Milky Way Math: Billions of Potentially Habitable, Earth-size Planets

November 5th, 2013

Based on a statistical analysis of all the Kepler observations, astronomers now estimate that one in five stars like the sun have planets about the size of Earth and a surface temperature conducive to life. Credit: Petigura/UC Berkeley, Howard/IfA, Marcy/UC Berkeley, Illumina Studios.

For those interstellar travelers hungry to hit high speed, there are places to visit!

New research indicates that there are several tens of billions of potentially habitable, Earth-size planets, in the Milky Way Galaxy alone.

Analysis by Univ. of Calif. Berkeley and University of Hawaii astronomers shows that one in five sun-like stars have potentially habitable, Earth-size planets.

Kepler spacecraft data

The key instrument used in the assessment is NASA’s Kepler space telescope. Launched in 2009, the spacecraft has been on the lookout for planets outside the solar system that cross in front of, or transit, their stars, which causes a slight diminution – about one hundredth of 1 percent – in the star’s brightness.

From among the 150,000 stars photographed every 30 minutes for four years, NASA’s Kepler team reported more than 3,000 planet candidates.

Many of these are much larger than Earth – ranging from large planets with thick atmospheres, like Neptune, to gas giants like Jupiter – or in orbits so close to their stars that they are roasted.

Statistical analysis

But based on a statistical analysis, accounting for missed planets, as well as the fact that only a small fraction of planets are oriented so that they cross in front of their host star as seen from Earth, that has allowed researchers to estimate that 22 percent of all sun-like stars in the galaxy have Earth-size planets in their habitable zones.

Astronomers now estimate that one in five stars like the sun have planets about the size of Earth and a surface temperature conducive to life.

UC Berkeley graduate student Erik Petigura, Andrew Howard, a former UC Berkeley post-doctoral fellow who is now on the faculty of the Institute for Astronomy at the University of Hawaii and Geoffrey Marcy, UC Berkeley professor of astronomy, have published their analysis and findings via the online early edition of the journal Proceedings of the National Academy of Sciences.


The team cautioned that Earth-size planets in orbits about the size of Earth’s are not necessarily hospitable to life, even if they reside in the habitable zone around a star where the temperature is not too hot and not too cold.

“Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms,” Marcy said. “We don’t know what range of planet types and their environments are suitable for life.”

According to Howard in a university press statement: “With this result, we’ve come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way Galaxy.”

For a video highlighting the new work, go to:

By Leonard David