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
.
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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