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During its four-year mission, NASA's Kepler space telescope discovered thousands of "planetary candidates" in our Milky Way galaxy -- the vast majority of which are almost certainly actual planets. But before scientists can deduce which planets might support life, a fundamental question must be answered: what are the planets made of?

To know this, each planet's mass first must be determined. Northwestern University's Yoram Lithwick now reports having measured the masses of approximately 60 exoplanets larger than Earth and smaller than Neptune. (An exoplanet, or extrasolar planet, is a planet outside our solar system.)

The measurements greatly expand scientists' knowledge of sub-Neptune exoplanets because once the mass and size of a planet are known, the density can be determined and the composition of the planet inferred.

"We were surprised to learn that planets only a few times bigger than Earth are covered by a lot of gas," said Lithwick, an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences.

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