Earth-like planets may be quite common, relatively close, study says


Is the Earth a “cosmic freak” or a planetary average Joe around our galactic neighborhood? UC Berkeley astronomer Geoff Marcy says we’re in good company.

With some clever sleight of hand, scientists using Kepler data have calculated that a whopping one in five Sun-like stars has an Earth-sized planet in the habitable zone – and, if they have the right chemical ingredients on board, could be capable of supporting life.

The kicker? The nearest one may lie just 12 light-years away.

The results, published Monday in the journal Proceedings of the National Academy of Sciences, also add 603 exoplanet candidates to Kepler’s tally, including 10 Earth-sized ones in the habitable zone.


“If we ever get star travel, we’d probably see a lot of traffic jams,” said William Borucki, Kepler’s lead scientist at NASA’s Ames Research Center in Mountain View, Calif.

Since its launch in 2009, the Kepler space telescope has uncovered a strange menagerie of alien worlds, some utterly unlike the inhabitants of our own solar system. Scientists have picked out what Borucki called a “bewildering variety” of planets, from mini-Neptunes to super-Earths. Some may be composed entirely of water; others have densities greater than iron or lower than Styrofoam. Some are smaller than Mercury and others are many times bigger than Jupiter.

For this study, the scientists had one driving question, said UC Berkeley astronomer and lead author Erik Petigura: Among all these different types of planets, how common were the ones that were sized like Earth?

The problem with answering that question is that it’s not easy to find such planets, Petigura said. A transit-watching telescope like Kepler waits for dips in brightness as a planet travels in front of its star and blocks a tiny fraction of its light. But that only works if we’re properly aligned and can catch them in the act of transiting.

Of those stars whose planetary transits we manage to see, many are simply too noisy, with their light patterns changing, to pick out a tiny dip in starlight from a transiting planet. (To put it in perspective, an Earth-sized planet would cause a 0.01% blip in its star’s light.)

The researchers decided to see if they could figure out how many planets they were missing. They injected synthetic planets into the starlight data, then ran the software that searched for those signals. Not all of them were found. Since they already knew how many fake planets they’d had to start with, the scientists now knew about how many planets were missing.


Based on that analysis, the researchers surmised that about 22% of stars like our sun have planets in the habitable zone that are just one to two Earth radii.

Of course, this doesn’t mean that these planets will actually be habitable – that depends on whether they’re made of rock and iron, have protective atmospheres and ingredients like water and other chemical building blocks for life as we know it.

But the researchers said they were encouraged by the discovery of Kepler-78b – an Earth-sized planet with an Earth-like density that also appears to be made of iron and rock. Though it was far too close to its star to be habitable, scientists said it was a sign that more promising discoveries would be on the way, as astronomers analyze the fourth and final year of data from Kepler, which suffered a malfunction this spring.


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