TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 7
7.1.2 Priority 2: Frequency of Terrestrial Planets
Improved knowledge of extrasolar planetary systems will allow us to predict with greater certainty the
scientific return from TPF-I. This question has highest priority once each mission enters Phase A, and
therefore should have a high priority in the preceding years.
• A comprehensive theoretical investigation into many aspects of the formation and evolution of
planets and planetary systems must provide the framework within which to understand necessarily
incomplete observational results. The combination of the existing and near-term radial velocity and
transit programs—with theoretical insights into the orbital stability of planets, planetary migration,
and the relationship between gas giants and rocky planets—will further enrich our understanding of
extrasolar planetary systems.
• Our current understanding of the frequency of Earth-like planets is based on observations of highermass
planets discovered through radial velocity surveys, transit surveys, and on inferences from
gravitational microlensing results. These highly successful programs should be further supported and
encouraged as new detections of lower-mass and longer-period planets continue to refine our estimate
of the frequency of Earths.
• Radial velocity surveys would be better supported through the development of new specialized highresolution
échelle spectrometers to offset the current demand for these instruments. Investments in
new equipment for radial velocity surveys, directed also at under-used 2–3-m class telescopes, would
represent an excellent strategy in the development of workhorse instruments for exoplanet detection.
• The space-based missions CoRoT (CNES/ESA, launched in 2006) and Kepler (NASA, launch in
2009) hold great promise for identifying transiting planets down to an Earth radius around stars
located 100–1000 pc away. With these missions we will determine the statistical incidence of
terrestrial planets and, with suitable follow-up, new insights into the physical state of these distant
planets. Ground-based transit surveys should also be encouraged. Of special interest is the
development of a worldwide network of dedicated wide-field transit or microlensing search
telescopes that would allow follow-up studies of bright targets.
7.1.3 Priority 3: Target Stars
The quality of science that will be derived from TPF-I will be partly determined by the stars included in
the final target list. A preliminary list of stars will greatly assist in judging the technical feasibility of the
mission concepts. This preliminary target list may include a larger number of stars than are retained in
the final list.
• The TPF-I project needs to determine which stellar parameters are most relevant to the search for life.
These parameters, once known, will need to be monitored over time for the stars included in the target
list. Spectroscopic observations over a broad range of wavelengths will be needed with coordinated
access to appropriate space observatories run by NASA and ESA. The TPF-I and Darwin projects
should work with NASA and ESA to ensure a coordinated observing program to observe target stars.
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