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.
Assumptions Used to Estimate the Results
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.
Summary of The Expected Results
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:
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.