The Nature of Planet Formation
Where to Look For Habitable Planets
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 which 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 believed 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
will not be able to detect terrestrial planets.
The Kepler Mission uses spacebased
photometry to detect planetary transits. It offers far greater
sensitivity for finding terrestrial and larger planets than ground-based
techniques. By providing a statistically robust census of the
sizes and orbital periods of terrestrial and larger 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.
The numerical modeling of Wetherill (1991)
shows that the accumulation of planetesimals during molecular
cloud collapse can be expected to produce, on the average, 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 an unbiased
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.