It doesn't pay to bet against Kepler. Even though the NASA spacecraft was hobbled a year and a half ago, the battered telescope has still managed to find a super-Earth. At 20,000 miles across, the newly discovered planet is 2.5 times as wide as Earth and holds about 12 times the mass.
The plucky little planet-hunter was thought by some to be dead in the water when the second of its four reaction wheels broke in 2013, crippling the space telescope.
Kepler was able to pick out tiny planets by looking for dips in starlight as the silhouetted planets traveled across the bright disk of their host stars. But it could only do so by staying very still, never wobbling -- and it needed at least three reaction wheels to stay in place.
With only two working wheels left, it looked like Kepler's mission was over for good.
But engineers at Ball Aerospace came up with a plan to use sunlight as the spacecraft's virtual "third wheel" -- the pressure from the photons (light particles) hitting the spacecraft would help to hold it in place.
Using this method, Kepler can now scan the skies for a new mission, named K2. Instead of staring into a single patch of fairly uniform stars, the spacecraft will look at a variety of targets across the night sky, including stars, proto-stars, galaxy clusters and supernovae.
Technically, the newly discovered planet didn't actually come out of a scientific search, said study lead author Andrew Vanderburg, an astronomer at the Harvard-Smithsonian Center for Astrophysics.
"This was just an engineering test," Vanderburg said. "It only lasted nine days, and we only looked at 2,000 stars."
In a standard science campaign, they'd look for 80 to 90 days at about 30,000 stars, he added.
"So this was actually a bit of a lucky find," Vanderburg said. "Of those few stars that they looked at, it just happened to transit once during the time they observed."
But even with the sun acting as the "third wheel," Kepler is still more wobbly than it used to be. So the researchers had to develop complex software that could correct for that predictable jitter, Vanderburg said. In the end, the researchers were able to reach about half the exquisite pointing precision of the original Kepler mission.
The team also used the HARPS-North spectrograph on the Telescopio Nazionale Galileo in the Canary Islands to confirm the results. (This instrument uses a different technique to detect planets, looking for the planet's tiny gravitational effect on its star.) Kepler only detected one transit in the nine days, but the researchers also used the Canadian MOST satellite to pick up hints of additional transits.
The planet in question, HIP 116454b, is a super-Earth -- a type of planet that doesn't exist in our solar system, Vanderburg said. It sits very close to its bright star, completing a "year" in just 9.1 days. Based on its mass, the planet could either be a watery world -- one-quarter rock, the rest water -- or a mini-Neptune with a thick, gassy atmosphere.
"We don't really have any of those planets in our solar system," Vanderburg said. "There's kind of a gap in our solar system of planet sizes between the Earth and Uranus, and this planet is right smack in the middle of this."
This planet could be a good future target for the Hubble Space Telescope, or for the James Webb Space Telescope, set for launch in 2018. In the meantime, more K2 data should be coming in the next month or so, Vanderberg said.