The Kepler space telescope is a photometer that measures the brightness of stars to unprecedented accuracy. The reason for such a task is to watch for the moment a planet passes between the telescope and a star – a transiting exoplanet. For this to happen, the planet’s orbit must be carefully aligned. During the course of an orbit, such a planet may block out some of the star’s light when passing in front and then reflect light, undergoing phases such as those of the Moon or more accurately Venus or Mercury. This reflected light can then be cut off suddenly as the planet vanishes behind the star. Rings and other orbitals can also affect these light curves. Kepler isn’t the only such telescope – CoRoT is another that announced the discovery of six new planets and a brown dwarf yesterday.
Following 43 days of staring at an area in the constellation of Cygnus containing 150,000 or so stars, the team behind Kepler have released part of their list of exoplanetary candidates so far, of which they expect half to be actual exoplanets and the rest either observational artifacts or binary stars. The top 400 most likely of the 706 released are being kept under wraps for further investigation, causing some consternation amongst others in the field, but the other 306 have been released along with five confirmed multiplanetary systems.
The exoplanetary candidates are all smaller than those generally found, reflecting the enhanced sensitivity of the probe and the scope of previous surveys. They average around half the radius of Jupiter, go down to around the size of the Earth and up to around the size of Jupiter.
The mission requires that at least two transits must be observed to confirm a planetary candidate, three to verify it, and the three year mission timeframe exists to allow an Earth-like candidate to be sighted over its course.