Solar System

The SUN and the family of PLANETS, natural SATELLITES, ASTEROIDS, METEORS, and COMETS that are its captives. The principal members of the sun's retinue are the nine major planets; in order of increasing distance from the sun, they are MERCURY, VENUS, EARTH, MARS, JUPITER, SATURN, URANUS, NEPTUNE, and PLUTO. All the planets orbit the sun in approximately the same plane (that of the ECLIPTIC) and move in the same direction (from west to east). Current theories suggest that the solar system was formed from a NEBULA consisting of a dense nucleus, or protosun, surrounded by a thin shell of a gaseous matter extending to the present edges of the solar sytem. Because of gravitational instabilities, the nebula eventually broke up into whirlpools of gas, called protoplanets, within the rotating mass. In time the protoplanets condensed and accreted to form the planets.

Sun

sun
intensely hot, self-luminous body of gases (mainly hydrogen and helium) at the center of the SOLAR SYSTEM. The sun is a medium-size main-sequence STAR. Its mean distance from the earth is defined as one ASTRONOMICAL UNIT. The sun is c.865,400 mi (1,392,000 km) in diameter; its volume is about 1,300,000 times, and its mass 332,000 times, that of the earth. At its center, the sun has a density over 100 times that of water, a pressure of over 1 billion atmospheres, and a temperature of about 15,000,000 degrees Kelvin

Jupiter

Jupiter
in astronomy, 5th PLANET from the sun, at a mean distance of 483.6 million mi (778.3 million km), and largest planet in the solar system, with an equatorial diameter of 88,700 mi (142,800 km). It is a largely liquid planet with an atmosphere composed mostly of hydrogen and helium, with traces of methane, ammonia, and other gases, and about five or six zones each of counterflowing eastward- and westward-flowing winds.