‘False positives:’ More than half of NASA’s Kepler exoplanets aren’t actually planets

A five-year study of exoplanet data captured by the Kepler space observatory found that half the exoplanet candidates aren’t planets at all. They are either too small (brown dwarfs) or eclipsing binary stars (failed stars).

The research, which was conducted between July 2010 and July 2015, started with all of the 8,826 objects of interest discovered by Kepler. The sample number was reduced to 129 Kepler objects of interest (KOI) on 125 target stars by removing false positives, stars too faint to be observed, and candidates with orbits of more than 400 days.

Researchers observed three orbits of an object around its star and found that 52.3 percent of the objects were failed stars, and another 2.3 percent were brown dwarfs.

“It was thought that reliability of the Kepler exoplanets detection was very good – between 10 to 20 percent of them were not planets,” said Alexandre Santerne of the Portugal-based Institute of Astrophysics and Space Sciences (IASS) in a statement. “Our extensive spectroscopic survey, of the largest exoplanets discovered by Kepler, shows that this percentage is much higher, even above 50 percent.”

In conducting their research, the astrophysicists used SOPHIE, a spectrograph for observing the dimming of starlight produced when an exoplanet crosses in front of its star, like a mini-eclipse. This transit helps to identify a potential planets’ radius.

“Detecting and characterizing planets is usually a very subtle and difficult task,” researcher Vardan Adibekyan of IASS said in a statement. “In this work, we showed that even big, easy to detect planets are also difficult to deal with. In particular, it was showed that less than half of the detected big transiting planet candidates are actually there. The rest are false positives, due to different kind of astrophysical sources of light or noise.”

Researchers announced the findings at the Extreme Solar System III conference in Hawaii on Thursday, and the study has been accepted for publication in the Journal of Astronomy & Astrophysics.

NASA’s Kepler space observatory has been orbiting around the sun since 2009, collecting data on 150,000 stars, all while searching for Earth-like planets in the Milky Way galaxy. Of the total 8,000-plus objects of interest, 4,696 are planet candidates, of which more than 1,000 have been confirmed as planets. Twelve of those have been confirmed as existing in habitable zones.

“If a planet candidate has an inferred size greater than that of Jupiter, there is a high probability that it is really due to an eclipsing binary, but there is no ‘nail in the coffin’ to definitely rule it out as a planet based on Kepler data alone, so we leave it designated as a candidate,” Jeffrey Coughlin, a Kepler researcher at NASA, told Popular Mechanics. “Santerne et al. collected follow-up observations of candidates with very large inferred sizes, and as expected, found that a large fraction are indeed false positives.”

“As far as I’ve read, their analysis only applies to KOIs, and they did not rule out any fully-confirmed planets,” Coughlin added.

Researchers at IASS said that after exploring planets as big as Jupiter around other suns, they still don’t understand “what is the physical mechanism that forms Jupiter-like planets with orbital periods as little as a few days.”

“It is like if our annual rotation around the Sun would last only a few days – imagine your age!” said Santerne.

Researchers know the radius of giant gas planets depends on their atmosphere and interior giant zone. When a star heats up their atmospheres, they become inflated or puffy, allowing their radius to be calculated. In the study, however, researchers were finding some giant gas planets were puffy, but others were not.

“The team also found a hint of connection between the density of these planets and the metallicity of the host starts, but this needs more confirmation,” said the statement. “Detailed characterization of the internal structure of these planets should shed new light on planet formation and evolution theories.”