Eight planets have been found existing in the “Goldilocks” zone around their stars, which is neither too hot nor too cold for liquid water. Two of these, on the characteristics we can measure, resemble Earth more closely than any other known planet.
While the flood of new planets being brought to light by the Kepler Space Telescope have included quite a few in the Goldilocks zone, most have been much larger than Earth – either as rocky super-Earths or as suspected gas giants. The eight announced by Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics at the American Astronomical Society's conference are all less than twice the Earth's diameter, doubling the number of planets of this size known in the potentially habitable zone around other stars.
The pick of the bunch for homes-away-from-home are Kepler-438b and Kepler-442b. Both orbit red dwarf stars, but make up for this by being much closer than the Earth is to the sun, making it likely their average temperatures are similar to ours.
Kepler-438b orbits a very small star 470 light-years away every 35 days and is 12% larger than the Earth in diameter. Without knowing its mass, Torres cannot be sure of its composition, but used the BLENDER program to calculate that there is a 70% chance it is a rocky planet.
The distant parent star for Kepler-442b is larger and 1,100 light-years away, and also keeps more of a distance with a 112 day orbit. It is nearly one-third larger than Earth, reducing the chance of it being predominantly rock to 60%.
"For our calculations we chose to adopt the broadest possible limits that can plausibly lead to suitable conditions for life," Torres said.
The Goldilocks zones for these planets are calculated on the amount of light they receive. However, the range where water can exist in liquid form also depends on the strength of the Greenhouse effect on that particular planet, something we can guess at from this distance, but not yet measure.
In the absence of atmospheric information, we can only say that Kepler-438b receives 40% more light than Earth. Since its larger size may well also indicate a greater Greenhouse effect, the chance that it is warm is quite high, but it has a 70% chance of hosting liquid water. Kepler-442b, on the other hand, gets only two-thirds our sunlight. If it has a thick atmosphere commensurate with its size, its temperature may be very similar to our own.
"We don't know for sure whether any of the planets in our sample are truly habitable," says co-author David Kipping of the CfA. "All we can say is that they're promising candidates."
Kepler-442b easily outstrips the two previous most Earth-like planets which, despite being slightly closer to our planet's diameter, receive just 32% and 41% of our sunlight respectively.
The masses of some of the planets discovered by Kepler could be measured by observing their influence on their parent stars. However, these objects are too small to test in this way with existing equipment.