Geology of solar terrestrial planets Earth, and Mars) have substantial atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanoes. The term "inner planet" should not be confused with "inferior planet", which refers to any planet that is closer to the Sun than the observer's planet is, but usually refers to Mercury and Venus. The Solar System is believed to have formed according to the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace. This theory holds that 4.6 billion years ago the Solar System formed from the gravitational collapse of a giant molecular
Solar System Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, at least four of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium