Study Says 40 Billion Planets In Our Galaxy Could Support Life

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A new study in the Proceedings of the National Academy of Sciences finds that roughly 1 in 5 stars, like our own sun, have an Earth-like planet orbiting around it. That's about 40 billion planets that could support life in the Milky Way galaxy. Melissa Block talks to co-author Geoff Marcy, an astronomy professor at the University of California-Berkeley, about the latest numbers.


As China and India race to Mars, we'll venture outside our solar system and consider this mind-expanding possibility: There could be 40 billion planets in our Milky Way galaxy that are orbiting stars in a habitable zone that could support life - 40 billion. Makes you kind of puny, doesn't it?

That number of potentially life-supporting planets comes from a new study of data sent back from NASA's Kepler space telescope, data that make the co-author of the new study, Geoff Marcy, feel a little tingly, as he puts it. Marcy is an astronomer at the University of California, Berkeley, and he joins me now. Geoff Marcy, just a little bit tingly, not a lot?


GEOFFREY MARCY: Well, the hairs are standing up on the back of my neck right now. It's really an extraordinary moment for all of us on planet Earth here.

BLOCK: Well, when you're looking for habitable - potentially habitable - planets, what are you looking for? What in particular are you trying to find in the data from the telescope?

MARCY: Well, there are a few properties of a planet that render it suitable for life. One is it ought to have a rocky surface so that the water, if any, on that planet, would puddle into ponds and lakes and oceans. Water, of course, being the important cocktail mixer for organic molecules, allowing them to recombine into amino acids and proteins of which our body is made.

Another key attribute of a habitable world would be its temperature. It can't be so cold that the water is frozen into ice nor so hot that the water would be in the form of steam. But instead we want, of course, liquid water so that the water can do its job as the cocktail mixer of life.

BLOCK: Now in terms of the planets that you think you have identified - maybe 40 billion if you extrapolate out from the data you have - is there any way of knowing how many would be the small, Earth-sized, rocky planet?

It's - we really have no idea. If I had to guess, I'd say something like a fifth of them, maybe a tenth.

I mean, that's still - we're still talking four billion planets.

MARCY: There's plenty to go around.

BLOCK: And the nearest of these potentially habitable planets, how close is the closest one to us?

MARCY: The closest habitable planet might be about 12 light years away. That's a stone's throw in the cosmic sense.

BLOCK: In the cosmic sense. Well, put it in some context in terms of our own solar system.

MARCY: You know, the Voyager spacecraft that we sent out in the 1980s has just now left our solar system. And at the speed it travels, to go to the nearest star, Alpha Centauri, would take something like 700,000 years. So that's really too long to pop over and borrow a cup of sugar.

BLOCK: Is there any danger, do you think, of overselling numbers like this?

MARCY: There is. We have actually no evidence at all of any life, even simple single-celled life. So the next generation of scientists has their work cut out for them to determine if life is actually out there, how common it is and especially whether there's any kindred spirits technologically out there.

BLOCK: Geoffrey Marcy is a professor of astronomy at the University of California, Berkeley. We were talking about the article he co-authored titled, "Prevalence of Earth-sized planets orbiting Sun-like stars." Professor Marcy, thanks so much.

MARCY: My pleasure.

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