Kepler-36b and Kepler-36c are as close as two planets get. The duo orbit just 1.9 million kilometers apart around a star named Kepler-36a, 1200 light-years from Earth. That's the smallest distance ever observed for two worlds , 20 times closer than the two nearest planets in our solar system—Venus and Mercury—and a mere five times the length from Earth to the moon. Kepler-36b is a rocky world like ours, but about 4.5 times more massive. Its companion (shown in Kepler-36b's sky in this artist's conception) is a gas giant roughly the size of Neptune and composed mostly of hydrogen, helium, and water, astronomers report online today in Science. Planets of such different weights and made up of such different elements do not typically orbit so close to each other. One bit of good news: this odd couple is in no danger of colliding.
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