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Follow-up Observations

For all candidate transit cases, complementary follow-up observations are made to confirm that the transits are due to planets and to learn more about the characteristics of the parent stars and planetary systems.

Elimination of non-Planetary Candidates

White dwarfs have radii similar to that of the Earth and can produce a light curve that mimics a transit by a terrestrial planet. Also, a grazing eclipse by a stellar companion can mimic a transit. In both cases, the stellar companions have masses that are usually >>100 MJ. The radial-velocity variations induced by such companions are typically >>1 km/s and can easily be detected, such as with the SAO Digital Speedometer available to Latham (1992). Thus, transit candidates where the companion is stellar can be eliminated. Similarly, brown dwarfs (10 MJ < M < 100 MJ, R~1 RJ) can be distinguished from giant planets.


The Smithsonian Institution and University of Arizona
6.5m MMT Observatory

Detection of Giant Planet Companions

Radial velocity data also serve to explore or delineate the structure of the planetary systems by detecting giant planets not seen in transit or by reflected light. Normally, such data only yield the quantity, M sini, where M is the planetary mass and i the orbital inclination. The presence of transits implies that i ~90°, which establishes the mass of the planet. By sampling the Doppler shift a few times during an orbit, the orbital parameters of those planets showing transits are completely determined and the planet mass is established to within 3%. The mass (from spectroscopy) and planet radius (from photometry) yield the density of the planet.

Kepler team member Marcy has developed the technique to measure velocities to 3 m/s on Keck (Marcy, et. al. 2000). Kepler team members also have institutional access to the proven Hamilton Echelle (Basri) and to new spectrometers on the Hobby-Eberly Telescope (Cochran) and the MMTO (Latham) all with <10 m/s capability.


McDonald Observatory: Hobby-Eberly Telescope

Stellar Mass, Size and Metalicity

For those stars found to have planets, high-resolution, high signal to noise ground-based spectroscopy is performed to clearly establish the spectral type and luminosity class. Stellar evolution models are used to estimate the mass, radius and metalicity of the parent star (e.g., Mazeh et al. 2000). The stellar mass is required to calculate the semi-major axis of the orbit and the stellar radius to calculate the planet's size. The frequency of planets with respect to spectral type and other stellar characteristics can then be established. There is now some preliminary evidence that planetary systems may be found more often around stars whose spectra show high metalicity (Gonzalez 1997, 1998) or depleted lithium (Cochran et al. 1997). Firmly establishing the existence of any such trends provides extremely valuable constraints on models of planetary system formation. Since the Kepler Mission FOV is along a galactic arm at the same galacto-centric distance as the Sun, the stellar population sampled is indistinguishable from the immediate solar neighborhood. Thus, these results can guide target selection for future planet searches by SIM and TPF for planetary systems in the very near solar neighborhood.

Additional constraints on the parent star's radius and other properties are obtained from p-mode oscillations (e.g., Brown and Gilliland 1994) using Kepler's 1-min sampling mode. In the Sun a series of modes with periods of about 3 min and equal spacing in the frequency domain are excited to a level of about 3 ppm in white light. This level of precision requires the detection of at least 1012 photons. Kepler provides the necessary photon levels in one month for the 3,600 dwarf stars brighter than mv=11.4 in the FOV. Two members of the Kepler team have considerable experience and scientific interest in this type of stellar seismology that can be done with Kepler (Gilliland and Brown)

Astrometric Observations

Where Kepler has found a planet, SIM can be used to find the masses (if they are jovian or larger) or set upper limits to the masses of the detected planets. SIM data complement the Doppler data since Doppler spectroscopy can not be used for stars hotter than F5. SIM can also be used to search for additional giant planets in wide orbits. For example, SIM can detect a 0.4 MJ planet in a 4-year orbit around a solar-like star at a distance of 500 pc

Using parallax, the geometric distance to a system at 500 pc can be determined to 0.2%, placing all inter-comparisons of discovered planetary systems on a very firm foundation. Two of the Kepler team members (Latham and Boss) are also members of the SIM key projects proposal team for planetary systems studies. The single-pointing field-of-regard of SIM of 15 deg. is nearly identical to that of Kepler and is a highly efficient use of SIM observing time.

Other Observations

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SOFIA: Stratospheric Observatory for Infrared Astronomy

From infrared measurements with SOFIA or NGST any IR excess or lack thereof will indicate the fraction of systems having terrestrial planets that are or are not embedded in large amounts of extrasolar zodiacal dust.

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