A deep blue arc peeks above the horizon of a lava-strewn alien world. As it slowly rises, it becomes a circle in the sky, looming three times as large as a full moon back on Earth. This is no moon, though. Despite its close proximity, it’s a planet.
Such a “planet-rise” is what you would see if you were standing on the rockier half of a planetary odd couple that has just been spotted orbiting a sun-like star. One is rocky and dense: a super Earth. Its companion is airy and majestic: a hot Neptune. These close-orbiting yet radically different worlds are the strangest planetary pair yet discovered.
About every three months, the sight from the smaller planet would be spectacular.
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. Learn more here, here, here or here.