Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing (original) (raw)

Main

Gravitational microlensing events can reveal extrasolar planets orbiting the foreground lens stars if the light curves are measured frequently enough to characterize planetary light curve deviations with features lasting a few hours6,7,8,9. Microlensing is most sensitive to planets in Earth-to-Jupiter-like orbits with semi-major axes in the range 1–5 au. The sensitivity of the microlensing method to low-mass planets is restricted by the finite angular size of the source stars10,11, limiting detections to planets of a few _M_⊕ for giant source stars, but allowing the detection of planets as small as 0.1_M_⊕ for main-sequence source stars in the Galactic Bulge. The PLANET collaboration12 maintains the high sampling rate required to detect low-mass planets while monitoring the most promising of the >500 microlensing events discovered annually by the OGLE collaboration, as well as events discovered by MOA. A decade of pioneering microlensing searches has resulted in the recent detections of two Jupiter-mass extrasolar planets13,14 with orbital separations of a few au by the combined observations of the OGLE, MOA, MicroFUN and PLANET collaborations. The absence of perturbations to stellar microlensing events can be used to constrain the presence of planetary lens companions. With large samples of events, upper limits on the frequency of Jupiter-mass planets have been placed over an orbital range of 1–10 au, down to _M_⊕ planets15,16,[17](/articles/nature04441#ref-CR17 "Dong, S. et al. Planetary detection efficiency of the magnification 3000 microlensing event OGLE-2004-BLG-343. Astrophys. J. (submitted); preprint at http://arXiv.org/astro-ph/0507079

             (2005)") for the most common stars of our galaxy.

On 11 July 2005, the OGLE Early Warning System18announced the microlensing event OGLE-2005-BLG-390 (right ascension α = 17 h 54 min 19.2 s, declination δ = -30° 22′ 38″, J2000) with a relatively bright clump giant as a source star. Subsequently, PLANET, OGLE and MOA monitored it with their different telescopes. After peaking at a maximum magnification of _A_max = 3.0 on 31 July 2005, a short-duration deviation from a single lens light curve was detected on 9 August 2005 by PLANET. As described below, this deviation was due to a low-mass planet orbiting the lens star.

From analysis of colour-magnitude diagrams, we derive the following reddening-corrected colours and magnitudes for the source star: (V - I)0 = 0.85, _I_0 = 14.25 and (V - K)0 = 1.9. We used the surface brightness relation20 linking the emerging flux per solid angle of a light-emitting body to its colour, calibrated by interferometric observations, to derive an angular radius of 5.25 ± 0.73 µas, which corresponds to a source radius of 9.6 ± 1.3_R_⊙ (where _R_⊙ is the radius of the Sun) if the source star is at a distance of 8.5 kpc. The source star colours indicate that it is a 5,200 K giant, which corresponds to a G4 III spectral type.

Figure 1 shows our photometric data for microlensing event OGLE-2005-BLG-390 and the best planetary binary lens model. The best-fit model has _χ_2 = 562.26 for 650 data points, seven lens parameters, and 12 flux normalization parameters, for a total of 631 degrees of freedom. Model length parameters in Table 1 are expressed in units of the Einstein ring radius _R_E (typically ∼2 au for a Galactic Bulge system), the size of the ring image that would be seen in the case of perfect lens–source alignment. In modelling the light curve, we adopted linear limb darkening laws21 with _Γ_I = 0.538 and _Γ_R = 0.626, appropriate for this G4 III giant source star, to describe the centre-to-limb variation of the intensity profile in the I and R bands. Four different binary lens modelling codes were used to confirm that the model we present is the only acceptable model for the observed light curve. The best alternative model is one with a large-flux-ratio binary source with a single lens, which has gross features that are similar to a planetary microlensing event22. However, as shown in Fig. 1, this model fails to account for the PLANET-Perth, PLANET-Danish and OGLE measurements near the end of the planetary deviation, and it is formally excluded by Δ_χ_2 = 46.25 with one less model parameter.

Figure 1: The observed light curve of the OGLE-2005-BLG-390 microlensing event and best-fit model plotted as a function of time.

Error bars are 1_σ_. The data set consists of 650 data points from PLANET Danish (ESO La Silla, red points), PLANET Perth (blue), PLANET Canopus (Hobart, cyan), RoboNet Faulkes North (Hawaii, green), OGLE (Las Campanas, black), MOA (Mt John Observatory, brown). This photometric monitoring was done in the I band (with the exception of the Faulkes R-band data and the MOA custom red passband) and real-time data reduction was performed with the different OGLE, PLANET and MOA data reduction pipelines. Danish and Perth data were finally reduced by the image subtraction technique19 with the OGLE pipeline. The top left inset shows the OGLE light curve extending over the previous 4 years, whereas the top right one shows a zoom of the planetary deviation, covering a time interval of 1.5 days. The solid curve is the best binary lens model described in the text with q = 7.6 ± 0.7 × 10-5, and a projected separation of d = 1.610 ± 0.008_R_E. The dashed grey curve is the best binary source model that is rejected by the data, and the dashed orange line is the best single lens model.

Full size image

Table 1 Microlensing fit parameters

Full size table

The planet is designated OGLE-2005-BLG-390Lb, where the ‘Lb’ suffix indicates the secondary component of the lens system with a planetary mass ratio. The microlensing fit only directly determines the planet–star mass ratio, q = 7.6 ± 0.7 × 10-5, and the projected planet–star separation, d = 1.610 ± 0.008_R_E. Although the planet and star masses are not directly determined for planetary microlensing events, we can derive their probability densities. We have performed a bayesian analysis[23](/articles/nature04441#ref-CR23 "Dominik, M. Stochastical distributions of lens and source properties for observed galactic microlensing events. Mon. Not. R. Astron. Soc. (submitted); preprint at http://arXiv.org/astro-ph/0507540

             (2005)") employing the Galactic models and mass functions described in refs [11](/articles/nature04441#ref-CR11 "Bennett, D. P. & Rhie, S. H. Simulation of a space-based microlensing survey for terrestrial extrasolar planets. Astrophys. J. 574, 985–1003 (2002)") and [23](/articles/nature04441#ref-CR23 "Dominik, M. Stochastical distributions of lens and source properties for observed galactic microlensing events. Mon. Not. R. Astron. Soc. (submitted); preprint at 
              http://arXiv.org/astro-ph/0507540
              
             (2005)"). We averaged over the distances and velocities of the lens and source stars, subject to the constraints due to the angular diameter of the source and the measured parameters given in [Table 1](/articles/nature04441#Tab1). This analysis gives a 95% probability that the planetary host star is a main-sequence star, a 4% probability that it is a white dwarf, and a probability of <1% that it is a neutron star or black hole. The host star and planet parameter probability densities for a main sequence lens star are shown in [Fig. 2](/articles/nature04441#Fig2) for the Galactic model used in ref. [23](/articles/nature04441#ref-CR23 "Dominik, M. Stochastical distributions of lens and source properties for observed galactic microlensing events. Mon. Not. R. Astron. Soc. (submitted); preprint at 
              http://arXiv.org/astro-ph/0507540
              
             (2005)"). The medians of the lens parameter probability distributions yield a companion mass of ![](http://media.springernature.com/lw50/springer-static/image/art%3A10.1038%2Fnature04441/MediaObjects/41586_2006_Article_BFnature04441_IEq1_HTML.gif) and an orbital separation of ![](http://media.springernature.com/lw49/springer-static/image/art%3A10.1038%2Fnature04441/MediaObjects/41586_2006_Article_BFnature04441_IEq2_HTML.gif) au from the ![](http://media.springernature.com/lw62/springer-static/image/art%3A10.1038%2Fnature04441/MediaObjects/41586_2006_Article_BFnature04441_IEq3_HTML.gif)_M_⊙ lens star, which is located at a distance of _D_L \= 6.6 ± 1.0 kpc. These error bars indicate the central 68% confidence interval. These median parameters imply that the planet receives radiation from its host star that is only 0.1% of the radiation that the Earth receives from the Sun, so the probable surface temperature of the planet is ∼50 K, similar to the temperatures of Neptune and Pluto.

Figure 2: Bayesian probability densities for the properties of the planet and its host star.

a, The masses of the lens star and its planet (M* and _M_p respectively), b, their distance from the observer (D L), c, the three-dimensional separation or semi-major axis a of an assumed circular planetary orbit; and d, the orbital period Ω of the planet. (In a, _M_ref refers to _M_⊕ on the upper x axis and _M_⊙ on the lower x axis.) The bold, curved line in each panel is the cumulative distribution, with the percentiles listed on the right. The dashed vertical lines indicate the medians, and the shading indicates the central 68.3% confidence intervals, while dots and arrows on the abscissa mark the expectation value and standard deviation. All estimates follow from a bayesian analysis assuming a standard model for the disk and bulge population of the Milky Way, the stellar mass function of ref. 23, and a gaussian prior distribution for _D_S = 1.05 ± 0.25_R_GC (where _R_GC = 7.62 ± 0.32 kpc for the Galactic Centre distance). The medians of these distributions yield a _M_⊕ planetary companion at a separation of au from a _M_⊙ Galactic Bulge M-dwarf at a distance of 6.6 ± 1.0 kpc from the Sun. The median planetary period is years. The logarithmic means of these probability distributions (which obey Kepler's third law) are a separation of 2.9 au, a period of 10.4 years, and masses of 0.22_M_⊙ and 5.5_M_⊕ for the star and planet, respectively. In each plot, the independent variable for the probability density is listed within square brackets. The distribution of the planet–star mass ratio was taken to be independent of the stellar mass, and a uniform prior distribution was assumed for the planet–star separation distribution.

Full size image

The parameters of this event are near the limits of microlensing planet detectability for a giant source star. The separation of d = 1.61 is near the outer edge of the so-called lensing zone7, which has the highest planet detection probability, and the planet's mass is about a factor of two above the detection limit set by the finite size of the source star. Planets with q > 10-3 and d ≈ 1 are much easier to detect, and so it may be that the parameters of OGLE-2005-BLG-390Lb represent a more common type of planet. This can be quantified by simulating planetary light curves with different values of q and θ (where θ is the angle of source motion with respect to the lens axis) but the remaining parameters are fixed to the values for the three known microlensing planets. We find that the probability of detecting a q ≈ 4–7 × 10-3 planet, like the first two microlens planets13,14, is ∼50 times larger than the probability of detecting a q = 7.6 × 10-5 planet like OGLE-2005-BLG-390Lb. This suggests that, at the orbital separations probed by microlensing, sub-Neptune-mass planets are significantly more common than large gas giants around the most common stars in our Galaxy. Similarly, the first detection of a sub-Neptune-mass planet at the outer edge of the ‘lensing zone’ provides a hint that these sub-Neptune-mass planets may tend to reside in orbits with semi-major axes a > 2 au.

The core-accretion model of planet formation predicts that rocky/icy 5–15_M_⊕ planets orbiting their host stars at 1–10 au are much more common than Jupiter-mass planets, and this prediction is consistent with the small fraction of M-dwarfs with planets detected by radial velocities3,5 and with previous limits from microlensing15. Our discovery of such a low-mass planet by gravitational microlensing lends further support to this model, but more detections of similar and lower-mass planets over a wide range of orbits are clearly needed. Planets with separations of ∼0.1 au will be detected routinely by the radial velocity method or space observations of planetary transits in the coming years24,25,26,27, but the best chance to increase our understanding of such planets over orbits of 1–10 au in the next 5–10 years is by future interferometer programs28 and more advanced microlensing surveys11,29,30.

References

1. Safronov, V. Evolution of the Protoplanetary Cloud and Formation of the Earth and Planets (Nauka, Moscow, 1969)
   Google Scholar
2. Wetherill, G. W. Formation of the terrestrial planets. Annu. Rev. Astron. Astrophys 18, 77–113 (1980)
   Article ADS CAS Google Scholar
3. Laughlin, G., Bodenheimer, P. & Adams, F. C. The core accretion model predicts few jovian-mass planets orbiting red dwarfs. Astrophys. J. 612, L73–L76 (2004)
   Article ADS Google Scholar
4. Ida, S. & Lin, D. N. C. Toward a deterministic model of planetary formation. II. The formation and retention of gas giant planets around stars with a range of metallicities. Astrophys. J. 616, 567–572 (2004)
   Article ADS Google Scholar
5. Rivera, E. et al. A ∼7.5 Earth-mass planet orbiting the nearby star, GJ 876. Astrophys. J. (in the press)
6. Mao, S. & Paczynski, B. Gravitational microlensing by double stars and planetary systems. Astrophys. J. 374, L37–L40 (1991)
   Article ADS Google Scholar
7. Gould, A. & Loeb, A. Discovering planetary systems through gravitational Microlenses. Astrophys. J. 396, 104–114 (1992)
   Article ADS Google Scholar
8. Wambsganss, J. Discovering Galactic planets by gravitational microlensing: magnification patterns and light curves. Mon. Not. R. Astron. Soc. 284, 172–188 (1997)
   Article ADS Google Scholar
9. Griest, K. & Safizadeh, N. The use of high-magnification microlensing events in discovering extrasolar planets. Astrophys. J. 500, 37–50 (1998)
   Article ADS Google Scholar
10. Bennett, D. P. & Rhie, S. H. Detecting Earth-mass planets with gravitational microlensing. Astrophys. J. 472, 660–664 (1996)
    Article ADS Google Scholar
11. Bennett, D. P. & Rhie, S. H. Simulation of a space-based microlensing survey for terrestrial extrasolar planets. Astrophys. J. 574, 985–1003 (2002)
    Article ADS Google Scholar
12. Albrow, M. et al. The 1995 pilot campaign of PLANET: searching for microlensing anomalies through precise, rapid, round-the-clock monitoring. Astrophys. J. 509, 687–702 (1998)
    Article ADS Google Scholar
13. Bond, I. A. et al. OGLE 2003-BLG-235/MOA 2003-BLG-53: A planetary microlensing event. Astrophys. J. 606, L155–L158 (2004)
    Article ADS Google Scholar
14. Udalski, A. et al. A jovian-mass planet in microlensing event OGLE-2005-BLG-071. Astrophys. J. 628, L109–L112 (2005)
    Article ADS Google Scholar
15. Gaudi, B. S. et al. Microlensing constraints on the frequency of Jupiter-mass companions: analysis of 5 years of PLANET photometry. Astrophys. J. 566, 463–499 (2002)
    Article ADS Google Scholar
16. Abe, F. et al. Search for low-mass exoplanets by gravitational microlensing at high magnification. Science 305, 1264–1267 (2004)
    Article ADS CAS Google Scholar
17. Dong, S. et al. Planetary detection efficiency of the magnification 3000 microlensing event OGLE-2004-BLG-343. Astrophys. J. (submitted); preprint at http://arXiv.org/astro-ph/0507079 (2005)
18. Udalski, A. The optical gravitational lensing experiment. real time data analysis systems in the OGLE-III survey. Acta Astron. 53, 291–305 (2003)
    ADS Google Scholar
19. Alard, C. Image subtraction using a space-varying kernel. Astron. Astrophys. Suppl. 144, 363–370 (2000)
    Article ADS Google Scholar
20. Kervella, P. et al. Cepheid distances from infrared long-baseline interferometry. III. Calibration of the surface brightness-color relations. Astron. Astrophys. 428, 587–593 (2004)
    Article ADS CAS Google Scholar
21. Claret, A., Diaz-Cordoves, J. & Gimenez, A. Linear and non-linear limb-darkening coefficients for the photometric bands R I J H K. Astron. Astrophys. Suppl. 114, 247–252 (1995)
    ADS Google Scholar
22. Gaudi, B. S. Distinguishing between binary-source and planetary microlensing perturbations. Astrophys. J. 506, 533–539 (1998)
    Article ADS Google Scholar
23. Dominik, M. Stochastical distributions of lens and source properties for observed galactic microlensing events. Mon. Not. R. Astron. Soc. (submitted); preprint at http://arXiv.org/astro-ph/0507540 (2005)
24. Vogt, S. S. et al. Five new multicomponent planetary systems. Astrophys. J. 632, 638–658 (2005)
    Article ADS CAS Google Scholar
25. Mayor, M. et al. The CORALIE survey for southern extrasolar planets. XII. Orbital solutions for 16 extrasolar planets discovered with CORALIE. Astron. Astrophys. 415, 391–402 (2004)
    Article ADS CAS Google Scholar
26. Borucki, W. et al. in Second Eddington Workshop: Stellar Structure and Habitable Planet Finding (eds Favata, F., Aigrain, S. & Wilson, A.) 177–182 (ESA SP-538, ESA Publications Division, Noordwijk, 2004)
    Google Scholar
27. Moutou, C. et al. Comparative blind test of five planetary transit detection algorithms on realistic synthetic light curves. Astron. Astrophys. 437, 355–368 (2005)
    Article ADS Google Scholar
28. Sozzeti, A. et al. Narrow-angle astrometry with the space interferometry mission: the search for extrasolar planets. I. Detection and characterization of single planets. Pub. Astron. Soc. Pacif. 114, 1173–1196 (2002)
    Article ADS Google Scholar
29. Bennett, D. P. in ASP Conf. Ser. on Extrasolar Planets: Today and Tomorrow (eds Beaulieu, J.-P., Lecavelier des Etangs, A. & Terquem, C.) Vol. 321, 59–68 (ASP, 2004)
    Google Scholar
30. Beaulieu, J. P. et al. PLANET III: searching for Earth-mass planets via microlensing from Dome C? ESA Publ. Ser. 14, 297–302 (2005)
    Article Google Scholar

Download references

Acknowledgements

PLANET is grateful to the observatories that support our science (the European Southern Observatory, Canopus, Perth; and the South African Astronomical Observatory, Boyden, Faulkes North) and to the ESO team in La Silla for their help in maintaining and operating the Danish telescope. Support for the PLANET project was provided by CNRS, NASA, the NSF, the LLNL/NNSA/DOE, PNP, PICS France-Australia, D. Warren, the DFG, IDA and the SNF. RoboNet is funded by the UK PPARC and the FTN was supported by the Dill Faulkes Educational Trust. Support for the OGLE project, conducted at Las Campanas Observatory (operated by the Carnegie Institution of Washington), was provided by the Polish Ministry of Science, the Foundation for Polish Science, the NSF and NASA. The MOA collaboration is supported by MEXT and JSPS of Japan, and the Marsden Fund of New Zealand.

Author information

Authors and Affiliations

1. PLANET/RoboNet Collaboration,
   J.-P. Beaulieu, D. P. Bennett, P. Fouqué, A. Williams, M. Dominik, U. G. Jørgensen, D. Kubas, A. Cassan, C. Coutures, J. Greenhill, K. Hill, J. Menzies, P. D. Sackett, M. Albrow, S. Brillant, J. A. R. Caldwell, J. J. Calitz, K. H. Cook, E. Corrales, M. Desort, S. Dieters, D. Dominis, J. Donatowicz, M. Hoffman, S. Kane, J.-B. Marquette, R. Martin, P. Meintjes, K. Pollard, K. Sahu, C. Vinter, J. Wambsganss, K. Woller, K. Horne, I. Steele, D. M. Bramich, M. Burgdorf, C. Snodgrass & M. Bode
2. OGLE Collaboration,
   A. Udalski, M. K. Szymański, M. Kubiak, T. Wiȩckowski, G. Pietrzyński, I. Soszyński, O. Szewczyk, Ł. Wyrzykowski & B. Paczyński
3. MOA Collaboration,
   D. P. Bennett, F. Abe, I. A. Bond, T. R. Britton, A. C. Gilmore, J. B. Hearnshaw, Y. Itow, K. Kamiya, P. M. Kilmartin, A. V. Korpela, K. Masuda, Y. Matsubara, M. Motomura, Y. Muraki, S. Nakamura, C. Okada, K. Ohnishi, N. J. Rattenbury, T. Sako, S. Sato, M. Sasaki, T. Sekiguchi, D. J. Sullivan, P. J. Tristram, P. C. M. Yock & T. Yoshioka
4. Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie UMR7095, 98bis Boulevard Arago, 75014, Paris, France
   J.-P. Beaulieu, A. Cassan, E. Corrales, M. Desort & J.-B. Marquette
5. Department of Physics, University of Notre Dame, Indiana, 46556-5670, Notre Dame, USA
   D. P. Bennett
6. Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, Université Paul Sabatier—Toulouse 3, 14 avenue Edouard Belin, 31400, Toulouse, France
   P. Fouqué
7. Perth Observatory, Walnut Road, Bickley, WA 6076, Perth, Australia
   A. Williams & R. Martin
8. Scottish Universities Physics Alliance, University of St Andrews, School of Physics and Astronomy, North Haugh, KY16 9SS, St Andrews, UK
   M. Dominik, K. Horne & D. M. Bramich
9. Niels Bohr Institutet, Astronomisk Observatorium, Juliane Maries Vej 30, 2100, København Ø, Denmark
   U. G. Jørgensen, C. Vinter & K. Woller
10. European Southern Observatory, Casilla 19001, 19, Santiago, Chile
    D. Kubas & S. Brillant
11. CEA DAPNIA/SPP Saclay, 91191, Gif-sur-Yvette cedex, France
    C. Coutures
12. University of Tasmania, School of Mathematics and Physics, Private Bag 37, TAS 7001, Hobart, Australia
    J. Greenhill, K. Hill & S. Dieters
13. South African Astronomical Observatory, PO Box 9, Observatory, 7935, South Africa
    J. Menzies
14. Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Observatory, Weston Creek, ACT 2611, Australia
    P. D. Sackett
15. Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, 8020, Christchurch, New Zealand
    M. Albrow, K. Pollard, T. R. Britton, A. C. Gilmore, J. B. Hearnshaw & P. M. Kilmartin
16. McDonald Observatory, 16120 St Hwy Spur 78 #2, Texas, 79734, Fort Davis, USA
    J. A. R. Caldwell
17. Department of Physics, Boyden Observatory, University of the Free State, PO Box 339, 9300, Bloemfontein, South Africa
    J. J. Calitz & P. Meintjes
18. Lawrence Livermore National Laboratory, IGPP, PO Box 808, California, 94551, Livermore, USA
    K. H. Cook
19. Institut für Physik, Universität Potsdam, Am Neuen Palais 10, 14469, Potsdam
    D. Dominis & M. Hoffman
20. Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482, Potsdam, Germany
    D. Dominis & M. Hoffman
21. Technische Universität Wien, Wiedner Hauptstrasse 8 / 020 B.A., 1040, Wien, Austria
    J. Donatowicz
22. Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Florida, 32611-2055, Gainesville, USA
    S. Kane
23. Space Telescope Science Institute, 3700 San Martin Drive, Maryland, 21218, Baltimore, USA
    K. Sahu
24. Astronomisches Rechen-Institut (ARI), Zentrum für Astronomie, Universität Heidelberg, Mönchhofstrasse 12–14, 69120, Heidelberg, Germany
    J. Wambsganss
25. Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, CH41 1LD, Birkenhead, UK
    I. Steele, D. M. Bramich, M. Burgdorf & M. Bode
26. Astronomy and Planetary Science Division, Department of Physics, Queen's University Belfast, Belfast, UK
    C. Snodgrass
27. Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Aleje Ujazdowskie 4, 00-478, Warszawa, Poland
    A. Udalski, M. K. Szymański, M. Kubiak, T. Wiȩckowski, G. Pietrzyński, I. Soszyński, O. Szewczyk & Ł. Wyrzykowski
28. Departamento de Fisica, Universidad de Concepcion, Casilla 160–C, Concepcion, Chile
    G. Pietrzyński & I. Soszyński
29. Jodrell Bank Observatory, The University of Manchester, Cheshire, SK11 9DL, Macclesfield, UK
    Ł. Wyrzykowski & N. J. Rattenbury
30. Princeton University Observatory, Peyton Hall, New Jersey, 08544, Princeton, USA
    B. Paczyński
31. Solar-Terrestrial Environment Laboratory, Nagoya University, 464-860, Nagoya, Japan
    F. Abe, Y. Itow, K. Kamiya, K. Masuda, Y. Matsubara, M. Motomura, Y. Muraki, S. Nakamura, C. Okada, T. Sako, M. Sasaki, T. Sekiguchi & T. Yoshioka
32. Institute for Information and Mathematical Sciences, Massey University, Private Bag 102-904, Auckland, New Zealand
    I. A. Bond
33. Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand
    T. R. Britton, P. J. Tristram & P. C. M. Yock
34. School of Chemical and Physical Sciences, Victoria University, PO Box 600, Wellington, New Zealand
    A. V. Korpela & D. J. Sullivan
35. Nagano National College of Technology, 381-8550, Nagano, Japan
    K. Ohnishi
36. Department of Astrophysics, Faculty of Science, Nagoya University, 464-860, Nagoya, Japan
    S. Sato

Authors

1. J.-P. Beaulieu
   You can also search for this author inPubMed Google Scholar
2. D. P. Bennett
   You can also search for this author inPubMed Google Scholar
3. P. Fouqué
   You can also search for this author inPubMed Google Scholar
4. A. Williams
   You can also search for this author inPubMed Google Scholar
5. M. Dominik
   You can also search for this author inPubMed Google Scholar
6. U. G. Jørgensen
   You can also search for this author inPubMed Google Scholar
7. D. Kubas
   You can also search for this author inPubMed Google Scholar
8. A. Cassan
   You can also search for this author inPubMed Google Scholar
9. C. Coutures
   You can also search for this author inPubMed Google Scholar
10. J. Greenhill
    You can also search for this author inPubMed Google Scholar
11. K. Hill
    You can also search for this author inPubMed Google Scholar
12. J. Menzies
    You can also search for this author inPubMed Google Scholar
13. P. D. Sackett
    You can also search for this author inPubMed Google Scholar
14. M. Albrow
    You can also search for this author inPubMed Google Scholar
15. S. Brillant
    You can also search for this author inPubMed Google Scholar
16. J. A. R. Caldwell
    You can also search for this author inPubMed Google Scholar
17. J. J. Calitz
    You can also search for this author inPubMed Google Scholar
18. K. H. Cook
    You can also search for this author inPubMed Google Scholar
19. E. Corrales
    You can also search for this author inPubMed Google Scholar
20. M. Desort
    You can also search for this author inPubMed Google Scholar
21. S. Dieters
    You can also search for this author inPubMed Google Scholar
22. D. Dominis
    You can also search for this author inPubMed Google Scholar
23. J. Donatowicz
    You can also search for this author inPubMed Google Scholar
24. M. Hoffman
    You can also search for this author inPubMed Google Scholar
25. S. Kane
    You can also search for this author inPubMed Google Scholar
26. J.-B. Marquette
    You can also search for this author inPubMed Google Scholar
27. R. Martin
    You can also search for this author inPubMed Google Scholar
28. P. Meintjes
    You can also search for this author inPubMed Google Scholar
29. K. Pollard
    You can also search for this author inPubMed Google Scholar
30. K. Sahu
    You can also search for this author inPubMed Google Scholar
31. C. Vinter
    You can also search for this author inPubMed Google Scholar
32. J. Wambsganss
    You can also search for this author inPubMed Google Scholar
33. K. Woller
    You can also search for this author inPubMed Google Scholar
34. K. Horne
    You can also search for this author inPubMed Google Scholar
35. I. Steele
    You can also search for this author inPubMed Google Scholar
36. D. M. Bramich
    You can also search for this author inPubMed Google Scholar
37. M. Burgdorf
    You can also search for this author inPubMed Google Scholar
38. C. Snodgrass
    You can also search for this author inPubMed Google Scholar
39. M. Bode
    You can also search for this author inPubMed Google Scholar
40. A. Udalski
    You can also search for this author inPubMed Google Scholar
41. M. K. Szymański
    You can also search for this author inPubMed Google Scholar
42. M. Kubiak
    You can also search for this author inPubMed Google Scholar
43. T. Wiȩckowski
    You can also search for this author inPubMed Google Scholar
44. G. Pietrzyński
    You can also search for this author inPubMed Google Scholar
45. I. Soszyński
    You can also search for this author inPubMed Google Scholar
46. O. Szewczyk
    You can also search for this author inPubMed Google Scholar
47. Ł. Wyrzykowski
    You can also search for this author inPubMed Google Scholar
48. B. Paczyński
    You can also search for this author inPubMed Google Scholar
49. F. Abe
    You can also search for this author inPubMed Google Scholar
50. I. A. Bond
    You can also search for this author inPubMed Google Scholar
51. T. R. Britton
    You can also search for this author inPubMed Google Scholar
52. A. C. Gilmore
    You can also search for this author inPubMed Google Scholar
53. J. B. Hearnshaw
    You can also search for this author inPubMed Google Scholar
54. Y. Itow
    You can also search for this author inPubMed Google Scholar
55. K. Kamiya
    You can also search for this author inPubMed Google Scholar
56. P. M. Kilmartin
    You can also search for this author inPubMed Google Scholar
57. A. V. Korpela
    You can also search for this author inPubMed Google Scholar
58. K. Masuda
    You can also search for this author inPubMed Google Scholar
59. Y. Matsubara
    You can also search for this author inPubMed Google Scholar
60. M. Motomura
    You can also search for this author inPubMed Google Scholar
61. Y. Muraki
    You can also search for this author inPubMed Google Scholar
62. S. Nakamura
    You can also search for this author inPubMed Google Scholar
63. C. Okada
    You can also search for this author inPubMed Google Scholar
64. K. Ohnishi
    You can also search for this author inPubMed Google Scholar
65. N. J. Rattenbury
    You can also search for this author inPubMed Google Scholar
66. T. Sako
    You can also search for this author inPubMed Google Scholar
67. S. Sato
    You can also search for this author inPubMed Google Scholar
68. M. Sasaki
    You can also search for this author inPubMed Google Scholar
69. T. Sekiguchi
    You can also search for this author inPubMed Google Scholar
70. D. J. Sullivan
    You can also search for this author inPubMed Google Scholar
71. P. J. Tristram
    You can also search for this author inPubMed Google Scholar
72. P. C. M. Yock
    You can also search for this author inPubMed Google Scholar
73. T. Yoshioka
    You can also search for this author inPubMed Google Scholar

Corresponding author

Correspondence toJ.-P. Beaulieu.

Ethics declarations

Competing interests

The photometric data set is available at planet.iap.fr and ogle.astrouw.edu.pl Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Rights and permissions

About this article

Cite this article

Beaulieu, JP., Bennett, D., Fouqué, P. et al. Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing.Nature 439, 437–440 (2006). https://doi.org/10.1038/nature04441

Download citation

• Received: 28 September 2005
• Accepted: 14 November 2005
• Issue Date: 26 January 2006
• DOI: https://doi.org/10.1038/nature04441