Astronomers are identifying more and more exoplanets, planets that exist in other star systems besides ours. Of the hundreds of exoplanets we know of, the one that most resembles earth is called GJ 1214b. It can be studied because it is only 42 light years away.

Based on its radius and mass — about 2.7 and 6.6 times that of Earth’s — Charbonneau and the other astronomers have calculated GJ 1214b’s density. It appears to be composed of extraordinarily deep oceans, surrounding a rocky core.

The planet’s atmosphere and precise composition remain a mystery, but it’s likely composed of many of the ...

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