Planet smallest known planet is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) slightly higher than that of the Moon. Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded
Planet Nine distant orbits around another star, or rogue planet, leaving one in a stable orbit around the Sun via a process similar to the capture of irregular satellites around the giant planets. If the planet originated in a system with a number of Neptune-massed planets, and without Jupiter-massed planets, it could be scattered into a more long-lasting distant eccentric orbit, increasing its chances of capture. Although the odds of the Sun capturing another planet from another star can be higher, a wider variety of orbits are possible, reducing the probability of a planet being captured on an orbit like that proposed