Kepler searches for planets by measuring stellar brightness dips caused when a planet passes in front of a star, briefly dimming the star’s light. This technique, called the transiting method, is more than 90 percent accurate, but sometimes a nonplanet can fool the telescope. One of the most common reasons for a “false positive” is an eclipsing binary—a pair of orbiting stars that sometimes cross in front of one another from our perspective—lying along the same line of sight as the foreground star Kepler is studying. Eclipsing binaries dim when one star passes in front of the other, mimicking the dimming effect a planet would have.
Stars with a single planet can be hard to distinguish from eclipsing binaries. But multiplanet systems are far less likely to be frauds. “It happens, but it’s unlikely that you have two eclipsing binaries in the background of the same star,” says Francois Fressin of the Harvard–Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., who was not involved in the study. “That simple fact tremendously increases the odds that they are bona fide planets.” It is also possible to have an eclipsing binary and a star with a planet lying right on top of one another, albeit extremely unlikely. “Based on that argument we started to get into the statistics to see if we can quantify that and see how many we can pull out and say with very good confidence they are validated planets,” Rowe says.
About 20 percent of the candidate planets Kepler finds inhabit systems with multiple worlds. Among this group, Rowe and his colleagues tried to weed out the small number that were likely to be false signals by examining the light signature of the candidate planets. The light from a single planetary system would be centered on one point, the parent star. An eclipsing binary in the background, however, would probably not lie exactly behind the main star, but would be offset by a small distance. When this binary blinks out as one star crosses the other, the center of the light in the field of view should shift over to the side, creating a signature called a moving centroid. “The moving centroids are the ones where we’re fairly sure they are false positives, and then we have a subset, the majority of them, that we are very confident are planetary systems and show no sign of blends,” Rowe says.
The idea that multiplanet systems are easier to validate is not new, and researchers have previously studied how to winnow out the small number of false positives. “I made this argument [in 2011] but now it has been worked out in careful detail,” says David Latham of the CfA. “Jason has done a really nice job.”
The new cache of planets is extremely unlikely to harbor imposters, but they are not “confirmed planets,” in the traditional sense. That requires measuring the parent star’s motion to determine how much the planets’ gravitational tugging causes it to wobble, revealing the planets’ mass. “Even though we can be very confident that these objects are real planets, the only information we have right now on their physical properties is their size (radius) and expected equilibrium temperature (which depends on the distance to their parent star, which is known),” says Guillermo Torres of the CfA.
Among the new trove of planets: a small, potentially rocky world; an odd binary star system where each star has planets of its own; and cramped systems where the multiple planets are each gravitationally tugging one another around. “Of course we have every type of planetary system in our validated set that people can think of, except the perfect Earth analogue,” Rowe says. For now, that remains Kepler’s holy grail.
*Editor’s Note (2/26/14): The tally of 719 exoplanet validations announced in the quote was restated as 715 by the Kepler team shortly after this story was posted.