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Earth-size Planets in Habitable Zone

Using an independent Kepler data processing program developed at UC Berkeley by graduate student Erik Petigura, astronomers at UC Berkeley and University of Hawaii, Manoa, estimate that 22 per cent of sun-like stars have Earth-size planets orbiting in their habitable zones. Their statistical analysis took account of planets that would normally be missed: those with tilted orbital planes that prevent dimming of their stars; and those escaping detection in Kepler photometry processing. Presented at the Second Kepler Conference, the results suggested that tens of billions of sun-like stars in the Miky Way galaxy have planets like Earth in their habitable zones, and prompted discussion on the location of these zones and the probability of life inside or outside of them.

The analysis focused on 42,000 stars that are like the sun or smaller, and found 603 candidate planets orbiting them. Only 10 of these were found to be Earth-size (defined as one to two times the diameter of Earth) and orbiting at a distance where light from the star allows liquid water to exist on the surface. This habitable zone was defined as from 0.5 AU to 2.0 AU from a sun-like star. In the solar system this would include Venus (0.7 AU), Earth (1.0 AU) and Mars (1.5 AU).


Size distribution of planets

The correction for missed planets increases as planet size decreases, the result is that Earth-sized planets tend to be missing from the Kepler results. For planet size in general, there are many more planets less than 2.8 Earth radii present. (Image credit: Erik Petigura)

All of the potentially habitable planets found in the team’s survey are around K stars, which are cooler and slightly smaller than the sun, Petigura said. But the researchers’ analysis shows that the result for K stars can be extrapolated to G stars like the sun.

If the stars in the Kepler field of view are representative of stars in the solar neighborhood, then the nearest Earth-size planet may be orbiting a star less than 12 light-years from Earth.

Petigura led the analysis of the Kepler data using the TERRA program he developed, publishing the results with Andrew Howard, Institute for Astronomy, University of Hawaii and Geoffrey Marcy, UC Berkeley professor of astronomy, in Proceedings of the National Academy of Sciences. The research was funded by UC Berkeley and the National Science Foundation, with the assistance of the W. M. Keck Observatory and NASA.