Third rock. Seen in the foreground in this artist impression, the outermost of the three planets orbiting the red dwarf star KOI-961 is barely larger than Mars.


More than half of purported giant alien worlds may not exist

When is an exoplanet not an exoplanet? A team of researchers, following a 5-year campaign investigating candidate alien worlds spotted by NASA’s Kepler mission, found that more than half of the giant exoplanets spotted by the orbiting telescope are not planets at all but a pair of stars orbiting each other, or a brown dwarf—or failed star (pictured)—orbiting another star. Kepler identifies exoplanets by staring at a large number of stars for extended periods and waiting for their brightness to dip periodically when a planet passes in front of them. But these dips can be caused by a number of effects so need to be confirmed by other methods. A team using the SOPHIE spectrograph on a 1.93-meter telescope at the Haute-Provence Observatory in France spent 5 years studying 129 of Kepler’s bigger candidates using a different method: looking for the slight movement of a star as a planet’s gravity tugs it around. As the researchers told the Extreme Solar Systems III conference in Hawaii today, nearly 55% of the candidates were false positives. This result will only add to the mysteries surrounding giant gaseous exoplanets, the researchers say, such as why so many of them seem to be in very close orbits around their stars—very unlike our own far-out gas giants, Jupiter and Saturn.