In 2009, a NASA spacecraft, fresh off the launchpad, drifted into an orbit high above Earth. The Kepler telescope would circle the sun, but its attention would be focused elsewhere, far beyond the edges of our solar system, on the Milky Way’s other stars. As Kepler settled into its perch, engineers on the ground commanded the spacecraft to overheat one of its wires until it snapped apart. The maneuver freed the oval-shaped lid they had placed over Kepler’s mirrors to protect them during the launch. The cover floated away.
Kepler, at last, could see. The light of thousands of stars flooded its mirrors.
But the telescope wasn’t designed to study the stars themselves. It was built to find the planets around them. Astronomers had predicted that there were many other planets in our galaxy, and they had already discovered about 300 exoplanets by the time Kepler launched, using a mix of terrestrial and space telescopes. They expected that the telescope would add more to the catalog.
“We were cautiously optimistic we would find planets,” Jessie Dotson, Kepler’s project scientist, recalled in a recent interview. “We really had no idea we were going to find so many.”
Kepler delivered a bounty: 2,681 exoplanets, with several thousand others awaiting confirmation. Some are massive and gaseous, like Jupiter. Others orbit in a solar system that has two stars, a scenario that astrophysicists had predicted couldn’t exist. Some are rocky and familiar; about 30 planets are the size of Earth and orbit in their star’s habitable zone, a region where temperatures are not too hot or too cold, but just right for liquid water to exist.
Of all the exoplanets known to astronomers today, about 70 percent were found by Kepler. In less than a decade, a mission that cost $600 million in taxpayer money—an incredible bargain for a space mission—became NASA’s most significant astrophysics mission of the 21st century.
That mission has now come to an end. On Tuesday, NASA announced that the telescope had run out of fuel. “This marks the end of science operations for Kepler,” Paul Hertz, the head of NASA’s astrophysics division, said in a press conference.
Like all spacecraft, Kepler launched with a limited amount of fuel. Scientists and engineers knew that the spacecraft would someday deplete its supply and lose the ability to orient itself in space, which means it would no longer be able to watch the stars. They predicted that would happen sometime this year, but they weren’t sure exactly when; the spacecraft wasn’t equipped with a gas gauge. In July, Kepler began to show the first real signs of running very low on fuel. Engineers shifted Kepler between awake and sleep modes several times in the last month, holding their breath each time they tried to rouse it, wondering if this time it wouldn’t work.
Dotson didn’t mind the torment. “I am more than happy to live through this uncertainty, given how fantastic of an instrument it is,” she said. “A small price to pay for the science we’re getting out of it.”
On October 19, Kepler slipped into another sleep mode, this time on its own, and for good.
The Kepler mission was named for Johannes Kepler, the 17th-century German astronomer who proposed three laws that govern the motion of planets around the sun. Kepler’s work relied on the theories of Nicolaus Copernicus, the 16th-century Polish astronomer who determined, much to the chagrin of religious leaders, that the Earth was not the center of the universe, but orbited the sun. Centuries later, the Kepler mission continued in these scientists’ footsteps in its own way. With each discovery of a planet around a distant star, the telescope seemed to scream, Here’s yet another reminder that we’re not the center of the universe, not even a little.
“Kepler has really nailed down how ubiquitous planets are,” Dotson said. “We knew they were out there, and now we know they’re everywhere.”
Kepler was the brainchild of Bill Borucki, a scientist who started out at NASA’s Ames Research Center in the 1960s, designing and testing materials for the heat shields of Apollo spacecraft, according to Universal Life, by Carnegie astronomer Alan Boss, a longtime friend of Borucki. Borucki took issue with a 1971 paper in a planetary science journal that proposed detecting exoplanets by measuring tiny changes in the color of a star’s light as a planet passed in front of it. The Earth’s atmosphere, he said, would warp and distort these observations. Plus, there were no telescopes strong enough to detect such minuscule fluctuations. But a space telescope, one that observed changes in the luminosity of a star, and not its color, would do the trick, he said.
Borucki’s idea had to wait for technology to catch up. The Nobel Prize–winning hardware that his particular telescope would require was invented by Bell Labs in 1969, but it didn’t reach the commercial sector until the 1980s.
Borucki eventually proposed his telescope to NASA in 1992. It was shot down, multiple times. It wasn’t until 2001 that the space agency formally approved the mission. On Tuesday, Hertz, the NASA official, praised Borucki for pushing the space agency for decades. “Boy, are we glad he did,” Hertz said.
The Kepler spacecraft blasted off from Florida’s Cape Canaveral on a humid day in March 2009. When the lid came off, the telescope stared continuously at about 150,000 stars, waiting for a hint of wobbling in the light.
Kepler searched for exoplanets using a technique known as the transit method. When a planet passes, or transits, its star, it blocks a tiny fraction of the star’s light. Kepler spent years staring at hundreds of thousands of stars, looking for this slight dimming in their brightness. If an alien telescope were aimed at our solar system from somewhere else in the Milky Way, it would observe the same effect as one of the big planets moved in front of the sun.
Three years into the mission, something went wrong. One of Kepler’s four reaction wheels, rapidly spinning devices that keep it oriented in space, failed. Within a year, a second followed. The spacecraft could no longer remain focused on its targets.
But NASA spacecraft are resilient, and their stewards are a creative bunch. When hardware breaks, engineers find ways to keep the mission going. Kepler’s engineers decided to take advantage of the spacecraft’s instability. Instead of staying focused on one part of the sky, the spacecraft would swing around every few months. In this new mode, Kepler could scan only 20,000 to 40,000 stars at a time, much fewer than before. But it was better than nothing.
The plan worked. The Kepler team announced batch after batch of new exoplanet discoveries. When it launched, the mission was supposed to run for about three-and-a-half years. Even with two broken reaction wheels, it lasted for more than nine.
There are many things that Kepler couldn’t tell us. While it could detect the size of an exoplanet and how long it takes to complete one trip around its star, it couldn’t reveal the planet’s mass, density, or composition. These properties are the ones that can help astronomers determine more closely the look of the planet and whether it possesses the conditions known to be conducive for life.
Kepler struggled to find rocky planets the size of our own, which would be the best candidates for this search. “These are really at the limit of what we can find with Kepler,” Dotson said, “but we still found some.” This means, she said, that there are likely many more Earth-sized rocky planets—which means many more chances of alien life. In its final moments, Kepler was trained on a star known to host seven Earth-sized planets.
The loss of Kepler is not the end of NASA’s exoplanet search. In April, NASA launched the Transiting Exoplanet Survey Satellite, or TESS, into space. Like Kepler, TESS will deploy the transit method in its scan of the skies, surveilling more than 200,000 stars. The mission is expected to last two years. But if it’s anything like its hardy predecessor, it will operate for much longer.
It wasn’t so long ago that there were no known exoplanets, that the only planets we knew of were our own; it’s been only 30 years since the first planet outside our solar system was detected. Dotson, Kepler’s project scientist, remembers this earlier era, which feels both recent and distant. She remembers peering through a telescope during a school outing in second grade and seeing Saturn, one of the nine planets anyone knew about.
“I think back to that little girl on that night,” Dotson said. “I can imagine just standing there next to her and saying, ‘You see all those other stars out there? They have planets around them, too.’”