NASA's Terrestrial Planet Finder project is designed to address one of the most fundamental questions in science: whether life exists on other worlds. Although the prospect of capturing detailed images of another Earth, with blue oceans and green forests, is beyond our current capabilities, recent technological advances have provided us with the means to locate such Earths and determine whether they have the ability to support life.
The Terrestrial Planet Finder observatories will capture the reflected and thermally emitted light from terrestrial planets that orbit nearby, solar-type stars, using coronagraphic (TPF-C) and interferometric (TPF-I) imaging, It will spectroscopically determine their potential to harbor life by identifying atmospheric molecular lines. It will also characterize their solar systems by observing exozodiacal dust and large planets.
How common are Earthlike planets?
Although current observations suggest that Earth-size rocky planets may be common, their abundance is quite uncertain. The information to-date, however, is encouraging:
- Roughly 7 percent of all nearby stars harbor a giant planet within 3 AU.
- The number of planets increases as mass decreases towards the mass of an Earth.
- Stars that contain higher abundance of metals are more likely to have planets.
- Multiple planets are common, often in resonant orbits.
- The number of planets increases with distance from the star.
- Eccentric orbits are common, with only 10 percent being nearly circular.
While some of these planets are gas giants similar to Jupiter and Saturn in our solar system, some of the newly discovered planets have masses as small as 7.5 times the mass of Earth. The increasing number of planets with smaller mass suggests that planets with masses below 7.5 Earth masses, currently undetectable, are even more numerous. Moreover, the correlation with heavy elements supports current planet-formation theory that suggests rocky planets would be more numerous than the gas giants. The observations suggest that many nearby stars harbor rocky planets.
The inherent limitations of the radial-velocity technique preclude detection of small, Earth-like extrasolar planets around stars of similar temperature to our Sun. Two precursor space telescopes, Kepler and SIM PlanetQuest, will set the stage for the TPF by determining the frequency and characteristics of lower-mass planets.
Kepler will photometrically monitor hundreds of thousands of distant stars up to 1,950 light-years away, looking for transits of planets as small as Earth as they pass in front of their parent stars. From Kepler we will derive a statistical knowledge of how rare or common planets like Earth are.
SIM PlanetQuest will perform the first census of Earth-like planets around the nearest stars - planets that we will be able to observe through direct detection of light to learn more about their physical properties, accurately measuring their masses and orbits. SIM PlanetQuest will have the ability to determine the complete catalog of planetary and orbital parameters needed to understand planetary systems down to terrestrial planet masses. The mission will also detect the "wobble" in the parent star's apparent motion as the planet orbits, to an accuracy of one millionth of an arcsecond - the thickness of a nickel, viewed at the distance of the Moon! Its census of nearby stars will optimize target selection for TPF-C, and will improve the latter's observing efficiency, speed the rate of discovery, and ultimately enhance our ability to characterize the planets we find.
In their respective roles, Kepler and SIM PlanetQuest are highly complementary to TPF: Kepler provides statistics on the frequency of Earth-size planets using distant stars, and SIM PlanetQuest surveys the nearest stars and finds targets suitable for subsequent observation by the Terrestrial Planet Finder missions.
