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Astronomers have discovered an extrasolar planet with the same mass as Earth, but the resemblance ends there. Not only is the planet too warm for liquid water to exist on its surface, but it also has a radius 60% larger than Earth, suggesting a vast, puffy atmosphere of hydrogen and helium.

“You’ve got a very small planet that is probably not rocky at all, and that’s frightening,” says Jacob Bean, an astrophysicist at the University of Chicago in Illinois.

What is scary, he says, is how the finding challenges the assumption that an Earth-mass exoplanet would have an Earth-like composition. With its thick atmosphere, the exoplanet is more like a scaled-down Neptune or Uranus, he notes. Its surprising density suggests that it will be even more important in future campaigns to measure both the size and the mass of exoplanets. Density, the ratio of mass and size, is known for only about 150 of the more than 3,000 known exoplanets.

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