Kepler’s false-positive rate for some planets is higher than thought
NASA’s Kepler spacecraft has found thousands of prospective planets around nearby stars. But the trick is often in confirming whether they’re real or fakes. Now, a new study of gas giant planets in the Kepler dataset finds that more than half of these candidates are false positives, a rate much higher than some originally thought.
But fear not, ye seekers of new worlds. The findings, presented at Extreme Solar Systems III conference in Hawaii this week and set to be published in the journal Astronomy & Astrophysics, don’t necessarily mean that there are fewer planets out there. But they could help peg down details that will eventually help scientists understand their frequency and distribution.
Before it was repurposed as the K2 mission, NASA’s Kepler spacecraft, launched in 2009, spent about four years staring at a patch of roughly 150,000 stars and looking for dips in the starlight that would signal a passing planet. These signals, however, can be mimicked by other interstellar phenomena, such as an eclipsing binary — a pair of stars circling each other at such an angle that, from our viewpoint, they regularly block each other’s light. So scientists typically need to use another telescope to try to verify that a detected dip in the light really does signal a planet, and not a false positive.
Kepler has discovered a number of gas giants sitting relatively close to their stars. They’re large and circle their stars frequently and so their periodic dips in light are easier to pick up. And for a good while, many researchers thought the false-positive rate in the Kepler data was fairly low, said lead author Alexandre Santerne, an astronomer at the Institute of Astrophysics and Space Sciences in Portugal.
But Santerne said he suspected otherwise, because of his experience with data from the French-led planet-hunting COROT mission, launched in 2006.
“When I looked at the Kepler data when they released the candidates, it was clear to me that it was not five or 10 percent of prospective [planets], it was much higher,” he said.
To get a handle on the false-positive rate among the in-close gas giants in the Kepler survey, the scientists used the SOPHIE spectrograph on a telescope at the Observatory of Haute-Provence in France to check 129 potential in-close gas giants over a period of five years. And they turned up a false-positive rate of 54.6%; that is, more than half of those prospective planets were not.
Does this mean that there are fewer planets out there than we previously thought? Not really, the scientists said.
David Ciardi, a Caltech astrophysicist who was not involved in the paper, pointed out that gas giants traveling alone around their stars are easily confused with eclipsing binaries. But rocky planets, which seem to arise in multipacks, don’t exhibit the same light patterns.
“It’s very difficult to mimic multiple planets with a set of eclipsing binaries,” Ciardi said, adding jokingly: “That would be highly evil of the universe to do that to us.”
So far, there appear to be far more small rocky planets around stars than there are in-close gas giants, and so a higher false-positive rate in this relatively rarer group of planets wouldn’t really affect the total, he added.
Besides, Kepler could only see planets that were quite close to their stars, hardly farther away than Earth is from the sun. If aliens on a planet around another star used a Kepler-like telescope to observe our solar system, they wouldn’t be able to pick up Jupiter, Saturn, Neptune or Uranus (not to mention demoted-but-still-fascinating Pluto).
“This paper in particular is just one step in a much bigger building process that has to take place — to understand [these planets’] evolution, to understand their frequency, to understand their distributions; it’s one piece in all of that,” Ciardi said.
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