Planets were assembled out of small "building bodies" called planetesimals, which themselves aggregated from the dust in the solar nebula. Planets and satellites are "built-in", and the program comes with some 7500 asteroids and comets. Planets with the perfect blend of molecular constituents orbiting at just the right distance from the sun enjoy what astronomers call a "Goldilocks" orbit. Planets require accretion to form, specifically accretion in aproto planetary disk around a young star. Planets, as we noted, move relative to the stars, and they do so, generally, moving from west to east in the sky.

Mercury

Mercury and Venus are the only planets with no known moons. Mercury, setting, is lost in the trees and glow along the horizon, while Jupiter is off the top of this view. Mercury Mercury is the closest planet to the Sun. Mercury's Moon, 1974Two days before the March 29, 1974, Mariner 10 flyby of Mercury, one instrument began registering bright, extreme ultra violent (UV) emissions that had "no right to be there. Since Mercury has no protective atmosphere, the temperature on the surface ranges from extremely hot on the side facing the Sun to extremely cold on the side that faces away from the Sun. Even then, however, summer temperatures in a mid-latitude location like Erie, Pennsylvania, in the United States would peak around 140 F (60 C) under a Sun that looks twice as large, as Earth moves within even Mercury's orbital path. Second, five of the new planets orbit very near their stars--closer, in fact, than Mercury orbits the sun.

Outer

The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25. Between the inner and outer planets is an asteroid belt. The planets in our solar system are classified as inner planets and outer planets. On the other hand, atmospheric carbon dioxide condenses at the cold outer edge of the HZ which eliminates its greenhouse warming effect. The outer planets are large gaseous spheres with rings and include Jupiter, Saturn, Uranus, and Neptune. In the solar system's outer region, though, it was chilly enough for ices to remain intact.

Planets are found much more commonly around stars like our sun that are rich in iron and other metals, hinting that such elements played an important role in planet formation. Since all the planets except Pluto orbit the Sun within a few degrees of the plane of the Sun's equator, we know very little about the interplanetary environment outside that plane. While it is still not known precisely how the planets of our solar system were formed, the currently accepted nine planets that orbit around and are illuminated by the sun include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Mercury, Venus, Earth, and Mars are the planets closest to the Sun. Each of the planets in the solar system also has at least one additional celestial body, known as a moon that orbits around it while it orbits the sun.

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