The Kepler spacecraft, with its 1.4-meter diameter telescope, was launched in March of 2009 to detect and study extrasolar planets. Since then it has been staring at over 150,000 stars, looking for the brightness dips that suggest a planet has passed across the face of the star (a transit). Once the telltale signatures are confirmed to be planets (and not other effects like sunspots), meticulous monitoring of repeated transits - their timing, dimming, and variations therein - allow astronomers to estimate the planets' orbital parameters, masses, sizes, and even properties of their atmospheres.
This month, after two years of observations, the Kepler team has published a paper summarizing the characteristics of the results from the first four months of the mission. Analysis of the data from that period discovered an astonishing 1235 planetary candidates (that is, events that seem to be genuine transits but still await rigorous confirmation). The results, the first large-scale study of the properties of planets, are of great importance. Although these first detections may not be representative of all planets because, for instance, these are objects that orbit close to their stars and have very short periods, they nonetheless provide a significant subset to compare against models of planet formation.
A group of twelve CfA astronomers, members of the Kepler team, helped co-author the new article. They report finding in the sample sixty-eight candidates approximately Earth-sized, and another 288 that are between 1.25 and 2 Earth-radii. Although none of these are yet known to be "Earth-like," with the right temperature, composition, and atmosphere, about six objects less than about two Earth-radii are in the habitable zone where water, if it exists, could be liquid. They also find another 662 candidates that are the size of Neptune (between 2 and 6 Earth-radii), 165 Jupiter-sized objects (between 6 and 15 Earth-radii), and 19 more that are up to twice the size of Jupiter (22 Earth-radii).
The current analysis is the first large overview of the state of this exciting new field of astronomy. With additional observations many of these candidates will no doubt have their parameters adjusted. One of their primary conclusions, however, after correcting for effects of geometry and sensitivity, is that Earth-sized candidates are perhaps roughly 5% of the total. That it is even possible to make measurements of this kind is a tribute to the research creativity, technical skill, and analytic prowess of modern astrophysics.