Planetshine - Search For Terrestrial Planets

Search For Terrestrial Planets

Scientists at NASA's Navigator Program, which specializes in the detection of terrestrial planets, have backed the launch of a Terrestrial Planet Finder (TPF) mission. TPF would detect planetshine from planets orbiting stars to investigate whether they could harbor life. It would use advanced telescope technologies to look for life-marks in the light reflected from the planets, including water, oxygen and methane.

The European Space Agency has a similar mission, named Darwin, under consideration. This will also study the earthshine of planets to detect the signatures of life.

Unlike many traditional astronomical challenges, the most serious challenge for these missions is not gathering enough photons from the faint planet, but rather detecting a faint planet that is extremely close to a very bright star. For a terrestrial planet, the contrast ratio of planet to its host stars is approximately ~10−6-10−7 in the thermal infrared or ~10−9-10−10 in the optical/near infrared. For this reason, Darwin and Terrestrial Planet Finder-I will work in the thermal infrared. However, searching for terrestrial planets in the optical/near infrared has the advantage that the diffraction limit corresponds to a smaller angle for a given size telescope. Therefore, NASA is also pursuing an Terrestrial Planet Finder-C mission that will search for and study terrestrial planets using the optical (and near infrared) wavelengths. While Terrestrial Planet Finder-C aims to study the planetshine of extrasolar planets, Darwin and Terrestrial Planet Finder-I will search for thermal infrared light that is reradiated (rather than scattered) by the planet, and most astronomers would not consider that to be planetshine.

In preparation for these missions, astronomers have performed detailed earthshine observations. Astronomers have paid particular attention to whether earthshine measurement can detect the red edge, a spectral feature that is due to the plants. The detection of a similar spectral feature in light from an extrasolar planet would be particularly interesting, since it might be due to a light-harvesting organism. While the red edge is almost certainly the easiest way to directly detect life on earth via earthshine observations, it could be extremely difficult to interpret a similar feature due to life on another planet, since the wavelength of the spectral feature is not known in advance (unlike most atomic or molecular spectral features).