The Nearest Potentially Habitable Planet
Most exoplanets have been discovered via the transiting technique used by the Kepler mission, as illustrated here. A new study has examined Kepler observations for exoplanet transits around lower mass, M-dwarf stars. These stars are much more numerous than solar-type stars. The study concludes that the nearest potentially habitable Earth-sized planet is likely to be about nine light-years away.
The Kepler mission has so far spotted an astonishing 4696 possible exoplanets; about one third of them, 1030, have been confirmed as exoplanets so far. A small percentage of this set are potentially habitable, by which astronomers generally mean that they have surface temperatures suitable for water to remain liquid, rather than frozen out as ice or suspended as vapor in the atmosphere. Exoplanets can exist around all kinds of stars, not only ones like the Sun. Sun-like stars are comparatively rare - the less massive ("M dwarf") stars are about ten times more common. They are also cooler and dimmer, and so their planetary systems will have habitable zones closer to the star than is the case in our solar system, but exoplanets orbiting in that zone are of great interest, especially rocky, Earth-sized ones large enough to retain an atmosphere.
CfA astronomers Courtney Dressing and David Charbonneau have reanalyzed the full Kepler mission dataset, using their own transiting planet detection software to search for exoplanets around M dwarf stars. They use both archival and followup data to refine the stellar sample and the transit details, explicitly address issues of sample completeness, and incorporate a sophisticated treatment of the habitable zone in these stars. After accounting for sampling effects that influence the transit detection (for example, the line-of-sight geometry to the stellar system), they conclude that Earth-sized planets (rocky planets between 1 to 1.5 times the Earth's size) are common: there is probably one with an orbital period less than fifty days for every 1.8 early M dwarf stars. With a somewhat expanded range of parameters, including sizes up to four Earth-radii and orbiting in less than two hundred days, there are on average an estimated 2.5 Earth-like planets per M dwarf star. Based on these statistics, the astronomers conclude that the nearest potentially habitable Earth-sized planet is likely to be a mere nine light-years away.
"The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity," Courtney D. Dressing and David Charbonneau, ApJ, 807, 45, 2015.