Astronomers have discovered an exoplanet smaller than Earth

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Worlds without end: Scientists have measured the size and mass of the smallest exoplanet yet, a Mars-sized planet orbiting a star about 200 light years away.

The planet, named Kepler-138 b, is the first exoplanet smaller than the Earth to have both its mass and its size measured. It is one of three planets that orbit the star Kepler-138 and that pass in front of it on every orbit as viewed from Earth — a maneuver that astronomers call a transit. “Each time a planet transits the star, it blocks a small fraction of the star’s light, allowing us to measure the size of the planet,” said Dr. Daniel Jontof-Hutter, a research associate in astronomy at Penn State who led the study.

“We also measured the gravity of all three planets, using data from NASA’s Kepler mission, by precisely observing the times of each transit,” Jontof-Hutter said. The astronomers also were able to measure the masses of these planets. “Each planet periodically slows down and accelerates ever so slightly from the gravity of its neighboring planets. This slight change in time between transits allowed us to measure the masses of the planets,” Jontof-Hutter explained. After measuring both the mass and size of an exoplanet, astronomers then can calculate its density and its bulk composition.

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