A few months ago, the Kepler space telescope discovered something new: A pair of planets, orbiting a pair of stars. Somebody standing on these worlds would see two suns in the sky, and could enjoy the science-fiction view of a double sunrise, and double sunset. The system, known as Kepler-47, is located in the constellation Cygnus (the Swan), and is about 5000 light years away from Earth. Astronomers using data from the Kepler telescope detected the planets using the transit method, in which the brightness of a star is measured with extreme accuracy for a long time. If there are any planets, and if they are orbiting the star in roughly the same plane as our view of the system, then they will pass between us and the star once each orbit. When this happens, the apparent brightness of the star will dip briefly. By measuring how long the light takes to dim, and how much time passes between transits, astronomers can calculate the size of the planet and how far it is from the star. Planets are quite tiny compared to stars, so this dimming is so slight that the Kepler telescope is one of the few instruments sensitive enough to measure it.
In the case of Kepler-47, the two planets orbiting the double star created a complex pattern of dimming cycles and it took extended observations before they could be interpreted. It seems to have been worth the wait, though, as it proves that double stars can and do form planetary systems – there was previously doubt that this could be possible, as the two stars might disrupt each other’s protoplanetary disks.