Data from NASA's Kepler space telescope shows two exoplanets that orbit closer to each other than any other planets yet discovered. The image is an artist's conception of what one planet might look like from the other planet. Both planets are too close to their star to contain liquid water.
The inner planet, Kepler-36b, orbits its host star every 13.8 days, and the outer planet, Kepler-36c, every 16.2 days. On their closest approach, the neighboring duo comes within about 1.2 million miles (1.9 million kilometers) of each other. This is only five times the Earth-moon distance and about 20 times closer to each other than any two planets in our solar system.
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(Image credit: Harvard-Smithsonian Center for Astrophysics)