Planets relatively common in the Milky Way; one in six stars has an Earth-sized world

credit: Francois Fressin & Christine Pulliam
credit: Francois Fressin & Christine Pulliam
A new analysis found that one in six stars has an Earth-sized planet in a close orbit. About a fourth of all stars in the Milky Way have a larger super-Earth, and the same fraction have a mini-Neptune.

It was a major feat last year when a team of astronomers announced it had detected two Earth-sized planets, circling very close to a star called Kepler-20, which is 950 light-years away. Now, scientists say that such planets aren’t uncommon at all—one in every six stars has an Earth-sized world.

A team from the Harvard-Smithsonian Center for Astrophysics announced the new result Monday at the meeting of the American Astronomical Society in Long Beach, Calif. The 17 billion Earth-sized worlds scientists say are lurking out there in the Milky Way are so close to their stars that they would be far too sweltering for life, at least as we know it. Now, scientists say it is only a matter of time and more observations before they will be able to make similar predictions about Earth-sized planets that are situated in their stars’ habitable zones.

The data may also fuel just a little bit of wonder for anyone gazing up at night, because the scientists predicted that at least 70 percent of stars have a planet, whether it is a small rocky planet or a gas giant.

Advertisement - Continue Reading Below

“Every time you look at the night sky, [virtually] each star you’re looking at has a planet. It will have a planetary system,” said Francois Fressin, a Harvard astronomer who led the new study. “It’s quite amazing.”

To detect planets, scientists use NASA’s Kepler telescope to detect planetary transits—tiny winks in the light that come from a star as a planet passes between the star and the telescope, creating a momentary dimming. But each wink of light is not necessarily a planet. Some variation in the light from a star could be caused by two stars orbiting one another. So scientists have to study these winking stars to confirm whether they indeed signal a planet. So far, out of 2,321 candidates, 105 are confirmed planets, according to NASA.

Fressin performed scientific simulations of the kinds of astrophysical phenomenon that could give rise to false signals when seen by Kepler to see how many false detections were likely, and found that false signals could account for only a small fraction—10 percent—of the signals.

In the next few years, Fressin said, scientists will begin to get enough data to detect Earth-sized planets in Earth-like orbits around their stars. That means they are inching closer to the “Goldilocks” worlds that would be not too hot, not too cold, and just the right size to be home to life.

Researchers from Yale University also reported the results of a different type of planetary census at the meeting, identifying more than a dozen giant candidate planets orbiting within the habitable zone, where temperatures could range from minus-126 to 86 degrees Fahrenheit. They also confirmed one Jupiter-sized gas giant of a star similar to the sun.

“We already have the first step,” Fressin said. “We just need more time.”