Could there be 17bn Earth-sized planets in the Milky Way?

8 Jan 2013

Scientists using NASA’s Kepler telescope have estimated that there are potentially 17bn other planets similar in size to planet Earth in our Milky Way.

The astrophysicists who have been using NASA’s Kepler telescope estimate that one in six stars has a planet similar in size to planet Earth.

The latest analysis of the Kepler data suggests that about 17pc of stars Earth-sized planet in an orbit closer than Mercury. And because 100 billion stars inhabit the Milky Way, the scientists believe that there are at least 17 billion Earth-sized planets out there.

Francois Fressin from the Harvard-Smithsonian Center for Astrophysics (CfA) presented the latest Kepler analysis at a conference during a meeting of the American Astronomical Society in California yesterday.

The findings are set to feature in an upcoming edition of The Astrophysical Journal.

Key findings from the scientists include that one in six stars hosts an Earth-sized planet in an orbit of 85 days or less, and that almost all Sun-like stars have a planetary system of some sort.

The Kepler telescope detects planet candidates using the transit method. This involves watching for a planet to cross its star to cause a mini-eclipse that dims the star slightly. The first 16 months of the Kepler survey detected around 2,400 planetary candidates.

Four habitable planets?

At the American Astronomical Society meeting, NASA also presented Kepler research. Christopher Burke from the SETI Institute presented the discovery of 461 new planet candidates based on Kepler observations carried out between May 2009 and March 2011.

According to NASA, four of these planets are twice the size of Earth and orbit in the sun’s habitable zone, the region in the planetary system where liquid water might exist on the surface of a planet.

Kepler data January 2013

Since the last Kepler catalogue was released in February 2012, the number of candidates discovered in the Kepler data has increased by 20pc and now totals 2,740 potential planets orbiting 2,036 stars. Image credit: NASA

Fressin said that, by simulating the Kepler survey, he and his colleagues were able to correct the list of candidates to recover the true occurrence of planets orbiting other stars, down to the size of Earth.

“There is a list of astrophysical configurations that can mimic planet signals, but altogether, they can only account for one tenth of the huge number of Kepler candidates. All the other signals are bona-fide planets,” said Fressin.

The astrophysicists claim to have found that 50pc of stars have a planet the size of the Earth or larger in a close orbit.

Five sizes

They grouped planets into five different sizes. They found that 17pc of stars have a planet between 0.8 and 1.25 times the size of Earth in an orbit of 85 days or less.

A quarter of stars have a ‘super Earth’, a planet that’s between 1.25 and two times the size of Earth, in an orbit of 150 days or less.

And a quarter of stars have a mini-Neptune that’s between two and four times the size of Earth in an orbit that last up to 250 days.

According to the scientists, larger planets are much less common. It seems that just 3pc of stars have a planet that’s between four and six times the size of Earth, while 5pc of stars have a ‘gas giant’ that’s between six and 22 times the size of Earth.

Carmel Doyle was a long-time reporter with Silicon Republic