A multiplicity of worlds

The day after my birthday the Kepler team announced (I like to think perhaps as a belated present) the discovery of 715 new exoplanets. This is a huge haul. It brings the tally of known planets to almost 1700.

The team was able to confirm the existence of such a large number of planets by making use of a new statistical approach to their analysis. Kepler worked by observing 160,000 stars and looking for periodic dips in brightness. The idea was that these periodic dips could be a sign of a transiting planet. The trouble is, these dips could also be caused by orbiting binary stars eclipsing each other. With the new technique, the Kepler team looked for multiple dips in brightness: this phenomenon must be caused by transiting planets rather than multiple eclipsing stars.

The technique works beautifully: those 715 planets just announced orbit only 305 stars. The Kepler data contains information on planetary systems, not just single planets.

Kepler tells us that planetary systems are common. It tells us small planets are common. And it tells us that some planets will orbit in the habitable zone. Deep-down we knew all those things anyway; but because of this announcement we can be sure.