Oddly Close Alien Planets Witness Huge 'Planetrise'

Two far-flung planets are orbiting the same star so closely that they can appear to each other as giant, colorful moons, Space.com reports. Spotted by the Kepler Space Telescope, a craggy "super-Earth" and a gaseous, Neptune-size world are 1,200 light-years away, but just 1.2 million miles apart, closer than any other known planets. Their orbits make them dramatically visible to each other every 97 days.

"This is unprecedented," says Eric Agol, co-author of a new study appearing in Science. "They are as different in density as Earth and Saturn ... yet they are 30 times closer than any pair of planets in our solar system." In fact, their proximity may force scientists to adjust theories on how planets form and migrate. The planets may also inspire renderings, for the Neptune-size planet appears in the super-Earth's sky 2.5 times bigger than our moon, and "more purple than Neptune," as one co-author put it. (More planets stories.)