A limestone slab engraved with an image of an aurochs, or extinct wild cow, discovered at Abri Blanchard in 2012. Ph. Jugie/Musée National de Préhistoire hide caption

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Ph. Jugie/Musée National de Préhistoire

NASA's Voyager 2 spacecraft gave humanity its first glimpse of Neptune and its moon Triton in the summer of 1989. This picture of Neptune was produced from the last whole planet images taken through the green and orange filters on the Voyager 2 narrow angle camera on Aug. 20, 1989. NASA hide caption

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NASA

A NASA poster imagines what it might be like to "planet hop" from the planet called TRAPPIST-1e, 40 light-years from Earth. NASA-JPL/Caltech hide caption

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NASA-JPL/Caltech