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Across the Milky Way, more planet Earths?

Two new discoveries suggest that sunlike solar systems like ours – with life-hosting planets – are more common than previously thought.

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Not alone: An artist's illustration of the idea that rocky, terrestrial worlds like the inner planets in our own solar system may be plentiful, and diverse, in the universe.

NASA

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Fresh discoveries of new worlds around other stars – and the prospect of finding more in our own solar system – are giving the Milky Way an increasingly life-friendly look.

In the process, these and other finds are upending long-held notions of what a solar system should look like and where a livable planet is most likely to be found. Even the definition of a planet is up for grabs, despite a recent attempt among the world's astronomers to settle the issue.

"In our own solar system, there's a pretty clear distinction based on size," says David Morrison, the senior scientist at the National Aeronautics and Space Administration's Astrobiology Institute in Mountain View, Calif. "But as we look at other solar systems, there's no reason to think that these distinctions will survive. We'll probably see all kinds of planets in all kinds of configurations. That's what makes this so exciting."

Two sets of discoveries announced in recent days suggest that the Milky Way hosts many more sunlike solar systems than previously believed.

One team found a solar system that resembles our own, although it's a scaled-down version. The astronomers detected two planets orbiting a star nearly 5,000 light-years away. The star is only half as massive as the sun and the two planets are smaller than Jupiter and Saturn. But after adjusting for these differences, the astronomers found that the two planets orbit their star at roughly the same relative distance as Jupiter and Saturn orbit our sun. Their relative masses are similar. And the estimated temperatures of the new planets are similar to Jupiter's and Saturn's.

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