New space telescope Kepler
A telescope in search for a "Second Earth"
Very rarely are telescopes built to observe just one kind of celestial body. However, this is exactly the case for the new telescope "Kepler" made in the U.S.A. It is supposed to stare at the same region of the sky. There are great hopes for the $600 Million project. For the first time astronomers want to spot planets outside of our solar system, which are similar to Earth. The observations are very sensitive, for that reason the reflecting telescope will be brought into orbit, where the atmosphere will not interfere. The start of the roughly 2200 pound Kepler is planned by NASA for late Friday, March 6th.
Search for habitable planets
Providing proof for planets of far away stars is always difficult. The huge difference in the brightness between the planet and it's central star usually prevents the direct detection of these extrasolar worlds. Nearly all of the 300 exoplanets were caught using indirect methods. None of them deserves the title "Second Earth", because several conditions need to be met simultaneously. For starters the mass of the planet has to be similar to Earth's. Then the distance to the central star and the type of that star matters. Both determine the temperature of the exoplanet. If these conditions allow for liquid water on the surface then the planet is in the so called habitable zone of its home planet.
100.000 stars at a glanceThe undivided attention of Kepler is focused on the 100,000 stars of the two constellations Cygnus and Lyra. Twice every hour 42 light sensitive chips, so called CCDs, will register their brightness. With all in all 95 megapixels it will be the biggest "digital camera" ever sent to space. If invisible exoplanets orbit around stars, they will reveal themselves by a slight dim out for a few hours. This will happen when the darker planet passes in front of the star from Kepler's perspective. Known as the transit method, it is a part of the repertoire of exoplanet chasers: it is used in observatories as well as space telescopes.
Solid statitical foundationKepler's primary mirror has a 1.4 meter diameter and will collect twelve times the light of its European counterpart, the French Corot Telescope (Convection, Rotation and planetary transits), which has been in space since 2006. Kepler is a Schmidt telescope and is especially fitting to observe a larger patch of celestial real estate. How many habitable Earth-like exoplanets found by the NASA satellite depends on their frequency of occurrence. Since that is still unknown astronomers must rely on estimates. If they exist frequently then Kepler could detect dozens. If they are rare then possibly none will be detected. In any event the space telescope will help to improve the knowledge about the allocation of exoplanets in the Milky Way. As a result the statistical model concerned with the allocation of exoplanets will be more solid, since Kepler will record the transits of all types of extrasolar planets.
Diminutive amount of blocked light during transitAn Earth-like transit only blocks a diminutive amount of light from the central star in the magnitude of 84 ppm (parts per million). The transit can last for up to 13 hours. To confirm an exoplanet three similar recordings of the transit are required. Since Kepler observes the same region for 3.5 years it could, unlike Corot, recognize transits that are a year apart. This is an important observation for planets like Earth, which need a full year for a complete revolution around the central star. An exoplanet with Earth-like mass, orbiting a sun-like star in one year - that would be a small sensation. The "Second Earth" would be found.
Thorsten Dambeck is a physicist and science writer in Germany. Translation: Raphael Steinbach.