledged by the astronomical community.

1. Anonymous. "Another extrasolar planet." Sky & Telescope 92(1):13, July 1996.

2. Baliunas, Sallie, et al. "Properties of sun-like stars with planets: Rho 1 Cancri, Tau Bootis, and Upsilon Andromedae." Astrophysical Journal 474:L119-122, January 10, 1997.

3. Butler, R. Paul, et al. "Three new "51 Pegasi-type" planets." Astrophysical Journal 474:L115-118, January 10, 1997.

Rho Coronae Borealis

On Thursday, April 24, 1997, a month after this paper was written, Robert Noyes of the Harvard-Smithsonian Astrophysical Observatory announced to the world that another planet had been discovered; in this case, around the star Rho Coronae Borealis. In addition, a press release (SAO Release 97-13) issued by three institutions the following day related the result of the findings. The institutions are the Smithsonian Astrophysical Observatory, the National Center for Atmospheric Research , and the Pennsylvania State University. Name recognition for the discovery was awarded to Noyes, Jha, Korsennik, Brown, Kennelly, and Horner. A paper on the discovery is soon to be published in Astrophysical Journal, Letters . For a pre-publication version of the paper click here .

The star, Rho Coronae Borealis can be found in the constellation known as the Northern Crown. This star can be seen with the naked eye. It is approximately 15.15 parsecs (50 light years) from Earth. According to the SAO Press Release (1) the radial velocity measurements for Rho Coronae Borealis were taken with the Advanced Fiber Optic Echelle spectrograph located at the 1.5-meter Tillinghast Reflector of the Fred Lawrence Whipple Observatory on Mt. Hopkins, Arizona. Based on the perturbations of the star, the planet has been determined to be 0.23 astronomical units from its sun, with a mass of 1.3 Jupiters. It revolves around its star every 39.6 days, and has a very small eccentric orbit. The eccentricity has been calculated to be about 0.028, an almost circular orbit. In addition, it has also been determined that the planets temperature is about 300 degrees Centigrade or 500 degrees Fahrenheit. What we have here is similiar to the findings elsewhere in this paper; i.e. a Jupiter sized planet close to its sun with an eccentricity defining an almost circular orbit. As of this time there seems to be strong evidence to indicate that this discovery will be confirmed by the astronomical community as a whole.

1. SAO Press Release No: 97-13

Tau Bootis

Tau Bootis, which is a star similiar to our sun, has been observed 19 times for doppler velocity perturbations. The time span for these measurements extend from 1995 through February 1996. Based upon these observations it has been determined that Tau Bootis has a planetary companion. Calculations seem to imply a companion with a mass of 3.87 times the mass of Jupiter orbiting its star at a distance of 0.0462 astronomical units. Its orbital period around Tau Bootis seems to be every 3.312 days. In addition, the radius of the companion is considered to be about 1.2 times the radius of Jupiter with a derived temperature of 1,400 K. (1)

Baliunas and her associates (2) feel that the velocity perturbations strongly suggest a planetary companion that is 0.046 astronomical units from Tau Bootis. Based on the star's radial velocity variations, it has been determined that it lies approximately 0.046 astronomical units from its star, Tau Bootis. The velocity variations strongly suggest a planet. (2)

It is worth noting that based on the observations and calculations it seems reasonable to assume that indeed a planetary companion is orbiting Tau Bootis.

1. Butler, R. Paul, et al. "Three new "51 Pegasi-type" planets." Astrophysical Journal 474:L115-118, January 10, 1997.

2. Baliunas, Sallie L., et al. "Properties of sun-like stars with planets: Rho 1 Cancri, Tau Bootis, and Upsilon Andromedae." Astrophysical Journal 474:L119-122, January 10, 1997.

T-Tauri Stars

Marsh and Mahoney (1) , (2) in a series of papers have suggested gaps in the interstellar dust clouds (circumstellar disks) that may infer the possibility of a planetary body. This work was carried out by observing the spectral energy distributions of a number of T-Tauri stars in Taurus the Bull. At the present time no conclusions can be drawn regarding what is causing the gaps. Faint stars, brown dwarfs, groups of planets, and planetesimals are all possibilities. The idea of gaps within the interstellar dust clouds was explored more fully in the section on Beta Pictoris .

1. Marsh, K.A., and M.J. Mahoney. "Evidence for unseen companions around T Tauri stars." Astrophysical Journal 395(2, pt.2):L115-18.

2. Marsh, K.A., and M.J. Mahoney. "Do the spectral energy distributions of GK Tauri and HK Tauri indicate the presence of planetary companions?" Astrophysical Journal 405(2, pt.2):L71-4, March 10, 1993.

Upsilon Andromedae

After a total of 18 Doppler measurements, the velocity variations suggest that a planetary companion may be orbiting Upsilon Andromedae, a star very similiar to our Sun. It has been calculated that the companion revolves about its star in about 4.611 days, and has a mass of 0.68 Jupiters. It lies at a distance of 0.057 astronomical units from Upsilon Andromedae. Furthermore, it has been suggested that the companion has a radius of 1.2 times the radius of Jupiter, and a derived temperature of about 1,300 K. (1)

Although there is no photometric data, researchers feel that velocity perturbations caused by convection may be large, thus suggesting a planetary companion. It may be useful to note that although this star is considered to be a spectroscopic binary in many star catalogs, Morbey and Griffin, two research astronomers, feel this is not so. (2)

With the evidence amassed within the last 6 months it seems reasonable to assume that indeed a planetary companion orbits Upsilon Andromedae.

1. Butler, R. Paul, et al. "Three new "51 Pegasi-type" planets." Astrophysical Journal 474:L115-118, January 10, 1997.

2. Baliunas, Sallie L. "Properties of sun-like stars with planets: Rho 1 Cancri, Tau Bootis, and Upsilon Andromedae." Astrophysical Journal 474:L119-122, January 10, 1997.

47 Ursae Majoris

Using doppler shifts to measure the radial velocities, Marcy and Butler (1) strongly suggest that a planet is revolving about 47 Ursae Majoris, which is a star similiar to our Sun. Iodine lines were used as a base reference point to measure the doppler shifts. A total of 34 observations were made spanning an 8.7 year period (1987-1996). According to the velocity curves amassed, Marcy and Butler feel that the planet in question has a minimum mass of 2.39 Jupiter masses, not to exceed 4.8 Jupiter masses, an orbital period of 2.98 years (amount of time it takes for the planet to revolve around its sun), an eccentricity of 0.03, which suggests an almost circular orbit, and an effective temperature of approximately 180 degrees Kelvin. In addition, the authors suggest that the planet is approximately 2.1 astronomical units from its sun. This star is 47 light years from Earth. (2)

What we have here is an extremely large planet, revolving about its star at a distance of 44.5 million miles more than Mars is to our Sun. According to Marcy (3) the effective temperature would be 90 degrees Centigrade, and that its atmosphere could contain liquid water. Unlike the planetary body found revolving about 70 Virginis, there is little doubt that due to its mass, this is more a planet-like body than a brown dwarf.

1. Butler, R. Paul, and Geoffrey W. Marcy. "A planet orbiting 47 Ursae Majoris." Astrophysical Journal 464(2, pt 2):L153-6, June 20, 1996.

2. Anonymous. "More extrasolar planets." Sky & Telescope 91(4):11, April 1996.

3. Cowen, Ron. "Two extrasolar planets may hold water." Science News

70 Virginis

Using doppler shifts to measure radial velocities, Marcy and Butler (1) strongly suggest that a giant planet is revolving about 70 Virginis , which is a star similiar to our Sun. It lies approximately 80 light years from Earth. Iodine lines were used as a base reference point to measure the doppler shifts. They made a total of 39 observations of the star spanning an 8 year period (1988-1996). According to the velocity curves they amassed, they feel that the planet has a mass of between 6.6 and 9.0 Jupiter masses, an orbital period of 116.6 days (amount of time it takes for the planet to revolve around its sun), an eccentricity of 0.40, and an effective temperature of approximately 90 degrees Centigrade (363.15 K). Because of its temperature Marcy (2) suggests that this planet may have oceans, and precipitation in the form of rain. In addition, the planet has been calculated to be 0.43 astronomical units from its sun (70 Virginis). (3)

Because this newly discovered planet is so massive there has been controversy as to whether this is a planet or a brown dwarf. Boss (4) contends that it cannot be a planet. This is based on the fact that a massive planet, such as described above, would not have an eccentric orbit as described by Marcy and Butler; massive planets would have circular orbits such as Jupiter and Saturn. (This is due to the way planets form in relation to the way stars form). In essence, Boss argues that the planetary body around 70 Virginis must be a brown dwarf; albeit a small one, but a brown dwarf nevertheless. There seems to be general agreement among the astronomical community that indeed a planetary body and/or brown dwarf is orbiting 70 Virginis.

1. Marcy, Geoffrey W., and R. Paul Butler. "A planetary companion to 70 Virginis." Astrophysical Journal 464(2, pt.2):L147-51, June 20, 1996.

2. Cowen, Ron. "Two extrasolar planets may hold water." Science News 149(4):52, January 27, 1996.

3. Naeye, Robert. "The strange new planetary zoo." Astronomy 25(4):42-49, April 1997. Pg. 47.

4. Cowen, Ron. "Two extrasolar planets may hold water." Science News 149(4):52, January 27, 1996.

SECTION 4: FUTURE CONSIDERATIONS

The future in the search for the discovery of extrasolar planets relies mainly with two premises. One, is the financial backing to continue programs and missions already in progress, and to develop new ones. Second is the continued development of new technologies and techniques that will enable astromers to detect and confirm the existence of these planetary bodies. It is the purpose of this section to provide some background on the programs that are now in place and to discuss what methodologies would be best in order to detect and confirm additional extrasolar planets. It should be noted that this is not a comprehensive survey of the development of astronomical instruments or programs. However, it is intended to present some information that will relate some idea as to the amount of time and effort now being placed in the search for planetary bodies outside our Solar System.

WHERE DO WE LOOK NEXT

An issue of extreme importance in determining the existence of extrasolar planets, both from a financial and time-saving aspect, is where in the universe should our efforts be concentrated. What stars should be examined? One possibility suggested by Janes (1) is to concentrate our efforts on old open star clusters. Effective photometric search programs are based on the number of stars that are monitored on a regular basis, and stars that are closely packed together. Both of these criteria are met when observing open star clusters. By using photometric techniques, one can observe the brightness variations of a large number of solar-type stars at one time in a star cluster, thereby determining whether the stars in question have been occulted by planets.

In short, the probability of discovering not only Jupiter-type planets but terrestrial ones as well, increases to a greater degree. Terrestrial planets can be better detected by using the transit method (occultation) than by any of the other methods available. Recent developments in ground-based stellar photometry has made this possible. Janes goes on to suggest that an effective program of this type would require at least one or more telescopes with a 4-m aperture. In addition to open star clusters, and based upon the detection of extrasolar planets thus far, stars that are close to us, stars that have large proper motions, stars exhibiting dust disks, stars similiar to our Sun, stars to which planets have already been ascertained, or any combination thereof, should also be considered candidates for future observations.

Another possibility which takes into account planet detection as well as possible life forms on the planet is to look for extrasolar planets that may lie within habitable zones, that is zones where life on a planetary body is possible. Kasting and others (2) , who define habitable zones as those areas in space having planets that are capable of maintaining liquid water on their surfaces, strongly recommend that mid-to-early K stars, as well as G stars (stars similiar to our Sun), should be scrutinized for planetary bodies. K and G stars are similiar in their basic characteristics and temperatures. K stars have temperatures around 4,000 K, while G star temperatures are about 5,600 K. The conclusion on studying these stars is based on the work that these researchers did on climatological modelling (i.e. determining the paramaters for where a habitable zone begins and ends), and what spectral star types would be best to insure habitable zone conditions. The habitable zone would also vary with respect to the age of the star. As stars age they become more luminous. In any event the types of stars listed above should show great promise for the detection of additional extrasolar planets.

FUTURE METHODOLOGIES AND INSTRUMENTATION

Another issue that needs to be examined for future extrasolar planetary discoveries is the type of methodology and instrumentation that needs to be used. One possibility is to extend the array of optical telescopes (i.e. add additional telescopes and spread them further apart for interferometric studies). This should be considered for ground-based and space-based operations . Labeyrie (3) feels that if this is accomplished then extrasolar planets can be resolved. His calculations suggest that to resolve Jupiter-like worlds 5 parsecs away, 10 kilometer arrays will be needed. Likewise to resolve Earth-like worlds 5 parsecs away, 100 kilometer arrays will be needed, and preferrably they should be positioned in space. As a footnote, 23 years ago Ronald N. Bracewell (4) suggested that by creating a system that consisted of two 1-meter telescopes separated by 20 meters, distant worlds could be observed. By observing a heavenly body with two or more telescopes the odds of resolving the body is greater. This type of device or arrangement is known as an interferometer, and has been employed for years for other purposes than the detection of extrasolar planets.

It is interesting to note and certainly something for future observations is an interferometer designed by Angel and Woolf (5) working out of the University of Arizona. They propose an interferometer having two pairs of mirrors arranged in a straight line. Because of this arrangement starlight can effectively be blocked out, thereby allowing this instrument to be 50 to 75 meters in length. Although the signals from distant planetary bodies recorded by this instrument will be complex and unique, the inventors suggest that by analyzing the results carefully one could foreseeably reconstruct an entire solar system, provided of course, that one exists. It has been suggested that this interferometer would cost slightly under 2 billion dollars to construct.

A second possibility focuses on the use of a telescope with a large aperture in high-precision astrometric studies. Using the Palomar 5-meter telescope on an open cluster (NGC 2420), and measuring the sources of noise present in an astrometric search, Pravdo and Shaklan (6) feels that a large telescope can be used to perform a statistically significant search for Jupiter-like planets orbiting nearby stars. They go on to suggest that a total equal to or greater than 100 stars are needed to be surveyed in order to enhance the probability of locating one or more extrasolar planets, provided our idea of solar system formation is correct.

A third prospect deals with the use of the Hubble Space Telescope (HST) . Although it has been determined that the Hubble Space Telescope's Planetary Camera can detect faint companions of stars, such as brown dwarfs, it does not have the capacity to detect planetary bodies smaller than brown dwarfs. (7) Nevertheless this exceptional instrument can still be used in the search for extrasolar planets, and with technological changes it is a good bet that a more sophisticated set of instruments will be devised to enhance HST's capabilities.

PROGRAMS

The Photometry Precision Method Program (PPM) is a substitute for the failed FRESIP Program. FRESIP (Frequency of Earth-Sized Inner Planets) was an extremely promising program that was not selected as a future space program. FRESIP's main responsibility was to determine Earth-sized planets by measuring small intensity changes in a star when it had been occulted by an inner Earth-like planet.

The Photometry Precision Method (PPM) Program has been scheduled to carry out, among other things, the same responsibility that was to be accomplished by the FRESIP Program. PPM will measure photometric variations with extreme precision with a medium size telescope, and an array of charge coupled devices (CCD's). Unlike FRESIP, PPM has received funding from NASA.

A promising and exciting program now under development is referred to as the Darwin Project . (8) This challenging ten-year program takes projects such as PPM, and carries it a step or two further. Its main purpose is to accumulate evidence that would indicate primitive life on an extrasolar planet. The European Space Agency plans to do this by designing an infrared observatory. The basis of this observatory would be to create a null interferometer with an IR spectrometer. This observatory is to be designed, built, and put into a solar orbit at a distance from the Sun at approximately 3.5 astronomical units. One of its main objectives would be to detect photosynthetic activity by detecting an abundant amount of oxygen or ozone in the atmosphere. At present this project is being considered. However, it will not be realized until sometime in the twenty-first century.

Another ambitious project, which is now underway, is the High Resolution Microwave Survey (HRMS) initiated by NASA in October of 1992. (9) Its main objective is to detect signals from approximately 1,000 selected solar-type stars that would indicate technical civilizations. By detecting these signals not only would advanced life be discovered, but extrasolar planet or planets, and of course, life itself would be confirmed.

Some other missions of note are COROT , and GAIA . (11) The Kepler Mission has been proposed by NASA but not selected. (12)

The COROT mission, which has been approved by CNES, a French Space Agency, should be launched in the early 2000's. The instrument to be launched will consist of a ~30-cm telescope with an array of CCDs. It will monitor the light curves of selected stars in order to study extrasolar planets.

ExNPS (Exploration of Neighboring Solar Systems), which is a study leading to the formation of NASA's Origins Program began operation in 1995. By detecting and studying exoplanetary systems, they will try to answer the question: "Are there other worlds in the universe capable of supporting life?" GAIA (Global Astrometric Interferometer for Astrophysics) will survey hundreds of thousands of stars in order to determine the existence of exoplanets. It is felt that this mission should be able to detect Jupiter-mass comnpanions out to 50-200 parsecs, and lower mass planets, such as the Earth, to several parsecs.

The Kepler Mission (12) , which as been proposed, but not selected by NASA at this point in time, will attempt to discover, and characterize hundreds of exoplanets similiar to that of Earth. They are proposing a space borne one-meter aperture photometer with a 12 degree field of view. In addition, it is expected that 80,000 dwarf stars brighter than 14th magnitude will be monitored for flux.

Although many of the programs are based on instrumentations, and platforms placed in Earth or solar orbit, David Tytler of the University of California at San Diego suggests that a large number of techniques and instruments can be used from the ground, rather than sent into space, for the purpose of detecting extrasolar planetary bodies. Although the number of useful techniques and instrumentation are too numerous to mention in this paper, many of the techniques considered relate to more sophisticated astrometry techniques, additional radial velocity measurements, microlensing, and optical imaging. New types of instruments corresponding to the above techniques include astrometric cameras, 2-aperture interferometers, and an IR Echelle spectrograph. Astrometry, radial velocity, and optical imaging have all been discussed in other parts of this paper.

The idea of microlensing has to do with the tenets of Einstein's General Theory of Relativity. Consider some type of astronomical body such as a star or a MACHO (Massive Compact Halo Object) that passes very close to the line of sight of a star more distant than the object. The light of the more distant object will be deflected by the gravity of the object that is closer. The closer object is called the lensing body, while the distant object is considered to be lensed. Martinez (14) suggests that this type of phenomena could be used to detect extrasolar planets. He feels that if a lensing star has a plane or planets orbiting it, the light curve recorded will be different from that of a star without any planetary companions. He goes on to suggest that the light curve will be similiar to that of a lone star but will produce a small perturbation due to the presence of a planet.

As was stated above, these are but a few of the types of instrumentations, methodologies, and missions that are either underway or may be underway shortly in the future. One thing is certain. As a result of the discovery of the exoplanets during the past year, we are bound to see a barage of studies, missions, and the like, to not only further confirm what we already know, but to detect a large number of additional planets, some of which may indeed contain life. As spoken by Dr. Wehinger (15) we have just seen the tip of the iceberg, we have been introduced to the hors d'oeuvres, we are waiting for the first course to arrive.

1. Janes, Kenneth. "Star clusters: Optimal targets for a photometric planetary search program." Journal of Geophysical Research 101 (E6):14853-14859, June 25, 1996.

2. Kasting, James F., Daniel P. Whitmire, and Ray T. Reynolds. "Habitable zones around main sequence stars." Icarus 101:108-128, 1993.

3. Labeyrie, A. "Resolved imaging of extra-solar planets with future 10-100 km optical interferometric arrays." Astronomy and Astrophysics Supplement Series 118(3):517-24, September, 1996.

4. Roger, J., P. Angel, and Neville J. Woolf. Scientific American 274(4):60-66, April, 1996.

5. Ibid 6. Pravdo, Steven H., and Stuart B. Shaklan. Astrophysical Journal 465(1, pt.1):264-77, July 1, 1996.7. Schroeder, Daniel J., and David A. Golimowski. "Searching for faint companions to nearby stars with the Hubble Space Telescope." Publications of the Astronomical Society of the Pacific 108(724):510-19, June 1996.

8. Leger, A., et al. "How to evidence primitive life on an exo-planet?--The Darwin Project. " Space Science Reviews 74(1-2):163-9, October, 1995.

9. Latham, D.W., and D.R. Soderblom. "SETI target selection." Acta Astronautica 35(9-11):741-4, May-June, 1995.

11. Global Astrometric Interferometer for Astrophysics .

12. Kepler Mission .

14. Martinez, Peter. "Searching for planets with gravitational microlensing." Monthly Notices of the Astonomical Society of Southern Africa 55(11 & 12):179-181, December, 1996.

15. Presented by Dr. Wehinger in a lecture February 19, 1997 in a class entitled "Origins: Cosmic and Biological Evolution" at Arizona State University.

SECTION 5: CONCLUSIONS

It is evident that great strides have been made during the past several years in detecting, and to some degree, confirming that planets outside our own Solar System exist. The work thus accomplished was by no means easy. Detections have been made using extremely precise instrumentation and precise analysis of data. The technology of the equipment and the ingenuity of the astronomers should not be overlooked. We are on the threshold of a new revolution. However, as is the case of many new discoveries, a monkey wrench has been thrown into the fray. Published in the February 27, 1997 issue of Nature , David Gray, an astrophysicist from the University of Western Ontario, concentrating specifically on the data and analysis of 51 Peg, disputes the discoveries of these exoplanets. In short, he feels that researchers were too fast to publish their results, which led to serious mistakes in the analysis of their spectral data.

Focusing in on the data collected for 51 Pegasi, Gray contends (1) that the radial velocity variations of the star measured by doppler shifts of the absorption lines in the spectra do not constitute the existence of a planetary body. He feels that it is the pulsation of the star itself (oscillatory upheavals in the star's atmosphere) causing the variations, and not the force of gravity exerted on the star by a planetary companion. (2) Gray states that (3) "It is important to realize that high spectral resolving power, greater than 100,000, and high signal-to-noise ratios are needed to see the asymmetries of spectral lines. Radial velocity investigations generally lack this essential ingredient."

Marcy and Butler, the astronomers who confirmed the existence of Peg 51, along with Mayor and Queloz, the original discoverers, do not buy into Gray's pulsation theory. In a retort paper , (4) , placed at Marcy and Butler's web site, they set forth a number of reasons to suggest that Gray is incorrect in his analysis of the situation. One of the reasons for refuting Gray's claim is that no variations in brightness in stars like Peg 51 were ever found, and yet, Gray is basing the non-existence of the planet on this fact. Another reason that Marcy and his colleagues dispute Gray's claim is that doppler variations that exhibit sine waves are shown to be the case in all Peg-like stars studied. They feel this is due to the planets gravitational pull on the star with the planet being in a circular orbit. Therefore, a planet exists around the star. Marcy and his colleagues, however, concede that the gravitational force of a planet could change the flow of gases on a star, which would indicate changes in the absorption line stated by Gray. These are but a few of the many reasons given by Marcy and others in refuting Gray's claims. However it should be realized that some astronomers think that Gray's alternative explanation should have its day in court, and that the issue should be resolved. (5)

Although the controversy will no doubt continue, the great strides made by astrophysicists in the last few years are exceptional. With the search continuing by more researchers using more sophisticated technology, coupled with the probability factor of other worlds existing, there are bound to be a mass of discoveries with a more solid footing than at present. We are indeed on the brink of a revolution. Not only is the search for the other worlds great, but the search for other words lying in habital zones having the possibility of fostering life is just as great.

One area of inquiry that no doubt will be studied in the future is the possibility of moons around some of the giant extrasolar planets that have been discovered. More than likely these giant extrasolar planets will be determined to have characteristics more like Jupiter or Saturn than that of the terrestrial ones, and that the possibility of life on these will be limited or non-existent. As we learn more about our own Solar System, our knowledge in this area will be extended to the extrasolar planets. It seems plausible, based on our probes, that Europa may have a liquid subsurface ocean, and that life may exist on it. (6) Consider the possibility that if there were moons around some of the newly discovered planets that are considered to lie in habitable zone orbits, then the possiblity of life on them will be greater than life on the moons of Jupiter which do not lie in a habitable zone area. Williams and others (7) suggest that the planetary companions to 16 Cygni B and 47 Ursae Majoris would be prime candidates for life to exist on their moons, provided of course they have moons.

The next several years before the new millenium will provide great excitement in the search for planets outside our Solar System and set the stage for astronomical discoveries in the twenty-first century that will be beyond our wildest dreams. Hopefully NASA's Origins Program will provide us with many of the new findings.

1. Gray, David F. "Absence of a planetary signature in the spectra of the star 51 Pegasi."
Nature 385(6619):795-796, February 27, 1997.

2. Walker, Gordon. "One of our planets is missing." Nature 385(6619):775-776, February 27, 1997.

3. Gray, David F. "Absence of a planetary signature in the spectra of the star 51 Pegasi." Nature 385(6619):795-796, February 27, 1997. Pg. 796.

4. Mayor, M, D. Queloz, G. Marcy, R.P. Butler. "Can the 51 Peg Doppler Variations be due to non-radial pulsations."

5. Glanz, James. "Is first extrasolar planet a lost world?" Science 275(5304):1257-1258, February 28, 1997.

6. Chyba, Christopher F. "Life on other moons." Nature 385(6613):201, January 16, 1997.

7. Williams, Darren M., James F. Kasting, and Richard A. Wade. "Habitable moons around extrasolar giant planets." Nature 385(6613):234-235, January 16, 1997.

INTRODUCTION AND EXPLANATION OF THE APPENDIX

What follows is a table providing data on the various planets mentioned in the paper. The name of the stars are listed alphabetically. In most cases the name of the star, its spectral type, and its distance from Earth is given in the first column. The second column represents the planets distance from its star in astronomical units. The third column sets forth the mass of the planet in either MJ (Jupiter masses) or ME (Earth masses). The fourth column describes the amount of time it takes for the planet to revolve about its star, while the fifth column considers the planets eccentricity. The sixth column refers to the temperature of the planet, and the seventh column discusses the type of astronomical methodology used in determining the existence of the planet. The eighth column provides the names of the astronomers associated with their discovery, while the ninth and last column describes the planet's acceptance or non-acceptance by most, if not all of the astronomical community. The information given in the Appendix is current as of May 5, 1997. With such a rapidly evolving field, these parameters may be subject to change.

It should be noted that by examining the table, the majority of the planets thus discovered were determined by radial velocity measurements, that the Jupiter mass planets, for the most part, have been found orbiting close to (<1 AU) stars similiar to our Sun, and therefore, orbit the stars in days rather than years. In addition, the majority of the planets discovered around pulsars are much smaller than the planets orbiting stars similiar to our Sun, and are measured in Earth rather than Jupiter masses.

Finally, it is also interesting to note from examining the table that the majority of the orbits of the newly discovered planets are nearly circular, thereby giving credence to the fact that the formation of these systems, in most likelihood, evolved similiar to our own Solar System (by the rotation of swirling gas clouds that eventually coalesced over time).

APPENDIX

STAR PLANET DISTANCE PLANET MASS PLANET ORBITAL PERIOD PLANET ORBIT E PLANET TEMP METHODOLOGY DISCOVERERS CONFIRM
Barnard's Star Red Dwarf (1.82 PC) (5.95 LY) 2.7 AU 0.7 MJ 12 years Circular N/A Astrometry van de Kamp No
Barnard's Star 3.8 AU 0.5 MJ 20 Years Circular N/A Astrometry van de Kamp No
Beta Pictoris A5V (17.17 PC) (58.68 LY) 20 AU 5 ME N/A 0.01 N/A Void in Circumstellar Dust Disk Roques Scholl Sicardy Smith No
CM-DraconisM Star Binary N/A N/A N/A N/A N/A Photometry (Occultation) N/A N/A
16 Cygni B Similiar to our Sun Wide Binary; 16 Cygni AB 1.8 AU 1.5-1.7 MJ 800.8 Days 0.63 N/A Radial Velocity Measurements Cochran No
Epsilon EridaniK2V(3.23 PC) (10.7 LY) 5 AU 2.0 MJ 5 Years N/A N/A Radial Velocity Measurments; Astrometry LawtonWright No
Gamma Cephei (15.4 PC)(51 LY) 4.5-5 AU 1.5-2 MJ N/A N/A N/A Radial Velocity Measurements Campbell No (Retracted)
HD3346 (HR152) K5III N/A 2 Planets (60 MJ and 10 MJ) N/A N/A N/A Radial Velocity Measurments Noyes No
HD114762F9V(27.27 PC) (90 LY) 0.40 AU 13 MJ N/A N/A N/A Occultation and Radial Velocity Measurments Robinson and Cochran No; Possibly a Star
Lalande 21185 (2.5 PC) (8.25 LY) 3.5 AU <1 MJ N/A N/A N/A Photoelectric Astrometry Gatewood No
Lalande21185 9.5 AU 0.5-2 MJ 35-50 Years N/A N/A Photoelectric Astrometry Gatewood No
Peg51G2.3(13.7 PC) (45 LY) 0.05 AU 0.5-1 MJ 4.23 Days 0.09 1300K Radial Velocity Measurements Mayor and Queloz Yes
PSR 0329+54 Pulsar N/A N/A 2 Planets (6140 Days and 1110 Days) N/A N/A Pulsar Timing Dagkesamanky No
PSR 1257+126.2 ms Pulsar(500 PC) (1,630 LY) 0.19 AU 0.015 ME 25.34 Days 0.00 N/A Pulsar Timing Wolszczan Frail Chakrabarti Swamy Yes
PSR 1257+12 0.36 AU 3.4 ME 66.54 Days 0.0182 N/A Pulsar Timing Wolszczan Frail Yes
PSR 1257+12 0.47 AU 2.8 ME 98.22 Days 0.0264 N/A Pulsar Timing Wolszczan Frail Yes
PSR 1620-26 .011 ms Pulsar (2 KPC) (6,000 LY) >10 AU? 5-10 MJ? or 1 MJ? 100-120 Years 0.3-0.5 N/A Anomalous Spin Period (2nd Derivative) Backer Thorsett Sigurdsson No
PSR 1829-10 ms Pulsar (10KPC) (30,000 LY) 0.7 AU 10 ME 6 Months 0.1 N/A Pulsar Timing Bailes Lyne No (Retracted)
Rho 1 Canceri (HR3522) G8 (76.8 PC) (253 LY) 0.11 AU 0.84 MJ 14.65 Days 0.05 700K Radial Velocity Measurements Marcy Butler Yes
Rho 1 Canceri (HR3522) N/A 5 MJ 8 Years 0.00 N/A Radial Velocity Measurements Marcy Butler No
Rho Coronae Borealis G0V (15.15 PC) (50 LY) 0.23 AU 1.13 MJ 39.6 Days 0.028 300C Radial Velocity Measurements Noyes, et al Yes
Tau Bootis F7V (54.5 PC) (180 LY) 0.0462 AU 3.87 MJ 3.312 Days 0.00 1400K Radial Velocity Measurements Marcy Butler Yes
Upsilon Andromedae F8V(56.8 PC) (187.4 LY) 0.057 AU 0.68 MJ 4.611 Days 0.00 1300K Radial Velocity Measurements Marcy Butler Yes
47 Ursae Majoris G0V (14.24 PC) (47 LY) 2.1 AU >2.39 MJ or <4.8 MJ 2.98 Years 0.03 180K Radial Velocity Measurements Marcy Butler Yes
70 Virginis G4V (24.24 PC) (80 LY) 0.43 AU 6.6-9 MJ 116.6 Days 0.40 363.15K Radial Velocity Measurements Marcy Butler Yes

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Copyright 1997, Arizona State University

Written and Compiled by George H. Bell
george.bell@asu.edu
May 1997; Last Revised; April 5, 2001
URL: http://www.public.asu.edu/\~sciref/exoplnt.htm