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THere are also asteroids,visiting our solor system once in a while on their gigantic orbital paths.
Mercury
Mercury, in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days. Mercury passes through phases similar to those of the moon as it completes each revolution about the sun, although the visible disk varies in size with respect to its distance from the earth. Because its greatest elongation is 28, it is seen only for a short time after sunset or before sunrise. Since observation of Mercury is particularly unfavorable when it is near the horizon, the planet has often been studied in full daylight, with the sun's light blocked off. Mercury has the second most elliptic orbit in the solar system, only Pluto's being greater among the planets. Its great eccentricity of orbit and its great orbital speed provided one of the important tests of Einstein's general theory of relativity. Mercury's perihelion (its closest point to the sun) is observed to advance by 43" each century more than can be explained from planetary perturbations using Newton's theory of gravitation, yet in nearly exact agreement with the prediction of the general theory. Mercury is the second smallest planet in the solar system, having a diameter of about 3,000 mi (4,800 km). Its mean density is comparable to that of the earth. Its small mass and proximity to the sun prevent it from having an appreciable atmosphere, although a slight amount of carbon dioxide has been detected. The surface of Mercury is much like that of the moon, as was shown during the Mariner 10 spacecraft flyby in 1974. Most of its craters were formed during a period of heavy bombardment by small asteroids early in the solar system's history. It was long thought that Mercury's period of rotation on its axis was identical to its period of revolution, so that the same side of the planet always faced the sun. However, radar studies in 1965 showed a period of rotation of 58.6 days. This results in periods of daylight and night of 90 earth days each, with the daylight temperatures reaching as high as 800 F (450 C). Night temperatures are believed to drop as low as -300 F (-184 C).
Venus
Venus, in astronomy, 2d planet from the sun; it is often called the evening star or morning star and is brighter than any object in the sky except the sun and the moon. Because its orbit lies between the sun and the orbit of the earth, Venus passes through phases like those of the moon, varying from a large bright crescent when the planet is near inferior conjunction (nearest the earth) to a smaller silvery disk when it is at superior conjunction (farthest from the earth). Since its greatest elongation (the angle made between the sun, the earth, and Venus) is 47°, it can never be seen much longer than 3 hr after sunset or 3 hr before sunrise. Venus revolves around the sun at a mean distance of c.67 million mi (107 million km) in a nearly circular orbit, and its period of revolution is about 225 days. It comes closer to the earth than any other planet, being c.26 million mi (42 million km) away at inferior conjunction. Venus is often referred to as the sister planet of the earth, because it is only slightly smaller in both size and mass. Several important differences, however, exist between the two planets. Although Venus is covered with a thick blanket of clouds that hides its surface from view, much has been learned of the conditions on Venus from U.S. and Soviet space probes. These probes indicate a surface temperature of about 890 F (475 C) and an atmospheric pressure as great as 100 times that at the earth's surface. The thick atmosphere is composed mainly of carbon dioxide, with a slight amount of water vapor and a trace of nitrogen and other elements. The high surface temperature is assumed to result partly from the greenhouse effect; radiation passing through the atmosphere heats the surface, but the heat is blocked by the enveloping carbon dioxide from escaping back out through the atmosphere. Studies also indicate that Venus rotates on its axis in a retrograde direction (opposite to the direction of revolution about the sun) with a period of about 243 days. Despite this slow rotation there is little observed temperature difference between the lighted and unlighted sides of the planet. The surface of Venus is thought to be erratic and stormy, although radio-wave data indicate the possible existence of two long mountain ranges. On May 15, 1991, the Magellan spacecraft completed the first of a series of radar maps of the Venusian surface, revealing details of a continentlike feature, called Aphrodite Terra, that crosses the planet's equator and is marked by geologic faults. A second such feature, Ishtar Terra, straddles the north polar region. Magellan also observed some craters, the results of impacts of asteroids and comets. Compared to the number of craters on other bodies of the inner solar system, this suggests that the surface of Venus is only about 800 million years old. No strong magnetic field comparable to that of the earth has been detected.
Earth
earth, in geology and astronomy, fifth largest planet of the solar system and the only planet definitely known to support life. Gravitational forces have molded the earth, like all celestial bodies, into a spherical shape. However, the earth is not an exact sphere, being slightly flattened at the poles and bulging at the equator. The equatorial diameter is c.7,926 mi (12,760 km) and the polar diameter 7,900 mi (12,720 km); the circumference at the equator is c.24,830 mi (40,000 km). The surface of the earth is divided into dry land and oceans, the dry land occupying c.57.5 million sq mi (148.9 million sq km), and the oceans c.139.5 million sq mi (361.3 million sq km). The earth is surrounded by an envelope of gases called the atmosphere, of which the greater part is nitrogen and oxygen.
Mars
Mars, in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth. Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice as bright as Sirius, the brightest star. Mars has a diameter of 4,200 mi (6,800 km), just over half the diameter of the earth, and its mass is only 11% of the earth's mass. The planet has a very thin atmosphere consisting mainly of carbon dioxide, with some nitrogen and argon. Mars has an extreme day-to-night temperature range, resulting from its thin atmosphere, from about 80 F (27 C) at noon to about -100 F (-73 C) at midnight; however, the high daytime temperatures are confined to less than 3 ft (1 m) above the surface.
Jupiter
Jupiter , in astronomy, fifth planet from the sun and largest planet of the solar system. Jupiter's orbit lies beyond the asteroid belt at a mean distance of 483.6 million mi (778.3 million km) from the sun; its period of revolution is 11.86 years. In order from the sun it is the first of the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—very large, massive planets of relatively low density, having rapid rotation and a thick, opaque atmosphere. Jupiter has a diameter of 88,815 mi (142,984 km), more than 11 times that of the earth. Its mass is 318 times that of the earth and about 2 1/2 times the mass of all other planets combined.
saturn
Saturn, in astronomy, 6th planet from the sun. Saturn's orbit lies between those of Jupiter and Uranus; its mean distance from the sun is c.886 million mi (1.43 billion km), almost twice that of Jupiter, and its period of revolution is about 29 1/2 years. Saturn appears in the sky as a yellow, starlike object of the first magnitude. When viewed through a telescope, it is seen as a golden sphere, crossed by a series of lightly colored bands parallel to the equator.
Uranus
Uranus , in astronomy, 7th planet from the sun, at a mean distance of 1.78 billion mi (2.87 billion km), with an orbit lying between those of Saturn and Neptune; its period of revolution is slightly more than 84 years. The first planet discovered in modern times with the aid of a telescope, Uranus was detected in 1781 by Sir William Herschel, who originally thought it to be a comet. Because the calculated orbit of Uranus did not compare accurately with the observed orbit, astronomers concluded that a disturbing influence was present. A study of this irregularity led to the discovery of Neptune in 1846. Uranus has a diameter of c.31,760 mi (46,700 km), roughly 4 times that of the earth, and a mass of about 15 times that of the earth. Like the giant planets Jupiter and Saturn, Uranus has a thick atmosphere of hydrogen, helium, and methane; a relatively low density; and a rapid period of rotation of about 17.9 hr, which causes a polar flattening of over 6%. However, its axis of rotation is tilted 98 to the plane of its orbit. The Voyager 2 space probe found that Uranus has the most inclined magnetic field in the solar system, and some astronomers interpret this as evidence that the magnetic field is reversing its polarity. Viewed through a telescope, Uranus appears as a greenish disk, slightly elliptical because of its rapid rotation. Its temperature is estimated to be about -330 F (-200 C), and at this temperature ammonia, the main constituent of the visible cloud cover, would exist in the form of ice crystals. Uranus has twenty known natural satellites with diameters ranging in size from 986 mi (1,578 km) down to 14 mi (20 km).
Neptune
Neptune, in astronomy, 8th planet from the sun at a mean distance of about 2.8 billion mi (4.5 billion km) with an orbit lying between those of Uranus and Pluto; its period of revolution is about 165 years. (Pluto, the 9th planet, has such a highly elliptical orbit that from 1979 to 1999 it was closer to the sun than Neptune; it will remain farther from the sun for 220 years, when it will again pass inside Neptune's orbit.) Neptune was discovered as the result of observed irregularities in the motion of Uranus and was the first planet to be discovered on the basis of theoretical calculations. J. C. Adams of Britain and U. J. Leverrier of France independently predicted the position of Neptune, and it was discovered by J. C. Galle in 1846, the day after he received Leverrier's prediction. Neptune has an equatorial diameter of about 30,700 mi (49,400 km), nearly four times that of the earth, and a mass about 17 times the earth's mass. It is much like Uranus and the other giant planets, with a thick atmosphere of hydrogen, helium, methane, and ammonia, a relatively low density, and a rapid period of rotation.
Pluto
Pluto, in astronomy, the 9th and most distant known planet from the sun, with an elliptical orbit usually lying beyond that of Neptune. Pluto's mean distance from the sun is 3.67 billion mi (5.91 billion km), and its period of revolution is about 248 years. Since Pluto has the most elliptical and tilted orbit of any planet (eccentricity .250, inclination 17 ), at its closest point to the sun it passes inside the orbit of Neptune; between 1979 and 1999 it was closer to the sun than Neptune was. It will remain farther from the sun for 220 years, when it will again pass inside Neptune's orbit. Its surface consists largely of frozen nitrogen. It is thought to have a rocky, silicate core; its thin atmosphere probably contains nitrogen, carbon monoxide, and methane. Its surface temperature is estimated to be about -360 F (-218 C), a temperature at which most gases exist in the frozen state.
PLANET DISTANCE FROM SUN (A.U.) MASS (earth=1.0) Mercury 0.39 0.06 Venus 0.72 0.82 Earth 1 365 1 Mars 1.52 0.11 Jupiter 5.20 317.89 Saturn 9.54 95.15 Uranus 19.2 14.54 Neptune 30.06 17.23 Pluto 39.44 0.002?