Expected Results
The Kepler Mission begins to collect
data immediately after launch and checkout and begins to produce
results in a progressive fashion shortly thereafter.
- The first results come in just a few months
when the giant inner planets are seen, those with orbital periods
of only a few days.
- Objects that are in Mercury-like orbits of
a few months are detected within the first year.
- Earth-size planets in Earth-like orbits require
nearly the full lifetime of the four year mission, although in
some cases three transits are seen in just a little more than
two years.
Other results that require the full four years of data are:
- Planets as small as Mercury in short period
orbits, which utilizes the addition of a dozens or more transits
to be detectable; and
- The detection of giant-inner planets that
do not transit the star but do periodically modulate the apparent
brightness due to reflected light from the planet.
The most exciting discovery from this mission
should be the detection of Earth-size planets in the habitable
zone of solar-like stars. However, we are prepared for many other
discoveries about the occurrence and characteristics of planets
around other stars. Even finding a few or no planets is important,
since it would lead to the conclusion that terrestrial planets
are rare and the origin of the Earth needs to be reconsidered.
The strength of the Kepler Mission is
its ability to address the unexpected with its capability to
monitor a large enough sample of stars to obtain a statistically
meaningful survey of terrestrial and larger planets with orbital
periods from a few days to over a year. We can only estimate
the expected results based on possible scenarios, since we have
no knowledge of the frequency and distribution of terrestrial
planets outside of our solar system. The mission has been designed
to gather enough information so that even a null result would
be meaningful and indicate that terrestrial planets were rare.
To quantitatively estimate the potential of
the results for the Kepler Mission, we assume that:
- One-hundred thousand main-sequence stars
are monitored;
- The average white-light variability of most
F-, G- and K-main-sequence stars on the time scale of a transit
is similar to that of the Sun after excluding the most active
25% of the dwarf stars in the FOV;
- Most main-sequence stars, including binaries,
have terrestrial planets in or near the habitable zone;
- On an average two Earth-size or larger
planets exist in the region between 0.5 and 1.5 AU, based
on our solar system and the accretion model of Wetherill (1996);
- The transit probability for planets in or
near the HZ is 1/2% per planet;
- The transit is near-grazing in a 1 year orbit;
- Each star has one giant planet in an outer
(jovian-like) orbit;
- On average, 1% of the main-sequence stars
have giant planets in orbits <1 week and comparable numbers
of giant planets in orbits of 1 week to 1 month and 1 month to
1 year;
- The detection efficiency is 84% with an expectation
of one false detection; and
- The mission life time is four years.
Based on these assumptions and the capability
of the Kepler Mission, we expect to to perform a census
of planets with periods from days to a few years and to detect:
Terrestrial inner-orbit planets based on
their transits:
- About 50 planets if most have R ~ 1.0 Re
- About 185 planets if most have R ~ 1.3 Re
- About 640 planets if most have R ~ 2.2 Re
(Or possibly some combination of the above)
- About 12% of the cases with two or more planets
per system
Giant inner planets based on the modulation
of their reflected light:
- About 870 planets with periods less than
one week
Giant planets based on their transits:
- About 135 inner-orbit planets along with
albedos for 100 of these planets
- Densities for 35 of the inner-orbit planets,
and
- About 30 outer-orbit planets.
Systems are expected with two or more terrestrial
planets seen in transit in or near the HZ. More than one planet
per system can be detected when a planetary system with small
relative orbital inclinations is viewed near either node of the
intersection of the orbital planes. The chance of seeing a second
planet is 12% when one planet has already been found (Koch and
Borucki, 1994) for systems having similar spacing and inclinations
as the Venus-Earth analog.
If binary stars do not have planets, then
the number of systems expected is about 46% less.
In summary, the Kepler Mission produces
a statistically valid sample, sufficient to establish the frequency
and distribution of planets in both single and multiple stellar
systems as stated in the Goals. The expected results are rich
enough that substantially different results still greatly enhance
the communities understanding of extrasolar planetary systems.
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