Earth-like Planets Could Be More Common Than Previously Believed
March 13, 2013

Habitable Zone Planets May Actually Be More Common

April Flowers for - Your Universe Online

A new study from Penn State's Department of Geosciences suggests that the number of potentially habitable planets is greater than previously thought, and that some of those planets are likely to be found around nearby stars.

"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."

Kopparapu recalculated the commonness of Earth-sized planets in the habitable zones of low-mass, also known as cool or M-dwarf, stars. There are several reasons scientists focus on M-dwarf stars, according to Kopparapu, such as the orbit of planets around M-dwarfs is very short, which allow 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. Another reason is that more M-dwarfs can be observed because they are also more common than stars like the Earth's Sun.

"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 Penn State study builds upon a recent study by a research team at the Harvard-Smithsonian Center for Astrophysics that analyzed 3,987 M-dwarf stars to calculate the number of Earth-sized planet candidates in cool stars' habitable zones. The habitable zone is the region surrounding a star where rocky planets are capable of sustaining liquid water and therefore life. The Harvard study used habitable zone limits calculated by Jim Kasting in 1993. Kasting is now an Evan Pugh Professor in Penn State's Department of Geosciences.

Based on data from NASA's Kepler satellite, the Harvard study didn't reflect the most recent estimates for determining whether planets fall within a habitable zone, according to Kopparapu.

The new estimates are based on a model developed by Kopparapu's research team using information on water and carbon dioxide absorption unavailable in 1993. These findings were applied to the Harvard team's study, using the same calculation method. The Penn State team found that there are additional planets in the newly determined habitable zones.

"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."

The findings are detailed in a paper scheduled to be published in an upcoming issue of Astrophysical Journal Letters.

Image 2 (below): This shows starlight on planet relative to sunlight on the Earth. Credit: Chester Harman