MercuryMercury is the closest planet to the Sun and the smallest of the Inner Planets, probably because the heat of the nearby Sun as Mercury formed, about 4.6 billion years ago, prevented most of the gases present in the vicinity from becoming part of the Proto planet. When the planet is visible on Earth's horizon just after sunset or before dawn, it is obscured by the haze and dust in our atmosphere. Only radar telescopes gave any hint of Mercury's surface conditions prior to the voyage of Mariner 10. The photographs Mariner 10 radioed back to Earth revealed an ancient, heavily cratered surface, closely resembling our own Moon.

So far, observations of Mercury from ground-based observers and the Mariner 10 spacecraft have not shown any evidence of a significant atmosphere. Mariner 10 did observe a tiny amount of helium (exerting a pressure 1/1,000,000,000,000 of the Earth's atmosphere) 1000 km above the surface, but this is probably produced by capturing particles from the solar wind and radioactive decay of Mercury's crust. Temperatures at the surface range between -180°C and 450°C (-290°F to 840°F - hot enough for lead to melt). This large range in surface temperature is possible because Mercury is so close to the Sun, a day 176 Earth days long, and does not have an atmosphere present to moderate the range in surface temperature.

Venus and Earth are almost the same size. Venus is the closest planet to Earth, but it does not have oceans or human life like Earth. Venus gets so hot during the day that it could melt a lead cannonball. The temperature rises to 484 degrees Celsius on the side facing the Sun. Venus has very thick, rapidly spinning clouds which cover its surface. These clouds hold heat in. That is why Venus gets so hot. These clouds also reflect sunlight. That is why Venus appears so bright to us here on Earth. There are constant thunderstorms in these clouds. The surface of Venus has many craters which were made by meteorites and asteroids crashing into the planet. Venus also has volcanoes. This planet is unusual because it rotates in a direction opposite that of all of the other planets. Venus spins very slowly as it orbits the Sun.

Venus is the second closest planet to the Sun and our nearest planetary neighbor. With a diameter of 12104 kilometers it is the closest in size to the Earth. Its orbit about the Sun takes 224.7 days with its distance from the Sun being almost three-quarters that of the Earth. The planet rotates retrogradely in 243 days with respect to the stars (117 days with respect to the Sun, the Venusian day). From the Earth the planet's surface is never seen as it is always covered by very dense layers of clouds. The upper clouds rotate with a period of four days at speeds of 350 km/hr (215 mph). Because the size and mass of Venus are close to those of the Earth it was supposed by many that Venus might be Earth like and might even have life-forms on its surface. The truth is that Venus is very different from the Earth and it is extremely unlikely that there is any possibility that life has formed on Venus.

Mars Facts (The God of War)Martians have four seasons like Earthlings, but have two satellites, Phobos and Deimos. Mars has a revolution period of 687 Earth days and one year is equivalent to 23 Earth months. Mars is about half the size of Earth, and temperatures range between 26 degrees Celsius in day to -111 degrees Celsius at night. Galileo first discovered Mars in 1610.

Physical Data

Equatorial Diameter 6794 kmEllipticity 0.0059

Mass 6.4191 x1023 kgVolume (Earth=1) 0.15

Density(Water=1) 3.93Equatorial Rotation 24.6229 hr.

Axial Inclination 23.98 degreesAlbedo 0.16

Surface Gravity (Earth=1) 0.379Escape Velocity 5.02 km s-1

Jupiter is the fifth planet from the Sun and the largest in the solar system. It is more than 10 times the diameter of Earth and more than 300 times its mass. In fact, the mass of Jupiter is almost 2.5 times that of all the other planets combined. Being composed largely of the light elements hydrogen and helium, its mean density is only 1.314 times that of water. The mean density of Earth is 5.245 times that of water. The pull of gravity on Jupiter at the top of the clouds at the equator is 2.4 times as great as gravity's pull at the surface of Earth at the equator. Jupiter rotates at a dizzying pace -- once every 9 hours 55 minutes 30 seconds, although the period determined by watching cloud features differs by up to five minutes due to intrinsic cloud motions. The massive planet takes almost 12 Earth years to complete a journey around the Sun. From the Earth, Jupiter can be seen to show a disc with polar flattening, this is due to a very rapid rotation.

Jupiter has a Great Red Spot (GRS), an enormous anti-cyclonic system which has lasted for hundreds of years. This hurricane-like storm in Jupiter's atmosphere is more than twice the size of the Earth. As a high-pressure region, the Great Red Spot spins in a direction opposite to that of low-pressure storms on Jupiter; it is surrounded by swirling currents that rotate around the spot and are sometimes consumed by it. Across the disc several bands of dark and light clouds can be seen and the GRS is visible during each rotation. Pictures returned by the Voyager probes have shown the complexity of the structures within these bands. It is thought that the brighter zones are cloud-covered regions of upward moving atmosphere, while the darker belts are the regions of descending gases. An elongated yellow cloud within the GRS is swirling around the spot's interior boundary in a counter-clockwise direction with a period of a little less than six days, confirming the whirlpool-like circulation that astronomers have suspected from ground-based photographs.

Saturn is the sixth planet from the Sun and the second largest: orbit: 1,429,400,000 km (9.54 AU) from Sundiameter: 120,536 km (equatorial)

Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically.

It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus and shortly thereafter around Jupiter and Neptune).

Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini, now on its way, will arrive in 2004.

Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune. orbit: 2,870,990,000 km (19.218 AU) from Sundiameter: 51,118 km (equatorial)

Uranus, the first planet discovered in modern times, was discovered by William Herschel while systematicly searching the sky with his telescope on March 13, 1781. It had actually been seen many times before but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). The name "Uranus" was first proposed by Bode in conformity with the other planetary names from classical mythology but didn't come into common use until 1850.

Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.

Neptune

Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus. orbit: 4,504,000,000 km (30.06 AU) from Sundiameter: 49,532 km (equatorial)

After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d’ Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery.

Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not

have been found anywhere near the predicted location.

Pluto is the farthest planet from the Sun (usually) and by far the smallest. Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)diameter: 2274 km

In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell.

Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.

After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no tenth planet.

Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of

light and dark areas on both bodies.