Astronomers are steadily zeroing in on Earth-like exoplanets: those transiting terrestrial-sized exoplanets located in their stellar habitable zones (the region where temperatures allow water to remain liquid) around Sun-like stars or more abundant, lower-mass stars. The first one discovered, by NASA's Kepler mission, was Kepler-22b, but its radius of 2.4 Earth-radii was a bit too large to facilitate an Earth-like composition. Other candidates, in the Kepler-20, Kepler-42, and Kepler-68 systems, orbit too close to their host stars to lie within the habitable zone. On the other hand, the star Kepler-62 hosts five planets, two of which are very likely to be solid and to reside in their star’s habitable zone; other candidates have been announced as well.
The problem is that the determination of an exoplanet's radius and equilibrium temperature hinges critically on the properties of the star it orbits, and measuring the properties of low-mass stars (like the ones mentioned above) is difficult and problematic. The direct comparison of stellar theory and observation, which is robust for deducing the properties of solar-type stars, is challenging for low-mass stars. The theory relies on detailed, computationally intensive modeling low-mass stellar interiors, and that includes knowing the complex array of molecules and grains that reside in the stellar atmospheres. For this reason, astronomers often appeal to empirical, rather than theoretical, methods for the physical characterization of low-mass stars -- but this is fraught with other possible assumptions such as about stellar activity or element abundances.
CfA astronomers Sarah Ballard, David Charbonneau, Francois Fressing, Guillermo Torres, Jonathan Irwin, and Elisabeth Newton, along with their colleagues, have come up with a promising alternative. In order to model the properties of the exoplanet around the low-mass star Kepler 61, they examine a set of other nearby low-mass stars. Using additional ground-based observations of the set, they compare the properties to derive the most-likely properties of Kepler 61, finding that it has a size of 0.62 solar-radii and a surface temperature of 4000 kelvin. They then model the transiting exoplanet to have a radius of 2.15 Earth-radii and an equilibrium temperature of 273 +-13 kelvin. They conclude that the exoplanet is probably too large to be terrestrial in composition, but has a moderate temperature and is likely to contain an atmosphere. Although the Kepler satellite has ceased regular operations, the dataset it has acquired is expected to continue to provide new results, and this new technique offers a way to obtain more reliable values for planetary parameters.
"Exoplanet Characterization by Proxy: A Transiting 2.15 R-Earth Planet Near the Habitable Zone of the Late K Dwarf Kepler-61," Sarah Ballard, David Charbonneau, Francois Fressin, Guillermo Torres, Jonathan Irwin, Jean-Michel Desert, and collaborators, ApJ, 773, 98, 2013.