Astronomers worldwide agree: there's a heck of a lot of planets out there. Thanks to tools like the Kepler Space Telescope, scientists have been discovering hundreds of new planets each year, and many believe it's only a matter of time until we find one very similar to our own earth. The big question now is, just how common are planets, and how many of them can we expect to look like Earth—or least hold liquid water?

In new study, researchers led by Steffen Jacobsen at the Niels Bohr Institute in Denmark have taken a crack at these questions and come up with some pretty stunning conclusions. By extrapolating on recent planet discoveries found though the Kepler mission, the astronomers have estimated that, on average, every star has between one to three planets nestled into its liquid water-supporting habitable zone. And of those planets, one in six should be rocky like the Earth, Jacobsen says.

Habitable zones for different kinds of stars NASA / Kepler

"In our galaxy alone, this would mean billions and billions of planets [in their star's habitable zones], with very good chances at finding an Earth twin," Jacobsen says. "But to be clear, these numbers are highly dependent on many assumptions at work here."

How they got those numbers

Since it started staring at the stars in 2009, Kepler has spotted hundreds of new planets and many more planet candidates that could be confirmed in time. It upended what we know and how we think about what's out there in the cosmos.

"In our galaxy alone, this would mean billions and billions of planets."

It also missed a crazy number of planets—many more than it ever saw. That's because the space telescope surveyed only a small swath the galaxy and could only see planets when they pass directly in front of their stars from our viewpoint. If the planets don't transit in front of their star from our point of view—or if they just take too long to do so, as Kepler only remained in full operation 5 years—it wouldn't find them. This is important, because any estimate on the number of planets in our galaxy will have to include planets that astronomers know satellites like Kepler are unable to detect with certainty.

Jacobsen's estimate is based on a modern reworking a 240-year-old astronomical observation called the Titius-Bode law. In short, the T-B law is a hypothesis that the planets in solar systems are spaced at mathematically specific intervals. (The upshot: If you know the distances between the Sun, Mercury, and Venus, then you can use math to estimate where Earth and Mars would be located). Jacobsen and his team used this law to infer planets where Kepler didn't our couldn't find them.

Our solar system, scaled to a football field Wikimedia Commons / Tdadamemd

The astronomers began their calculation by considering 151 star systems found by Kepler that had three or more planets. For 124 of these systems, Jacobsen found that the T-B law fit even better than it does in our own solar system, and so he used it to infer the existence and location of the next few planets. For the other star systems, Jacobsen and his colleagues found that inserting planets Kepler may have missed into specific gaps actually aligned the system to fit the mathematical proportions of the T-B law.

Big ifs

Jacobsen puts two caveats on his prediction. First, the numbers are correct only if those planets he assumes to be there really are there. That much is obvious. Secondly, though, his model assumes that the Kepler planetary systems are the norm, and representative of what the rest of the galaxy is like. Those are two big ifs.

"These predictions are not crazy—they just have a pretty high error [range] attached to them."

Jeff Coughlin, a SETI astronomer working with and who was not involved in this research, says that the trouble with Jacobsen's type of estimation is that with the information we have now, it's difficult to prove or disprove how valid the inserted planet predictions really are.

"It's certainly very interesting," says Coughlin, "but until we have actual data showing us how much we should trust these predications, we should be careful. These predictions are not crazy—they just have a pretty high error [range] attached to them."

Jacobsen wholeheartedly agrees. To this end, he provided a list of 77 planets whose existence he inferred in this study, and which he hopes might be found by meticulously combing through the Kepler data in the upcoming months. Finding them could provide a solid clue as to whether he's right about the great abundance of new worlds out there waiting to be discovered.

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