The Nature of Planet Formation
The formation of stars and planets is complex, making it almost impossible to predict the diversity of planetary systems from first principles (Boss, 1995, Lissauer 1995). Most modern theories describe planetary growth beginning with small solid grains within circumstellar disks. Such disks are believed to form as part of the star formation process and are observed around many young stars. Dust collides and agglomerates into larger bodies, eventually producing planets. Sufficiently massive bodies that form while gas remains in the disk can accrete substantial amounts of hydrogen and helium to become gas giants, whereas smaller planets are primarily composed of condensed material. According to this scenario, planets are expected to be common and a broad range of planetary sizes and masses is produced, including rocky planets several times as massive as the Earth.
The characteristics of a particular planetary system depend upon the interaction of a vast array of physical and chemical processes, involving magnetic fields, turbulence and viscosity in disks composed of gas and dust, sticking and growth of small grains, torques between growing planets and their surrounding disk, etc. Theory cannot definitively predict the frequency of planet formation nor the distribution of planetary sizes and orbits. One theory predicts that Jovian mass planets are formed from dynamic instabilities in accretion disks. A more popular theory envisions the formation of terrestrial planets that grow large enough to attract a massive gaseous envelope. Large cores should be seen in the second case, but not the first.
Theory is most useful for extrapolating from known systems. Models based on the single example of our Solar System cannot say whether our system is typical or anomalous. The discovery of short-period giant planets using Doppler spectroscopy results implies that at least a few percent of solar-like stars have systems quite different from our own. However, such surveys are not be able to detect large numbers of terrestrial planets.
The Kepler Mission uses spacebased photometry to detect planetary transits. It offers far greater sensitivity for finding terrestrial and smaller planets than ground-based techniques. By providing a statistically robust census of the sizes and orbital periods of terrestrial and smaller planets orbiting a wide variety of stellar types, results from this mission will allow us to place our Solar System within the continuum of planetary systems in the Galaxy and develop theories based on empirical data.
What is a habitable planet?
Many factors influence the conditions that make a planet habitable. The most common definition starts with having liquid water on the surface of the planet. This is predominantly determined by the stellar type, that is the size and effective temperature of the star and the orbit of the planet. This defines the habitable zone (HZ) for a particular star, which is illustrated in the figure below. Planets interior to the HZ (closer to their parent stars) are so hot that any water on the surface would vaporize and boil away, as is the case for Mercury. Planets outside the HZ are so cold that any water on the surface is constantly frozen, as is the case for Mars. For our Sun, the HZ is roughly between the orbits of Venus and Mars.
The second major factor is the size and mass of the planet. Planets that form with less than about one quarter of an Earth mass don't have enough surface gravity to hold onto a life sustaining atmosphere, which is the case for Mars. On the other hand, planets that form with a mass of about 10 Earth masses have enough surface gravity to hold onto even the lightest and most abundant elements, hydrogen and helium, and grow into gas giants. Such is the case for Jupiter, Saturn, Uranus and Neptune.
Thus, the goal of detecting habitable planets requires finding those planets with masses between about one-half to ten Earth masses, assuming a similar density and composition for terrestrial planets, about 0.7 to 2.0a Earth radii. This is precisely what the Kepler Mission is designed to do.
Additional factors affect the habitable question. The amount and composition of the atmosphere is critical as it influences the temperature, provides UV and cosmic ray protection, etc., The effects of moons and giant planets in the planetary system are also important as they may provide protection from comet and asteroid impacts, etc.
Where to Look For Habitable Planets
The numerical modeling of Chambers (2001) shows that the accumulation of planetesimals during molecular cloud collapse can be expected to produce, on the average, three or four inner planets. Two of these are approximately Earth-size and two are smaller. These results indicate that the position of the terrestrial planets can be anywhere from the position of Mercury's orbit to that of Mars'. Therefore, a search for terrestrial planets should include a wide range of orbits.
The Terrestrial Accretion Zone and The Habitable Zone for Various Stellar Types
The continuously habitable zone is bounded by the range of distances from a star for which liquid water would exist and by the range of stellar spectral types for which planets had enough time to form and complex life had enough time to evolve (less massive than F) and for which stellar flares and atmospheric condensation due to tidal locking do not occur (more massive than M). The figure shows the continuously habitable zone as calculated by Kasting, Whitmire, and Reynolds, (1993) for main-sequence stars as a function of spectral type.
The Kepler Mission performs a search for all orbital periods less than two years, that is, out to a Martian orbit, and for all spectral types of stars. It is not affected by solar or extrasolar zodiacal background and can detect planets within binary star systems.