美国国家公共电台 NPR Earth's 'Bigger, Older Cousin' Maybe Doesn't Even Exist

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Earth's 'Bigger, Older Cousin' Maybe Doesn't Even Exist

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Ever since astronomers² started to detect planets beyond our solar system, they've been trying to find another world just like Earth. A few years ago, they announced that they'd found a planet that was the closest match yet. NPR's Nell Greenfieldboyce reports that scientists have now taken a new look at this discovery and say it may not be what it seemed.

NELL GREENFIELDBOYCE, BYLINE³: It was the summer of 2015, and NASA held a press conference. Its Kepler Space Telescope had detected a new planet named Kepler-452b, and it was big news.

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UNIDENTIFIED REPORTER: The space agency says it has found what it's calling Earth's cousin, the most similar planet to our own they've ever found.

GREENFIELDBOYCE: TV shows and newspapers ran artist depictions of the alien world, and NPR weighed in, of course.

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MELISSA BLOCK, BYLINE: It orbits in the so-called Goldilocks Zone where liquid water, and possibly life, could exist.

GREENFIELDBOYCE: This was the first near-Earth-sized planet orbiting in the habitable zone around a star very similar to our sun. What could be finer? Trouble is now some astronomers say it's not possible to know for sure that this planet actually exists.

FERGAL MULLALLY: There's new information that we can know quantify, which tells us something that we didn't know before.

GREENFIELDBOYCE: Fergal Mullally used to be an astronomer¹ on the Kepler team. He says the original science wasn't shoddy. It's just that researchers have learned more about the telescope's imperfections.

MULLALLY: And I kind of hate saying that because Kepler was an absolutely wonderful instrument. It was exquisite⁴ in the quality the data could detect. But nothing is perfect.

GREENFIELDBOYCE: Kepler stared at stars for years, looking for telltale dips in brightness that meant a planet was passing in front of a star and blocking some of its light. Not every drop in brightness came from a planet, though. Scientists already knew to look for false alarms caused by things like two stars going around each other. But increasingly, researchers have learned that other random⁵ stuff was happening. Maybe a star's brightness naturally varied⁶ or maybe the telescope got hit by a piece of dust. And sometimes this random stuff happened in just the right pattern to mimic⁷ an orbiting planet. So Mullally and some colleagues decided⁸ to go back and take another look at 452-b.

MULLALLY: We've told people there's a planet there. How confident are we actually that there's a planet there now that we're aware of this other noise source and we can sort of characterize it?

GREENFIELDBOYCE: The answer is they are not super confident. What are the chances that this planet is real?

MULLALLY: I would say it's higher than 50 percent and less than 90 percent. That's my gut⁹ feeling on it.

GREENFIELDBOYCE: For a bonafide, confirmed planet, astronomers like to see more like 99 percent certainty, so this is rather awkward. Chris Burke of MIT was on the research team.

CHRIS BURKE: You sort of don't want to be the person to deliver bad news, but sometimes the bad news is the truth of it (laughter).

GREENFIELDBOYCE: He says there's still hope. They're not saying a planet definitely isn't there, and there's other ways to look for it.

BURKE: One possibility that we're looking into is using bigger telescopes, so like the Hubble Space Telescope. Could it be used to independently verify Kepler 452-b?

GREENFIELDBOYCE: Even if it can never be verified, don't be too sad.

BURKE: You know, you can't hear about all of them in the popular press, but there is tons of awesome¹⁰ other planets out there.

GREENFIELDBOYCE: Like Kepler-442b, another world that could be rocky and have temperatures cozy¹¹ enough for life. It orbits a star a little cooler than our sun, so it's not exactly Earth-like, but Burke says that discovery is still reliably confirmed. Nell Greenfieldboyce, NPR News.

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