The most crowded alien planetary system found yet possesses five worlds all orbiting a star at least 12 times closer than Earth does the sun, researchers say. KOI-500 is a super-compact planetary system, the most tightly packed one seen yet, hosting at least five planets ranging from 1.3 to 2.6 times the size of Earth. These planets orbit so near KOI-500 that their “years,” or the time it takes to circle their star, are only 1.0, 3.1, 4.6, 7.1, and 9.5 days long. The planets are so close together that their mutual gravity slightly pushes and pulls on their orbit

Recent theories for the formation of the giant planets of our outer solar system involve planets moving during the formation process, as scientists suspect happened with KOI-500. As these giants shifted their orbits, researchers suggest their gravitational pulls hurled asteroids and comets toward the inner solar system, causing the so-called Late Heavy Bombardment about 4.1 billion to 3.8 billion years ago, which pummeled Earth, the moon and the inner planets with a barrage of countless impacts. As scientists have discovered more and more exoplanets, they have found that most observed worlds orbit much closer to their stars than any planet in our solar system orbits the sun, including so-called hot Jupiters, which are giant planets orbiting closer to their stars than Mercury does the sun. Scientists still don’t understand why most observed alien planetary systems look so unlike ours.