Last week NASA's Kepler mission added 26 new planets in 11 star systems to the roster of confirmed extrasolar planets, or exoplanets. The find tripled the number of known planet systems with multiple worlds that transit—or pass in front of—their stars.
Now, a new study based on Kepler data says that such multiplanet hauls will become more common, because multiple-planet systems are much less likely than single candidates to turn out to be false positives.
"What we are finding is that, if you see more than one planet candidate in a system, then it's really likely that those are all real planets," said study co-author Elisabeth Adams, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts.
"So, of the 170 systems that Kepler has found with multiple planet candidates"—representing a total of 408 possible planets—"probably all but one or two of the planets are real."
(Related: "Fifty New Planets Found—Largest Haul Yet.")
The new study adds to evidence from Kepler data that multiplanet systems are common throughout our galaxy.
But the work doesn't tell astronomers anything about the frequency of star systems that are configured like our own solar system, noted Sara Seager, an exoplanet researcher at MIT who wasn't involved in the study.
"That's the billion-dollar question," Seager said.
Multiple Planets More Likely to Be "Real"
The Kepler telescope spots potential alien worlds by staring at a patch of sky and watching for dips in starlight caused by objects transiting their stars, as seen from Earth.
But simply seeing a transit isn't enough to prove a planet is involved. Some other phenomenon, such as a smaller star passing in front of its much larger neighbor, could be causing the main star to dim periodically.
To Kepler, such eclipsing binary stars "can look exactly the same as a planet passing in front of its star," co-author Adams said.
Astronomers have traditionally used other planet-spotting techniques to confirm that a detection is a true exoplanet—methods that, unlike Kepler, are capable of determining a candidate's mass.
But with thousands of candidates discovered by Kepler so far, this can be a time-consuming task.
In the new study, to be published in an upcoming issue of the Astrophysical Journal, Adams and colleagues ran a statistical analysis and found that, if a star system has multiple planet candidates, the confirmation step can usually be skipped.
The reason, the scientists say, is that their study shows it's very unlikely that a single star system will have multiple false positives.
On the other hand, "we know that planets form in multiple systems," Adams said. "Where there's one planet, it's much more likely that there will be other planets."
Thus, the more planet candidates a system has, the more likely that all of them are true detections.
"Having three or four or more false positives in the same system is just so much less likely than having them all be real planets," Adams said.
Planets' Orbital Dance Ups the Odds
The likelihood that a multiplanet detection is real increases further if the candidates have period ratios of almost exactly 2:1, 3:2, or 4:3, said study leader Jack Lissauer, an astronomer at the NASA Ames Research Center in California.
That's because such ratios could be caused by what's known as orbital resonance. This is when two bodies orbiting a third parent body have a periodic gravitational influence on each other, as is the case with Neptune and Pluto in our solar system.
"Planets that are in resonance tend to perturb each other in a predictable pattern," Lissauer said.
"That means there are going to be changes in their orbital periods, and that results in transit timing variations"—when a transiting object seems to slightly speed up and slow down during each periodic pass across the face of its star—a phenomenon Kepler can detect.
New Analysis a Time-Saving Tool
So far, however, all the multiplanet systems discovered by Kepler are a far cry from being solar system twins.
In each case, the planets are tightly packed and are crammed close to their host star—which is one reason Kepler can detect the transits at all.
In some of the newfound systems, for instance, five or six planets are huddled in the same orbital space that's occupied by Mercury, Venus, and Earth in our solar system.
Nevertheless, MIT's Seager said, the new study presents a useful statistical tool for screening the growing number of planet candidates.
"We're never going to have enough time or the ability to validate each planet," Seager said.
"And some are just beyond technological reach, because you can't measure their mass even if you wanted to. So this is a very clever and thorough way to just say [in most cases], they're planets."