Since rocketing into space in 2009, NASA’s planet-hunting Kepler space telescope has discovered more than 4,500 confirmed or candidate worlds, in the process reshaping our entire view of the prospects for life in the universe. Thanks to Kepler, we can now conjecture that planets circle essentially every star in the sky, perhaps 10 percent of those might be habitable, and our solar system’s familiar architecture of small inner worlds and outer giants is rather rare in the cosmos.

And yet despite all these revolutionary results, Kepler’s most sought-after quarry—a mirror Earth around another sunlike star—has proved elusive. At least, that is, until now. At a NASA press conference today that also unveiled more than 500 other new candidate planets, Kepler’s mission scientists announced they have finally found and confirmed what looks to be the mission’s long-sought holy grail, a near-twin of Earth called Kepler 452 b. The discovery is detailed in a paper to be published in The Astronomical Journal. “Yes, this is the first small, possibly rocky planet in the habitable zone of a sunlike star,” says lead author Jon Jenkins, an astronomer and 20-year veteran of the Kepler mission at the NASA Ames Research Center. Kepler 452 b is estimated to be 1.6 times the size of our own world, and resides in a clement, life-friendly orbit around a star in the constellation of Cygnus some 1,400 light-years away that is eerily similar to our own sun.

The discovery marks the end of a long road. Before reaching the launch pad, Kepler endured decades of developmental woes as its advocates struggled to convince NASA the mission would actually work as planned. After Kepler finally launched, the setbacks continued. Most of the sunlike stars it surveyed for planets proved to be far less placid than our own star, contaminating the spacecraft’s delicate datasets with astrophysical “noise” that would require years of extra observing time to overcome. Even worse, the reaction wheels used to point the spacecraft wore out earlier than planned, bringing the primary mission to a premature end in 2013.

Early in its mission, Kepler managed to find some tantalizing worlds, a handful of supersize cousins of Earth, most of them in clement orbits around smaller, cooler, quieter stars than the sun called M and K dwarfs, but all the setbacks made finding smaller Earth-sized planets around sun-like G stars a very tall order. “We thought perhaps that our hopes of finding small, rocky habitable worlds orbiting sunlike stars were dashed,” Jenkins recalls. But thanks to a host of ingenious analytic techniques and observation methods developed on the fly, with each new pass through Kepler’s data mission scientists have managed to wring out ever-smaller planets. And as those smaller, cooler planets pile up, astronomers are coming ever closer to pinning down the number of potentially habitable, potentially Earth-like planets in our galaxy, a value they call “eta-Earth.”

“We’re watching Kepler zero in on the Earth analogues in slow motion,” says study co-author Natalie Batalha, an astrophysicist at Ames who is also Kepler’s mission scientist. “The closer we get, the harder it gets. We’re tromping through the weeds, looking for the most precious stones…. Some said Kepler couldn’t find small habitable-zone planets orbiting G-type stars. Now that we have, I’m confident that Kepler will determine eta-Earth not just for K and M stars but also for G stars.” Knowing eta-Earth, Batalha says, will allow astronomers to estimate how nearby the closest Earth twins are and thus how large future space telescopes will have to be to image those planets and study them for signs of life.

After today’s data release, however, there will be only one more official Kepler release sometime next year. Although astronomers hope to wrest further discoveries from the mission’s archives for generations to come, the end is near for Kepler’s hunt for habitable worlds. “We are reaching the limit of what the Kepler project has to offer regarding the prevalence of potentially Earth-like planets,” says Kepler team member Joe Twicken, an astronomer and study co-author at the SETI Institute.

According to Kepler scientist Jeff Coughlin, also of SETI, today’s announcement still provides a preview of several “hidden gems” that may be fully revealed in next year’s final data release. In addition to Kepler 452 b, the team has discovered 11 yet-to-be-confirmed candidate planets that appear to be small, rocky and potentially suitable for life. One of them, presently known only as KOI-7235.01, looks to be only 15 percent larger than Earth, and orbits right in the middle of its star’s habitable zone. If confirmed, it would surpass even Kepler 452 b to become the most Earth-like world astronomers have ever found beyond our solar system.

The shaky status of Kepler’s finds, the confusing mixture of “candidates” and “confirmed” planets, comes from how it looks for worlds in the first place. Kepler detects planets through “transits,” watching for the telltale dips in starlight caused by the shadows of worlds that happen to periodically flit across the faces of their stars as seen from Earth. But many things besides transiting planets can cause stars to dim periodically, and to validate any candidate, Kepler’s scientists must rule out all of them. Once confirmed, a transit allows astronomers to confidently measure a planet’s orbital period—its year—as well as to estimate its size, by comparing the depth of its shadow with the estimated dimensions and luminosity of its star. Based on these scant data points, they then guess what the world’s composition and climate might be like.

In the case of Kepler 452 b, the team performed extensive simulations as well as ground-based observations of the star and its immediate surroundings to rule out the possibility of anything spoofing the planetary signal. Based on the planet’s estimated size, the researchers give it a better than even chance of being rocky like Earth. Depending on its exact composition, it could have a mass of anywhere between one to five times that of our own planet, which would powerfully influence its environment and fate.

Fittingly, Kepler 452 b may represent an end in more ways than one, being a denouement not only for the Kepler mission, but also for any biosphere the faraway world might harbor. The planet’s host star is estimated to be 1.5 billion years older than our sun, and the planet itself receives some 10 percent more starlight than our own world. Because sun-like stars gradually increase in luminosity as they age, this could mean that Kepler 452 b is a fading, geriatric world, once thriving with life but now withering beneath the slowly brightening light of its sun. If the planet is only one Earth mass, Jenkins says, any life there might be near its end; the world would be on the verge of a runaway greenhouse effect, with gravity too weak to prevent its life-giving water from boiling off into space due to rising surface temperatures.

Study co-author Douglas Caldwell, a SETI astronomer, says that Kepler 452 b is more likely to be about five times more massive than Earth, which could give it enough gravity to hold on to its water and maintain a flourishing biosphere. The planet “could have a thick atmosphere with lots of water, either in the atmosphere, in oceans or both,” Caldwell says. “Given the larger size and thus longer core-cooling time, we think there would likely still be volcanism.” All that put together would give any robust biosphere on the planet another 500 million years of life—about the same amount of time, it turns out, that our own planet may have before it begins losing its oceans to space, too.

“Given the concerns about our own environment and climate change, finding an exoplanet that might experience the runaway greenhouse much sooner than we will made us reflect on the Earth’s near- and long-term prospects for habitability,” Jenkins says. “It is bittersweet to see Kepler coming to a close,” he adds, “but we’ve managed to change the world’s view of our place in the universe. When I was a child, I used to lay in the grass on summer evenings wondering whether there were people on unseen worlds orbiting the stars I gazed up at. Now we know for certain that there are many small, rocky worlds in the habitable zones of sunlike stars and can move on to the next important steps in answering the question, ‘Are we alone?’ What a great way for this Kepler party to end!”