Dark Earths: Initial Goals for Interstellar Exploration (original) (raw)

The presence of unseen mass in the solar neighbourhood has prompted modelling of, and searches for, a population of cool, low mass stars to make up the deficit. Such brown dwarfs are thought to exist within a mass range of 0.01 M⊙ < M < 0.08 M⊙. In this paper the possibility of the existence of interstellar planets (ISPs) of mass range 5x10^-9 M⊙ < M < 0.01 M⊙ is examined. Six potential modes of formation of ISPs are identified, although some are mutually exclusive, depending of different cosmogonic hypotheses. ISPs are of two basic types: those formed solitary within molecular clouds and those formed within, and subsequently unbound from, planetary systems. While the existence of the former is uncertain, interstellar planets of the unbound variety almost definitely exist, although not in sufficient quantity to account for the unseen mass. The number density of unbound planets in the solar neighbourhood may be of a similar, or greater, order of magnitude to that of stars, the majority of them being massive planetesimals ejected from planetary systems in formation. The nearest extra-solar planet may thus be closer to the solar system than the nearest star.

Gravitational microlensing has revealed an extensive population of “nomadic” planets not orbiting any star, with Jupiter-mass nomads being more populous than main sequence stars. Except for distant objects discovered through microlensing, and hot, young nomads found near star formation regions, to date only a small number of nomad candidates have been discovered. Here I show that there should be significant numbers of mature nomadic exoplanets close enough to be discovered with existing or planned astronomical resources, including possibly dozens of massive planets closer than the nearest star. Observational data are used to derive models relating mass, radius, heat flux and magnetic dipole moment; these are used to show the observability of nomads in the IR, due to thermal emissions, and at radio frequencies, due to cyclotron maser instabilities. These neighboring nomadic planets will provide a new exoplanet population for astronomical research and, eventually, direct exploration by spacecraft.

Nomadic worlds are objects not bound to any star(s), and are of great interest to planetary science and astrobiology. They have garnered attention recently on account of their detection in microlensing surveys and also from the recent discovery of interstellar planetesimals. In this paper, we evaluate the prevalence of nomadic worlds with radii of 100 km ≲ R ≲ 10 4 km, which might permit habitable conditions. The cumulative number density n > (> R) appears to follow a heuristic power law given by n > ∝ R −3. Therefore, smaller objects should be much more numerous than the largest rocky nomadic planets, and thus statistically more likely to have members relatively close to the inner Solar system. Our results suggest that tens to hundreds of planet-sized nomadic worlds may populate the spherical volume centered on Earth and circumscribed by Proxima Centauri, and thus may comprise closer interstellar targets than any stellar planetary system. For the first time, we systematically analyze the feasibility of exploring these unbounded celestial bodies via deep space missions. We investigate what near-future propulsion systems could theoretically enable us to reach nomadic worlds (of radius > R) on a 50-year timescale. Objects with R ∼ 100 km are within the purview of multiple propulsion methods such as electric sails, laser electric propulsion, and solar sails. In contrast, nomadic worlds with R ≳ 1000 km are accessible by laser sails (and perhaps nuclear fusion), thereby underscoring their vast potential for deep space exploration.

A review is presented of the scientific benefits of rapid (v 0.1 c) interstellar spaceflight. Significant benefits are identified in the fields of interstellar medium studies, stellar astrophysics, planetary science and astrobiology. In the latter three areas the benefits would be considerably enhanced if the interstellar vehicle is able to decelerate from its interstellar cruise velocity to rest relative to the target system. Although this will greatly complicate the mission architecture, and extend the overall travel time, the scientific benefits are such that this option should be considered seriously in future studies.

The study of planets beyond the solar system and the search for other habitable planets and life is just beginning. Ground-based (radial velocity and transits) and space-based surveys (transits and astrometry) will identify planets spanning a wide range of size and orbital location, from Earth-sized objects within 1 AU to giant planets beyond 5 AU, orbiting stars as near as

"A lengthy analysis of the output of the "Silicon Creation" model is presented. Seven thousand five hundred microcomputer simulations of stars in the mass range 0.6 - 1.3M⊙ suggest that habitable planets are to be expect to occur around stars of between 0.8 - 1.2 M⊙ and most commonly around stars between 0.95 - 1.05 M⊙. The characteristics of these planets range between extremes of mass ~ 0.4 - 2.8 M_e; surface gravity ~ 0.7 - 1.5 g; atmospheric pressure ~ 500 - 2000 mb; average surface temperature ~ 1 C - 21 C; hydrosphere ~ 50 - 99%. An extended simulation of an evolving volume of Galactic disc space, 270 ly in diameter, containing 100,000 star systems of maximum age 10^10 years was performed, assuming constant stellar birthrate and rising metallicity with time. 86 habitable planets were produced giving a value for the ratio of habitable planets to stars in the disc N_HP / N_*disc = 8.6x10^4. The average separation between habitable planets is therefore ~ 50 ly and the number of habitable planets in the Galaxy is approximately 90 million. Comparison of these results is made with those of other authors and uncertainties inherent in the "Silicon Creation" model are briefly discussed."

The probable abundance of planets possessing suitable conditions for the evolution of technologically capable forms of life has been assessed by an analysis of the series of runs of the "Silicon Creation" computer model. An evolutionary simulation of 100,000 disc stars of varying mass, metallicity and age was performed. Not only did the computer search for potential civilisation sites, but also for civilisations that had come into existence on those planets over the past 10^10 years of galactic disc history. The frequency of such planets was determined to be N_sites/N_*disc = 2.92x10^-3. The frequency of planets actually developing a technological civilisation was N_civ/N_sites = 0.031, which gives N_civ/N_*disc= 9x10^-5. These figures are two orders of magnitude lower than the most optimistic manipulations of the Drake Equation, but not low enough to resolve the "Fermi Paradox."