Giant Planets in Outer Orbits
About 30 single transits of giant planets in outer orbits are expected while observing more than 100,000 main-sequence stars for 4 years (a>1.6 AU, or average distance to the planet is greater than 1.6 times the Earth's average distance from the Sun). The probability that a planet in a Jovian (12-year) orbit will produce a transit is only 9x10-4. The chance of one transit in 4 years is 3x10-4 yields 30 detections, if each system has on the average one outer giant planet. (See Table on About Transits)
These planets have orbital periods too long to be re-observed during the mission, but individual transits are unmistakable with a significance of approximately 400 sigma for Jovian-size or 40 sigma for Uranian-size planets. Subsequent transits can be detected from the ground to confirm the discoveries. For stars cooler than spectral type F5, Doppler spectroscopy measurements can be made to determine the planetary orbit and mass, as the transit observations determine the planet's orbital inclination. Since the size is known from the transit depth, the density of each planet can be calculated as has been done for several Kepler Discoveries. This basic information is needed to understand planetary system structure.
Giant Planets in Inner Orbits
The Kepler Mission can record the modulation of the light reflected by close-in giant planets as they orbit their stars with their phases changing between superior and inferior conjunction. For periods between one and five days, the fraction of reflected light for a Jovian-size planet falls from 10-4 to 10-5. Although the amplitudes are small, the periodic nature of the signal and the hundreds of repetitions observed during the four year mission allow these signals to be detected with a statistical significance of greater than 6 sigma for stars no noisier than the Sun and for orbital periods less than seven days. For larger planets or quieter stars, planets with even longer periods can be detected. Follow-up confirmation can be made using Doppler spectroscopy.
The discovery of giant planets in short period orbits (see figure) with the Doppler spectroscopy technique shows that about 1% of the stars that Marcy and Butler (1996) monitor have giant planets with orbital periods less than one week (a<0.1 AU). In the case of the Kepler Mission about 1,000 of our target stars should have giant inner planets. Taking into account that randomly the orbital poles of 87% of the estimated 1,000 planets are greater than 30º from the line-of-sight and have detectable modulation, many giant planets are detectable from reflected light.
This mission provides important information on the geometric albedos of extrasolar planets. About 100 of the planets detected by reflected light are expected to show transits, since the transit alignment probability for the expected 1000 inner-orbit giants, d*/2a, is 10%. From the area of the planet, semi-major axis and amount of reflected light, the planetary albedos can be derived. In addition to defining the albedo of the planet, its reflected light signature contains diagnostic information on the scattering phase function of its atmospheric constituents. If the atmosphere contains aerosols, the scattering peak of the planet's phase function is likely to be pronounced (Goody and Yung 1989).
There are about 35,000 stars in the FOV between spectral types F5 and K5 brighter than mv=14 which are suitable for making Doppler spectroscopy measurements. For the 0.1% of these stars that show transits of inner-giant planets, team members Cochran, Latham and Marcy, have the experience and facility access to determine the planet's masses. Hence, the densities of giant planets can be determined.
In addition to the approximately 1% of the stars having giant planets with orbits <1 week, there are a comparable number of planets in 1 week to 1 month (a ~0.2) and 1 month to 1 year (a ~0.5) orbits (See this figure,Marcy & Butler, 2000). For these two cases, the alignment probabilities are 2.5% and 1% respectively. Hence, these two cases yield an additional 25 and 10 inner-planet detections for a total of 135 expected inner-giant planets.
A summary of the potential results for the mission including that for giants is given in the next section.