More Earth-Sized Planets in Habitable Zones And Closer Than Thought

The number of potentially habitable, earth-sized exoplanets is larger than the current estimate when extrapolating the total numbers using an updated habitability zone, instead of the old definition from 1993. The scientist who calculated the habitable zone--the region around a star where rocky planets are capable of sustaining liquid water and therefore life--for small stars 20 years ago updated the lower and upper distance limits of the zone in a paper just 6 weeks ago in January. James Kasting, a professor at the Penn State University who studies planetary atmospheres, came up with new parameters for his model in order to offer a more precise calculation of the habitable zones that can be discovered around the star.

Also a researcher at Penn State University, Ravi Kopparapu applied the new habitability definition to the existing estimates of potential exoplanets  recalculating the commonness of Earth-sized planets in the zones of low-mass stars, also known as cool stars or M-dwarfs. His paper was accepted for publication in Astrophysical Journal Letters.

"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," said Ravi Kopparapu, a post-doctoral researcher in geosciences. "That is a conservative estimate," he added. "There could be more."

Scientists focus on M-dwarfs for several reasons, he explained. The orbit of planets around M-dwarfs is very short, which allows scientists to gather data on a greater number of orbits in a shorter period of time than can be gathered on Sun-like stars, which have larger habitable zones. M-dwarfs are also more common than stars like the Earth's Sun, which means more of them can be observed.

According to his findings, "The average distance to the nearest potentially habitable planet is about seven light years. That is about half the distance of previous estimates," Kopparapu said. "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 work follows up on a recent study by researchers at the Harvard-Smithsonian Center for Astrophysics which analyzed 3,987 M-dwarf stars to calculate the number of Earth-sized planet candidates in cool stars' habitable zone.

The newer estimates used are based on an updated model developed also by Kopparapu and collaborators, using information on water and carbon dioxide absorption that was not available in 1993.

"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," Kopparapu said. "This means Earth-sized planets are more common than we thought, and that is a good sign for detecting extraterrestrial life."