Later tonight, if all goes well, NASA will launch a new mission to hunt for alien worlds. A rocket is waiting on a launch pad in Cape Canaveral, Florida, and its cargo is a giant telescope. From space, it will spend years searching part of our Milky Way galaxy for Earth-like planets - ones that might have liquid water and even life. NPR's Nell Greenfieldboyce reports.

NELL GREENFIELDBOYCE: As a kid, you probably memorized all the planets that orbit our sun. There's eight of them, if you don't count poor, little Pluto. But beyond our solar system, scientists have found more than 300 planets orbiting distant stars. Ed Weiler is head of science missions at NASA. He says because it's so hard to detect these far-off worlds, the planets we've found so far are mostly big ones - enormous balls of gas. They're not like our small, rocky home. Weiler says that's why NASA is spending $600 million on something called the Kepler mission.

Dr. ED WEILER (Head of Science Missions, NASA): If you ask me are there other Earths out there, I'd say absolutely. There has to be. We can't be so special. But if you ask me to prove it, I can't.

GREENFIELDBOYCE: He hopes Kepler will get that proof.

Dr. WEILER: This is a historical mission. It's not just a science mission. I maintain it really attacks some very basic human questions that have been part of our genetic code since that first man or woman looked up in the sky and asked the question: Are we alone?

GREENFIELDBOYCE: The Kepler telescope will spend over three years just staring at one patch of our galaxy. William Borucki is chief scientist for the mission. He says every half-hour, the telescope will measure the brightness of over 100,000 stars. Whenever an orbiting planet passes between its star and the telescope, the planet will block a tiny bit of the star's light.

Dr. WILLIAM BORUCKI (Chief Scientist, Kepler Mission, NASA): The bigger the planet, the more light it blocks. So we get the size of the planet from the size of the dimming.

GREENFIELDBOYCE: He says they'll also be able to figure out how close the planet is to its star. That will tell them the temperature on the planet's surface. What they want to find is planets in the so-called Goldilocks Zone, where it's not too hot and not too cold.

Dr. BORUCKI: We're looking for planets where the temperature is just about right for liquid water on a surface of the planet. And that's the area we think might be conducive to life.

GREENFIELDBOYCE: Borucki says it will take a few years, but Kepler could potentially find dozens of Earth-like planets.

Mr. BORUCKI: And if we find that many, it certainly will mean that life may well be common throughout our galaxy.

GREENFIELDBOYCE: But on the other hand, if they don't find any planets like Earth...

Mr. BORUCKI: It will mean that Earth must be very rare. We may be the only extant life in our universe. In fact, it'll mean there will be no "Star Trek."

GREENFIELDBOYCE: So far, the scientists haven't started placing bets on exactly how many alien Earths the telescope will find.

Dr. ALAN BOSS (Researcher, Carnegie Institution): We don't have a pool yet.

GREENFIELDBOYCE: Alan Boss is on the mission's science team. He's a researcher at the Carnegie Institution of Washington.

Dr. BOSS: We've all spent many years of our lives into this, especially the principal investigator; he's put over two decades of his life into this. And we're all betting that Kepler will find Earths, and so we all hope to win the same bet.

GREENFIELDBOYCE: He says Kepler won't be able to detect any signs of life, so it won't find E.T. But if it finds some places that E.T. might conceivably call home, NASA could then start building another telescope, one that could analyze the atmosphere of these planets, looking for things like oxygen and water - things that would mean the planet is not only habitable, but maybe even inhabited.

Nell Greenfieldboyce, NPR News.

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