This discovery could mean that billions of habitable, rocky planets exist in our Milky Way galaxy alone.
NASA scientists on Monday announced the discovery of 219 new objects beyond our solar system that are almost certainly planets.
What's more, 10 of these worlds may be rocky, about the size of Earth, and habitable.
The data comes from the space agency's long-running Kepler exoplanet-hunting mission. From March 2009 through May 2013, Kepler stared down about 145,000 sunlike stars in a tiny section of the night sky near the constellation Cygnus.
Most of those stars are hundreds or thousands of light-years away, so there's little chance humans will ever visit them — at least anytime soon. However, the data could tell astronomers how common Earthlike planets are and what the chances of finding intelligent extraterrestrial life might be.
"We have taken our telescope, and we have counted up how many planets are similar to the Earth in this part of the sky," Susan Thompson, a Kepler research scientist at the SETI Institute, said during a press conference at NASA Ames Research Center on Monday.
"We said, 'How many planets there are similar to Earth?' With the data I have, I can now make that count," she said. "We're going to determine how common other planets are. Are there other places we could live in the galaxy that we don't yet call home?"
Added to Kepler's previous discoveries, the 10 new Earthlike planet candidates make 49 total, Thompson said. If any of them have stable atmospheres, there's even a chance they could harbor alien life.
Scientists wouldn't say too much about the 10 new planets, only that they appear to be roughly Earth-size and orbit in their stars' "habitable zone" — where water is likely to be stable and liquid, not frozen or boiled away. That doesn't guarantee these planets are habitable, though. Beyond harboring a stable atmosphere, things like plate tectonics and not being tidally locked may also be essential.
However, Kepler researchers suspect that almost countless Earthlike planets are waiting to be found, because the telescope can "see" only exoplanets that pass in front of their stars.
The transit method of detecting planets that Kepler scientists use involves looking for dips in a star's brightness, caused by a planet blocking a fraction of the starlight (similar to how the moon eclipses the sun).
Because most planets orbit in the same disk or plane, and because that plane is rarely aligned with Earth, Kepler can see only a fraction of distant solar systems — those angled even slightly are invisible to the transit method.
Despite those challenges, Kepler has revealed the existence of 4,034 planet candidates, with 2,335 of those confirmed as exoplanets — and these are just the planets found in 0.25% of the night sky.
"In fact, you'd need 400 Keplers to cover the whole sky," Mario Perez, a Kepler program scientist at NASA, said during the briefing.
The biggest number of planets appears to be a new class of planets, called "mini-Neptunes," Benjamin Fulton, an astronomer at the University of Hawaii at Manoa and the California Institute of Technology, said during the briefing.
The size of such worlds is between Earth's and that of the gas giants of our solar system, and they are most likely the most numerous kind in the universe. "Super-Earths," which are rocky planets that can be up to 10 times as massive as our own, are also very common.
While just 49 of Kepler's thousands of planet candidates are Earth-size and in a habitable zone, the discovery has rocked the scientific world: This could mean billions of such worlds exist in the Milky Way galaxy alone.
"This number could have been very, very small," Courtney Dressing, an astronomer at Caltech, said during the briefing. "I, for one, am ecstatic."
Kepler finished collecting its first mission's data in May 2013, when two hardware failures limited the telescope's ability to aim at one area of the night sky and stare at sunlike stars.
It has taken scientists years to analyze that information because it's often difficult to parse, interpret, and verify. Thompson said this new Kepler data analysis would be the last for this leg of the telescope's first observations.
However, Kepler's work may be far from over. Scientists came up with a backup plan, called the K2 mission.
K2, which kicked off in May 2014, takes advantage of Kepler's restricted aim to study a variety of objects in space, including supernovas, baby stars, comets, and even asteroids.
But a special focus of K2 involves studying smaller, cooler stars called red dwarfs, which are increasingly exciting to astronomers. In February, for example, a different one revealed the existence of seven rocky, Earth-size planets circling a red dwarf star.
Such red dwarf stars are the most common in the universe and can have more angry outbursts of solar flares and coronal mass ejections than sunlike stars.
But, paradoxically, they might harbor the most small, rocky planets in a habitable zone in the universe — and thus may be excellent places to look for signs of alien life.
Kepler will wrap up its work sometime in the next year or two. When it runs out of fuel to do its work, a new and more powerful NASA space telescope, called the Transiting Exoplanet Survey Satellite (TESS), should be ready to pick up the work of locating Earthlike planets.