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Detection of Giant Planets
Giant Planets in Outer Orbits

About 30 single transits of giant planets in outer orbits (a>1.6 AU) are expected while observing 100,000 main-sequence stars for four years. The probability that a planet in a jovian (12-year) orbit will produce a transit is only 9x10-4 (See Table on Transit Properties). The chance of one transit in 4 years is 3x10-4 yielding 30 detections, if each system has on the average one outer giant planet.

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. Doppler spectroscopy measurements of those stars cooler than spectral type F5 can be made to determine the planetary orbit and mass, as there is little uncertainty in the orbital inclination. SIM can be used for stars hotter than F5. Since the size is known from the transit depth, the density of each planet can be calculated, as was done for the case of HD209458b. This basic information is needed to understand planetary system structure.

Giant Planets in Inner Orbits

The Kepler Mission readily records the modulation of the light reflected by about 870 close-in giant planets as their phases change 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 1000 of our target stars should have giant inner planets. Taking into account that randomly the orbital poles of 87% of the estimated 1000 planets are greater than 30° from the line-of-sight and have detectable modulation, about 870 giant planets are detectable from reflected light.


Radial Velocity Variation of 51 Peg
(Other Doppler velocity curves by Marcy and Butler)

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 about 35 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.

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