There could be a lot more potentially habitable planets out there than previously thought, and some of them could be very nearby.
Penn State researcher Ravi Kopparapu recalculated the commonness of Earth-sized planets in the habitable zones of low-mass stars, also known as cool stars or M-dwarfs.
"We now estimate that if we were to look at 10 of the nearest small stars we would find about four potentially habitable planets, give or take," he says. "That is a conservative estimate - there could be more."
There are good reasons for focusing on M-dwarfs, he says. Planets tend to have very short orbits around such stars, allowing scientists to gather a lot more data in a shorter period of time than is the case with sun-like stars, which have larger habitable zones. There are also a lot more of them.
And, says Kopparapu: "The average distance to the nearest potentially habitable planet is about seven light years. That is about half the distance of previous estimates."
"There are about eight cool stars within 10 light-years, so conservatively, we should expect to find about three Earth-size planets in the habitable zones."
The new estimates are based on an updated model developed by Kopparapu and collaborators, including new information on water and carbon dioxide absorption.
"I used our new habitable zone calculations and found that there are nearly three times as many Earth-sized planets in the habitable zones around these low mass stars as in previous estimates," he says. "This means Earth-sized planets are more common than we thought, and that is a good sign for detecting extraterrestrial life."