Half Of Kepler's Giant Exoplanets May Not Be Planets

Artist's impression of a giant exoplanet orbiting a yellow star. IA

A new study has indicated that more than half of the potential giant exoplanet candidates found by NASA's Kepler Space Telescope are actually not exoplanets at all. According to the study, accepted for publication in Astronomy & Astrophysics, 52.3 percent were eclipsing binaries (two stars orbiting one another) and 2.3 percent were failed stars known as brown dwarfs (although the distinction between a brown dwarf and a giant planet is not entirely clear).

The team started by selecting a representative sample from all the 8,826 candidates from the Kepler object of interest list. The final selection was 129 large objects that block more than three percent of the star's light that could be detected multiple times as they orbit their star. The parent star has to be bright enough to be observed by SOPHIE, the spectrograph used for this particular analysis at the Haute-Provence Observatory in France.

"It was thought that the reliability of the Kepler exoplanets detection was very good – between 10 and 20 percent of them were not planets," lead author Alexandre Santerne from the Institute of Astrophysics and Space Sciences said 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. This has strong implications in our understanding of the exoplanet population in the Kepler field."

Potential exoplanet discoveries are often followed by detailed spectroscopic observations that allow astronomers to confirm if the object is a planet or not, as well as providing estimates for the mass. A smaller star orbiting a large star or brown dwarfs can mimic a giant transiting exoplanet, so a spectroscopic survey like this one is very important in confirming the true nature of exoplanet candidates.

"After 20 years of exploring planets as big as Jupiter around other suns, we still have a lot of questions left open," Santerne added. "For instance, we 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! We also don't understand why some of these giant planets are so puffy."

By using the size information from Kepler and the mass from SOPHIE, the team was able to calculate the bulk density of the confirmed exoplanets, and it turns out that not all giant planets are as puffy as astronomers thought they were. More data is necessary to understand these hot Jupiters and to precisely describe them in planetary formation and evolution theories.  

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