The Kepler telescope was launched into an Earth-trailing orbit around the Sun by NASA in March 2009. Its principal mission was to continuously monitor large numbers of stars over long periods in search of the very slight periodic dimming that might indicate the repeated transit of an exoplanet across the face of its host star. To that end, the telescope has photometrically measured the brightnesses of about 150 000 nearby stars in a patch of northern sky (the Kepler field) every half hour for the past 4 years. That prodigious undertaking has yielded some 3500 planet candidates.
From any given vantage point, transiting exoplanets are rare. And they’re hard to recognize by photometric monitoring. Easiest to spot are giant planets in tight, short-period orbits around relatively small stars. To obtain a reliable census that reflects the true abundance of planets and the distribution of their characteristics, one must correct transit...
