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The Milky Way might harbor 20 billion Earth-sized planets

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Planets the size of Earth are 'a general feature of nature' according to new research

Habitable zones
Habitable zones

Gazing up at the stars is something of a shared human experience. We look and we wonder. Are those stars like ours? Do they have planets around them? Are we alone? While ET hasn't phoned home yet, a new study from astronomers at the University of California at Berkeley and the University of Hawaii at Manoa sheds light on how many planets could resemble our own.

Finding Earth-sized planets that are in an Earthlike orbit is a challenge. The larger a planet is and the closer it is to a star, the easier it is to detect. So it isn't a surprise that some of the first exoplanets found were gas giants, such as Jupiter, that were fairly close to their star. But scientists have gotten better at spotting planets outside of our solar system. In large part, they do this by detecting when a planet moves in front of its star, which causes the light from that star to dim. A larger planet, or one closer to its host star, will yield a more pronounced drop in light.

Spotting planets outside of our solar system

Unfortunately, Earth-sized planets are relatively small and won't block much light from a star. And while scientists have just identified an Earth-sized planet with an Earthlike composition, that planet isn't in what's known as the habitable zone — it's too hot for life. The habitable zone is the not-too-hot, not-too-cold distance from a star where water can exist in liquid form.

In their new study, scientists wanted to find out how many planets are the size of Earth and orbit in the habitable zone of their sunlike stars. To do that, they created a computer program to analyze starlight gathered by the Kepler space observatory, in an effort to find the characteristic dimming of planets crossing in front of their stars.

For every planet researchers do see, there could be 100 more

Of the 42,000 stars analyzed, researchers detected only 10 that harbored Earth-sized planets. While this may seem disappointingly small, these few planets actually point to a host of other, undetected ones. As it turns out, most planets will not cross in front of their star, so this dimming is uncommon. For every planet researchers do see, there could be 100 more in orbits that keep them hidden. And that's not the only way the planets are undercounted.

In an effort to boost the accuracy of their estimation, the team then created a test set of "mock" dimmings — essentially allowing them to distinguish between how many planets their program should find and how many it actually did. They concluded that the program only isolates a small number of dimmings, meaning that the 10 Earthlike planets directly detected may represent over 9,000 similarly sized planets in habitable zones. "The high abundance of Earth-sized planets appears to be a general feature of nature," says lead study author Erik Petigura. "With about 100 billion stars in our Milky Way galaxy, that's about 20 billion such planets."

"The high abundance of Earth-sized planets appears to be a general feature of nature."

Of course, being both Earth-sized and within the habitable zone aren't the only factors that matter. "Some significant share of those planets are going to be uninhabitable for lots of different reasons," says study co-author Andrew Howard. A planet might be uninhabitable because of a gas-giant composition or an elliptical orbit, for instance. This latest research, however, ensures scientists will have plenty of planets to study more closely.