The habitable zone corresponds to the range of orbital distances where liquid water can exist on a planet's surface. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.

Astronomers from the University of Hawaii at Manoa and the University of California, Berkeley now estimate that one in five stars like the sun have planets about the size of Earth and a surface temperature conducive to life. This conclusion is based on a statistical analysis of all observations from NASA’s Kepler Space Telescope.

Though Kepler is now crippled, it nevertheless provided enough data to complete its mission objective: to determine how many of the 100 billion stars in our galaxy have potentially habitable planets. A habitable planet is defined as one that is approximately the size of Earth and that is the right temperature for liquid water.

"What this means is, when you look up at the thousands of stars in the night sky, the nearest sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye. That is amazing," said Erik Petigura, who led the analysis of the Kepler data. Petigura is a UC Berkeley graduate student working at UH Manoa for a year.

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"It's been nearly 20 years since the discovery of the first extrasolar planet around a normal star. Since then we have learned that most stars have planets of some size and that Earth-size planets are relatively common in close-in orbits that are too hot for life," said Andrew Howard, an astronomer at the Institute for Astronomy at UH Manoa. "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."

Petigura, Howard and Geoffrey Marcy, a UC Berkeley professor of astronomy, will publish their findings online this week in the journal Proceedings of the National Academy of Sciences. In addition to the public data from Kepler, their analysis depended on spectra of the planet-hosting stars from the W. M. Keck Observatory on Mauna Kea in Hawaii.

"For NASA, this number – that every fifth star has a planet somewhat like Earth – is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are," Howard said. "An abundance of planets orbiting nearby stars simplifies such follow-up missions."

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

Last week, however, Howard, Marcy and their colleagues provided hope that many such planets actually are rocky like the Earth. They reported that one Earth-size planet discovered by Kepler – albeit, an uninhabitable planet with a temperature of more than 2,000 Kelvin – is the same density as Earth and likely composed of rock and iron, like Earth.

"This gives us some confidence that when we look out into the habitable zone, the planets Erik is describing may be Earth-size, rocky planets," Howard said.

What distinguishes the team's analysis from previous analyses of Kepler data is that they subjected Petigura's planet-finding algorithms to a battery of tests to measure how many habitable zone, Earth-size planets they missed. Petigura actually introduced fake planets into the Kepler data to determine which ones his software could detect and which it couldn’t. Their study is also the first census of Earth-size planets from Kepler to accurately estimate the host star’s habitable zones using data from the Keck Observatory.

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 and therefore could be detected by Kepler, allowed them to estimate that 22 percent of all sun-like stars in the galaxy have Earth-size planets in their habitable zones.

All of the potentially habitable planets found in their survey are around K stars, which are cooler and slightly smaller than the sun, Petigura said. But the team's analysis shows that the result for K stars can be extrapolated to G stars like the sun.

"If the stars in the Kepler field are representative of stars in the solar neighborhood, … then the nearest (Earth-size) planet is expected to orbit a star that is less than 12 light-years from Earth and can be seen by the unaided eye," the researchers wrote in their paper. "Future instrumentation to image and take spectra of these Earths need only observe a few dozen nearby stars to detect a sample of Earth-size planets residing in the habitable zones of their host stars."

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Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaii.