How many of the 200 billion stars in the Milky Way have potentially habitable planets? Astronomers at the University of California, Berkeley, and University of Hawaii, Manoa, have analyzed all data NASA’s Kepler space telescope, gathered during its four-year mission and can answer this question.
They estimate that one in five stars like the sun have planets about the size of Earth and a surface temperature favorable for life.
Given that about 20 percent of stars are sun-like, the researchers say, that amounts to several tens of billions of potentially habitable, Earth-size planets in the Milky Way Galaxy.
“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 UC Berkeley graduate student Erik Petigura, who led the analysis of the Kepler data.
NASA launched the Kepler space telescope in 2009 to look 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, including objects much larget than Earth or in orbits so close to their stars that they are roasted.
To sort them out, Petigura and his colleagues are using the Keck telescopes in Hawaii to obtain spectra of as many stars as possible. This will help them determine each star’s true brightness and calculate the diameter of each transiting planet, with an emphasis on Earth-diameter planets.
Astronomers focused on the 42,000 stars that are like the sun or slightly cooler and smaller, and found 603 candidate planets orbiting them.
Only 10 of these were Earth-size, that is, one to two times the diameter of Earth and orbiting their star at a distance where they are heated to lukewarm temperatures suitable for life.
Subsequently, they subjected Petigura’s planet-finding algorithms to a battery of tests in order to measure how many habitable zone, Earth-size planets they missed.
Petigura actually introduced fake planets into the Kepler data in order to determine which ones his software could detect and which it couldn’t.
“What we’re doing is taking a census of extrasolar planets, but we can’t knock on every door. Only after injecting these fake planets and measuring how many we actually found could we really pin down the number of real planets that we missed,” Petigura said.
All of the potentially habitable planets found in the team’s survey are around K stars, which are cooler and slightly smaller than the sun, Petigura said.
But the researchers’ analysis shows that the result for K stars can be extrapolated to G stars like the sun. Had Kepler survived for an extended mission, it would have obtained enough data to directly detect a handful of Earth-size planets in the habitable zones of G-type stars.
“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.”