Mission science

The Kepler spacecraft in operation. Image: NASA In November 2008 NASA plans to launch the Kepler mission – a space-based observatory that will look for rocky Earth-like (terrestrial) planets around other stars.

Extrasolar planets occasionally pass in front of and block a small fraction of the light from their parent star in transits. Kepler will use a 0.95 metre aperture optical telescope and instruments to look for sets of three transits around each star. If each event in a set has the same duration and change in brightness and the events are spaced at equal time intervals, then are almost certainly due to an orbiting planet.

Anlaysing the properties of the transit allows scientists to deduce the characteristics of the planet. From the change in brightness the planet's size can be found whilst the time interval gives its orbital period. The two together should allow the temperature to be calculated. This is a rigorous method that should be sensitive enough to discover and confirm hundreds of Earth-sized planets.

In order to view repeated transits, the stars under study must be far from the plane of the Earth's orbit around the sun, otherwise they will be blocked by sunlight at certain times of year. The chosen field of view is in the constellation of Cygnus and contains stars within the Orion arm of our galaxy.

Mission lifetime

Kepler will be placed in an orbit centred on our Sun that trails behind the Earth-Moon system and has a period of 372.5 days. This will make it a very stable platform that does not suffer from the effects of the Earth's gravity, magnetosphere and atmospheric drag.

The mission lifetime is intended to be four years, long enough to detect planets with the orbital period of the Earth. A possible two-year extended mission would search for terrestrial planets in Mars-like orbits.

The discovery of Earth-type planets would be a key step in finding life elsewhere in the universe. However, even finding none would be an important result. Astronomers would then conclude that terrestrial planets are rare and need to reconsider theories for the formation of the Earth.