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

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

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Ever since astronomers 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, BYLINE: 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 astronomer 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 exquisite 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 random stuff was happening. Maybe a star's brightness naturally varied or maybe the telescope got hit by a piece of dust. And sometimes this random stuff happened in just the right pattern to mimic an orbiting planet. So Mullally and some colleagues decided 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 gut 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 awesome other planets out there.

GREENFIELDBOYCE: Like Kepler-442b, another world that could be rocky and have temperatures cozy 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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