Kepler launched in 2009 with a very specific goal: Find out just how common planets are in our cosmic neighborhood, and how many of those planets share characteristics with Earth -- such as small size and proximity to a bright star -- that might indicate they're worth a closer look. For four years, Kepler stared at one spot in the sky, looking at 150,000 stars at once to see if any planets passed in front of them.
But in May 2013, one of Kepler's four reaction wheels (which kept it centered so precisely on its target) failed. Without it, the spacecraft was unstable, and any outside force could knock it totally out of position. It seemed like the mission was over.
Instead, scientists decided to harness the power of the sun.
By using the light from the sun as a physical force pushing hard against the craft's solar panels, the mission team turned the star into a virtual fourth reaction wheel. The other wheels push back against the force of the sun, and that tension keeps Kepler firmly in place.
It's not exactly the same as it was during its first mission. Now Kepler has to be positioned so that the sun hits its panels just so and can stay focused on one point only for about 80 days before it's time to readjust, but NASA has found new ways to use the craft.
"We developed this concept of how to operate the telescope in a different mode," said Steve Howell, Kepler/K2 project scientist at NASA's Ames Research Center. Instead of staring at lots of stars for a long time to observe the sheer number of planets, Howell said, K2 focuses on potentially interesting objects in the hope of finding a planetary jackpot.
That's why he's so excited about K2's first finding. "This was the promise of K2," he said. "We had great hopes that it would find these kind of planets."
The planet in question is a Super Earth (a planet about 2.5 times larger than Earth) 180 light years away. Because it sits in between Earth (a rocky planet) and Neptune (a gaseous one) in size, Howell says that many scientists will want to study it. They want to understand how planet size and type are related.
To find it, a graduate student at the Harvard-Smithsonian Center for Astrophysics named Andrew Vanderburg compared thousands of publicly available images from Kepler.
"We get an image of the star over and over," Howell said, "And in those images the star is very slightly fainter at times because the planet has blocked its light."
Kepler's work on this new planet is now done: K2 will never be pointed at the planet again, Howell said. But as it continues to look for new bodies worth exploring, scientists using other space telescopes -- and ones on the ground -- will turn their attention to the first big find of its second life.