At the AAS meeting, the discovery team announced that all three planets orbiting KOI-961 whip around the star in less than two days. The outermost body is the tiniest, with a diameter half that of Earth, or about the same as Mars, and a temperature of about 400 degrees Celsius. The inner two planets are larger, with diameters about three-fourths that of Earth. But that is still smaller than Venus. Because the planets are all small and close to their star, much of the atmosphere they may once have had would have evaporated, leaving behind bare rock, Marcy says.
Marcy calculates that Kepler could find planets around dwarf stars that are even smaller — with a diameter only 20% that of Earth, smaller than our Moon.
Finding a multitude of rocky, Mars-sized exoplanets may help shed light on a long-standing problem in understanding the formation of the inner Solar System, comments Dimitar Sasselov, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was not part of the study. Scientists have no clear explanation for why Mars, which lies outside the orbits of Venus and Earth, is so much smaller than its siblings.
The diminutive planetary trio is just the latest in a flurry of discoveries by Kepler. In December, researchers reported that the orbiting telescope had found the first known Earth-sized planets beyond the Solar System, although these bodies were also too hot for water to exist on its surface. Scientists also reported last month that the craft had found its first exoplanet in the habitable zone — although in that case, the orb is much bigger than Earth.
The new findings demonstrate that “Kepler is surely robust at finding truly Earth-sized planets,” says Marcy. The next goal, he notes, is the mission’s reason for being: “to find an Earth-sized planet that is lukewarm, where water would be in liquid form to host biology.”
This article is reproduced with permission from the magazine Nature. The article was first published on January 11, 2012.