This 6,000-planet milestone comes 30 years after the first exoplanet was discovered orbiting a star similar to our sun. Researchers estimate that rocky planets appear to be most common in the universe. They’ve also discovered planets the size of Jupiter that orbit closer to their star than Mercury does to our sun, or planets that have two stars. Some discovered worlds are covered in lava; others are surrounded by clouds of precious stones. This is because light from faraway worlds is often so faint that it blends into the light emitted by the parent star.