Scientists announced on Feb. 26 that NASA’s Kepler space telescope has discovered 715 new exoplanets. These newly discovered worlds double the current number of confirmed exoplanets, bringing the total tally to roughly 1,700.
These exoplanets, which are planets outside of our solar system, were found to orbit 305 stars—revealing multiple-planet systems much like our own solar system. Nearly 95 percent of these 715 planets are smaller than Neptune, which is four times the size of Earth, marking a large increase in the number of small-sized planets. Many previously identified exoplanets were closer to the size of Jupiter, which is 11 times the size of Earth. Four of these newly verified planets are less than 2.5 times the size of Earth and reside in their system’s habitable zone, often called the Goldilocks zone, where the temperature is just right to allow for liquid water—and potentially life—to exist.
Launched in 2009, Kepler was tasked with locating such planets and determining how common Earth-like planets are in our galaxy. Its mission ended last May with the failure of two out of four reaction wheels, leaving the telescope unable to orient itself. But during the four years leading up to this, Kepler flagged over 3,600 planet candidates. Mission team members continue to work through the data sent back by Kepler. The recently verified 715 planets were only part of the first two years’ worth of data.
How does Kepler and its team find and verify these planets? The telescope focuses its gaze on stars and watches for a dip in brightness, denoting a planet is there. As exoplanets orbit their stars, they transit, or pass in front of them and obstruct the star’s light. It’s a minor obstruction, though, due to significant differences between the size of stars and their planets; it would look similar to a bug flying in front of a flashlight. Scientists watch for these dips in light. If they occur regularly, say every 50 days, it means that blip is a planet that completes one trip around its sun in 50 days.
It is a laborious process that goes planet-by-planet. Mission scientists developed a new method of verification that was used to confirm these new planets. The team’s technique is called ‘verification by multiplicity’ and relies in part on the logic of probability.
Kepler watches 150,000 stars and has found that only a few thousand of them have planet candidates orbiting them. If the candidates were randomly distributed among all of Kepler’s observed stars, only a handful of them would have more than one planet candidate orbiting it. Kepler has detected hundreds of stars, however, that have multiple planet candidates around them. It was through a careful study of this sample that the 715 planets were confirmed.
NASA likens the method to the behavior of lions and lionesses. Let the savannah be space, the lions be the stars Kepler watches, and the lionesses be the planet candidates. Lionesses are sometimes seen grouped together whereas lions tend to roam alone. If you see two lions together, it could be a lion with a lioness or it could just be two lions. But if more than two felines are gathered, then it is very likely that it is a lion with his pride. Thus, through multiplicity the lionesses can be reliably verified in the same way multiple planets can be found around the same star.
The multiplicity method should help scientists confirm hundreds more candidates, says Jason Rowe, who co-lead the research. It should also cause a higher percentage of these findings to be in the habitable zone, since the transit method takes longer to detect distantly orbiting planets than planets that revolve around their sun in a matter of days or weeks.
This recent discovery by Kepler shows that systems with multiple planets around one star—like our solar system—are quite common across the galaxy. It also shows that the planets in these systems are small and have orbits that are flat and circular, like pancakes, Rowe says. “The more we explore the more we find familiar traces of ourselves amongst the stars that remind us of home.”