美国国家公共电台 NPR Get Ready For the Next Big Thing In NASA's Search For Earth's Twin（在线收听）

LULU GARCIA-NAVARRO, HOST:

Tomorrow, a rocket is scheduled to blast off with a NASA satellite. It's a planet hunter that will spend two years looking for alien worlds. NPR's Nell Greenfieldboyce reports on how it will help scientists in their quest to find another Earth.

NELL GREENFIELDBOYCE, BYLINE: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus. For a long time, those were the only planets scientists knew about. In recent years, beyond our solar system, they've discovered over 3,000 more, thanks in large part to a NASA mission called Kepler.

GEORGE RICKER: The most exciting things we've learned from Kepler has been that planets are extremely common. There are far more planets in the Milky Way than there are stars.

GREENFIELDBOYCE: That's George Ricker, an astronomer at MIT. He says Kepler was really good at finding planets but not ones that scientists could study in detail.

RICKER: There's really not much more we can say other than that they exist.

GREENFIELDBOYCE: And that is very unsatisfying because after all, what scientists really want to do is find another planet like our own, one that some form of life could call home. I asked Sara Seager, also at MIT, how close we are to finding Earth's twin.

SARA SEAGER: Far. We're very far from knowing that something is like Earth out there.

GREENFIELDBOYCE: That's why she and Ricker are part of a team that's working on NASA's next mission called TESS, for Transiting Exoplanet Survey Satellite. TESS will scan almost the entire sky, looking for telltale signs of planets around closer, brighter stars. This will make it easier for scientists to do follow-up measurements.

SEAGER: And so with the planets that TESS finds, we're going to be able to use a different set of telescopes and then try to find out if any of them are indeed somewhat like Earth.

GREENFIELDBOYCE: She says the goal is to find small, probably rocky planets orbiting in a sweet spot around the star that's not too hot and not too cold. Those planets could be studied using a big, new space telescope that NASA is supposed to launch in a couple years. It could analyze the planet's atmospheres.

SEAGER: The most important thing I think would be to find signs of water vapor in a small planet atmosphere because all life as we know it needs liquid water.

GREENFIELDBOYCE: Astronomers are excited by the idea that tests will give them a massive new catalog of relatively easy-to-study planets beyond our solar system. The follow-up work is sure to go on for years. Ruth Angus is an astronomer at Columbia University.

RUTH ANGUS: If we detect life signatures on any extrasolar planet in the next ten years, then that planet will almost certainly have been discovered by TESS.

GREENFIELDBOYCE: But she says that's a big if because there's reason to think the planets found by TESS might not be super-cozy little places. The mission will mainly be targeting planets orbiting small common stars called red dwarfs. And red dwarfs are known to send out scorching blasts of ultraviolet and X-ray radiation that could fry any life. Still, Angus says she's skeptically hopeful.

ANGUS: There are certain conditions that we need for the life that we're familiar with. But who knows if those conditions apply across the galaxy?

GREENFIELDBOYCE: She says they've got to start looking somewhere, and TESS will tell them where. Nell Greenfieldboyce, NPR News.