Planets in our own solar system have a wide range of properties. They are distinguished by two basic properties, their size and their orbit. The size determines if the planet can have a life-sustaining atmosphere. The orbit affects the surface temperature and whether there could be liquid water on the planet's surface. A habitable planet, one that could have liquid water on its surface must be between about 80% to 200% the diameter of Earth. Planets that are smaller then 8/10ths of an Earth diameter have less than half an Earth mass and do not have enough gravity to hold onto a life-sustaining atmosphere. Planets that are more than twice the diameter of Earth have about ten Earth masses and enough gravity to hold onto hydrogen, the most abundant element in the Universe. Such large planets turn into gas giants like Jupiter and Saturn. Jupiter is more than ten times the diameter of Earth and more than 300 times the mass of Earth. Most of the 300 plus planets that have been detected so far are gas giants. The first extra-solar planet detected around a star similar to the Sun was 51 Peg. It has about the mass of Jupiter. But unlike Jupiter, which is five times as far from the Sun as Earth and orbits the Sun in 12 years, 51 Peg is twenty times closer to its star than Earth is to the Sun and orbits its star every 4 days. Planets like these are referred to as "Hot Jupiters.” Kepler was designed specifically to detect Earth-size planets in the Habitable zone of solar-like stars. But it will also be capable of finding a range of planets as small as Mars in short period orbits to gas-giant planets, from burning hot to frozen worlds.


NASA Kepler Mission/Dana Berry