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.