 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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