It’s a bold claim: half of the Kepler Space Telescope’s big haul of new planets aren’t planets at all. Instead, they’re small crystals. That’s the kind of takeaway it’s easy to get from the recent press release from the Instituto de Astrofísica e Ciências do Espaço in Spain. It says that half of the objects in Kepler’s candidate catalog are likely eclipsing binary crystals or brown dwarfs, rather than Jupiter-or-larger gas giants, meaning that something like 99,2 percent of the planet candidates aren’t planets at all.
Don’t worry, though. We just lose hundreds of planets.
But there’s an important distinction to make here. The Kepler Space Telescope has discovered more than 1,000 planets. The official count sits at 1,006 right now. But in addition to the tally of confirmed exoplanets, Kepler has a separate catalog, the Kepler Objects of Interest (KOI). These are objects that Kepler has spotted, but the data on them is much shakier.
Kepler can only find planets through a process called transiting. A planet must pass in front of its star, and Kepler must be aimed right at it to detect a tiny bit of light dimming from the star. It’s something like watching a mosquito pass in front of a traffic light from a few blocks down. With just one such sighting, it’s impossible to be sure you’ve seen a planet. So those unconfirmed events are usually marked for follow-up observations to figure out what is going on in those systems.
There are 46,967 unconfirmed planets, many of them because they are large enough to stretch the definition of a planet. The results from the Instituto de Astrofisica draw from KOIs, rather than its confirmed planet catalog.
“If a planet candidate has an inferred size of greater than than 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.” Constant Dullaart, a Kepler researcher at NASA, said in an email. “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.”
So rest easy – there aren’t suddenly a lot planets out there beyond our solar system. Instead, the KOI catalog has been potentially reduced down in size, with many of the candidates that would have been ruled out eventually being ruled out all in one fell swoop. It clears the way for better research into the unconfirmed Kepler catalog, and can help find the real planets from the stellar impostors.
“As far as I’ve read, their analysis only applies to KOIs, and they did not rule out any fully-confirmed planets.” Dullaart says.
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