NASA has found new evidence of 219 planets outside our Solar System. Ten of those exoplanets appear to be similar to the size of the Earth and orbit their stars in the habitable zone — just far enough away to develop water, but not so far that they freeze. If confirmed, they would be added to a small but growing list of Earth-sized planets that occupy our corner of the Milky Way galaxy, supporting the idea that rocky worlds are more common than we once thought.

The potential discoveries are part of the final catalog of results being released from the first Kepler space telescope mission. Kepler has been surveying the Cygnus constellation since 2009, and during that time scientists have found more than 5,000 potential exoplanets in an area of the sky about 3,000 light-years away from Earth. Today’s announcement whittled those candidates down to 4,034. A total of 2,335 of those have been verified as exoplanets, 21 of which are Earth-sized and orbit in their star’s habitable zone. Earth-sized planets are of particular interest because they can teach us about how our own planet formed, and because there's a small chance they could harbor life.

2,335 exoplanets confirmed, and 21 of them Earth-sized and in the habitable zone — so far

Kepler spots planets by looking for dips in the brightness of the stars they orbit, known as a “transit.” When scientists see this happen, they then study each signal to confirm that it’s coming from a planet passing in front of the star and not some other anomaly. If it is a planet, that Kepler data can be used to determine its mass, size, and orbital period, or how long it takes to go around the star.

To get these newly refined results, the team moved away from identifying each signal by hand — an inconsistent method according to Susan Thompson, a Kepler research scientist for the SETI Institute. “You'd walk in and you go, ‘Looks like a transit. Looks like a transit. Looks like a variable star.’ You know what I mean? It was like, ‘Junk, junk, variable star — ooh, planet!’ It would be like that. We stopped doing it that way,” she said during a NASA podcast interview.

So to fix this, the Kepler team simulated their own positive and false signals of planet transits and compared them to the actual data from the mission. This allowed the team to figure out where they might have overcounted or undercounted a particular type of planet. The result is that this final catalog should be the most accurate ever released by the Kepler team.

“A major new division in the family tree of exoplanets”

NASA is getting better at identifying Earth-sized exoplanets in other ways, too. The team spent five years working with the operators of the ground-based Keck telescope in Hawaii to study 1,300 stars that hosted planets of these sizes. From that effort, they were able to surmise what Benjamin Fulton, the lead author on this study, called “a major new division in the family tree of exoplanets” during a press conference about the news. He compared the identification of these new planet classes to the discovery that mammals and lizards are separate branches on the tree of life.

The division now is between two categories: “super-Earths,” or rocky planets about 1.5 times the size of our own, and “mini-Neptunes,” gassy planets more than 2.5 times Earth’s size. They’re terms were already used but were sometimes considered interchangeable. And It’s an important distinction to make, because Neptune-sized planets are almost always inhospitable to life as we know it. “Our result sharpens up the dividing line,” Fulton said.

Better knowledge of how to find planets like our own will help scientists sift through this final catalog of data from Kepler’s original mission. It will also allow scientists to write better lists of promising targets that more powerful telescopes can study in the future.

With the final catalog of planetary candidates from Kepler’s original mission released, NASA will now focus on the “K2” mission, which began in 2014. K2, which has found over 100 different exoplanets so far, was started as a way to give the telescope a second life after Kepler suffered multiple mission-threatening technological setbacks. Two of the telescope’s four reaction wheels had failed — one in 2012, and one in 2013 — and at least three are needed to properly aim the telescope. Luckily, team scientists devised a clever workaround by using the Sun’s pressure on the telescope’s solar panels as a stand-in for the broken wheel.

Not an ending, but a new beginning

Kepler can still hunt for planets, but photons from the Sun don’t generate as much force as a reaction wheel can. The limits of the K2 mission mean scientists have to go about their searches more methodically. It’s akin to picking up leaves in your yard one by one instead of raking them all into a bag. The telescope will continue on with K2 until it runs out of fuel, likely sometime in 2018.

When asked during the press conference how the team felt about Kepler’s first mission coming to a close, Thompson explained that she sees it more as a new beginning. “It’s amazing the things that Kepler has found,” Thompson said. “It has shown us these terrestrial worlds, and we still have all this work to do to really understand how common Earths are in the galaxy.”