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Planets and their    moons
Having formed in a single rotating disk, all of the planets orbit the    Sun in the same direction (anticlockwise when viewed from above) and pretty    much the same plane. Most, including most of any moons they may have, all rotate the same way too (also anticlockwise when viewed from above). Exceptions are    generally thought due to the high number of collisions which took place between    the planets and their moons and other objects during the Solar Systems early    history. The Sun is the only object in the Solar System to produce its own light.    The planets and their moons are seen because they reflect the Suns light. Distances    within the Solar System are measured in Astronomical Units, the mean    distance between the Sun and the Earth (1AU = 150 million kilometres). Our knowledge    and understanding of the planets and their moons continues to increase dramatically    as Earth and space-based observations and remote probes provide us with more    and more information.
Mercury
Mercury is the nearest planet to the Sun. At a mean distance of          58 million miles (0.4AU), and travelling at about 48km/s, it takes          Mercury about 88 days to orbit the Sun once. Mercury rotates          on its axis once every 59 days. With a diameter of about          4,878km (no polar flattening), Mercury is 0.4 times the size          of the Earth. 60% of Mercurys surface is heavily cratered,          the remaining 40% consists of relatively flat lava plains.          The largest surface feature observed on Mercury is known          as the Caloris Basin, a lava filled impact crater about 1,300km          across. Mercurys surface is also covered in wrinkles or          rupes, formed as the planet contracted as it cooled shortly          after formation. With daytime highs of 430C and nightime          lows of -170C, Mercury experiences the greatest temperature          range of all the planets (though Venus is the hottest).          Mercury possesses a very thin and temporary atmosphere          of helium, sodium and oxygen with traces of argon, potassium          and hydrogen. Mercury has no moons.
 
Venus
Venus is the second planet from the Sun. At a mean distance of          108 million kilometres (0.7AU), and travelling at about 35km/s,          it takes Venus about 224.7 days to orbit the Sun once. Venus          exhibits retrograde rotation (it rotates the opposite way to          other planets), spinning on its axis once every 243 days. With          a diameter of about 12,102km (no polar flattening), Venus is          about 0.9 times the size of the Earth. Venus possesses a dense,          cloudy atmosphere dominated by carbon dioxide with minor amounts          of nitrogen, water vapour, sulphur dioxide and sulphuric acid.          Atmospheric pressure is about 90 times that on Earth. The          surface of Venus is known to be cratered and mountainous          with evidence of much volcanic activity. With highs of about          480ï¿½C, Venus is the hottest planet in the Solar system (runaway          Greenhouse Effect). Venus has no moons.
 
Earth
The Earth is the third planet from the Sun. At a mean distance of 150 million kilometres (1.0AU), and travelling at about 30km/s, it takes the Earth about 365.25 days to orbit          the Sun once. The Earth rotates on its axis once every 23          hours 56 minutes. The Earth is not perfectly spherical.          Its equatorial diameter of 12,756km is slightly greater than          its diameter measured from pole to pole (0.3% polar flattening).          The Earth possesses a protective atmosphere dominated by nitrogen          and oxygen with traces of carbon dioxide, water vapour and          other gases. About 70% of the Earthï¿½s surface is covered by          water making it unique in the Solar System. Surface features          on its land masses are wide and varied. The Earths crust and          upper mantle are divided into about 12 mobile tectonic plates,          the edges of which are defined by earthquake and volcanic          activity. Surface temperatures range from about 60ï¿½C to -90ï¿½C.          The Earth has one moon, the Moon. Many astronomers          consider the Earth and the Moon to be a binary system. The          Earth is the only planet known to support life for sure.
 
Mars
Mars is the fourth planet from the Sun. At a mean distance of 228 million kilometres (1.5AU), and travelling at about 24km/s, it takes Mars about 687 days to orbit the Sun once. Mars rotates          on its axis once every 24 hours 37 minutes. With an equatorial          diameter of about 6,786km (0.5% polar flattening), Mars is          about 0.5 times the size of the Earth. Mars possesses a thin          atmosphere dominated by carbon dioxide with minor amounts          of nitrogen, argon and other gases. Surface features on Mars          are wide and varied and include mountains, deserts, canyons,          volcanoes, craters and polar caps of frozen carbon dioxide.          Many features were once produced by flowing water which has          since ï¿½disappearedï¿½. Two impressive features on Mars are Olympus          Mons, an extinct volcano about 600km across and 25km high          (the largest volcanic feature in the Solar System), and Valles          Marineris, a canyon system over 4,000km long and up to 7km          deep. Surface temperatures range from about 20C to -140C.          Mars has two irregularly shaped moons, Phobos (22km across)          and Deimos (13km across). The controversial discovery of microfossils          in a meteorite thought to have come from Mars has recently          rekindled the life on Mars debate.
 
Jupiter
Jupiter is the fifth planet from the Sun and the first of the          gas giants. At a mean distance of 778 million kilometres (5.2AU), and travelling at about 13km/s, it takes Jupiter about 11.86 years to orbit the Sun once. Jupiter rotates on its axis          once every 9 hours 48 minutes. With an equatorial diameter          of about 142,980km (6.7% polar flattening), Jupiter is about          11.2 times the size of the Earth and the largest planet in          the Solar System. As a gas giant, Jupiter has no real surface          as such. Its atmospheric cloud layers, dominated by hydrogen          and helium with traces of methane, ammonia and water vapour,          give Jupiter a banded and colourful yet turbulent appearance.          The temperature at Jupiterï¿½s cloud tops is about -110C.          Jupiters most prominent feature is its phosphorous-rich Great          Red Spot, a long lived, rotating, anticyclonic storm reaching          up to 40,000km across. Jupiter has a narrow and faint ring          system of very fine grained dust particles and 16 known moons,          the largest of which, the Galilean moons, are Io, Europa,          Ganymede and Calisto. Io is known to be volcanically active.          Europas icy surface layers are thought by some astronomers          to hide underlying oceans of water which might support          life
 
Saturn
Saturn is the sixth planet from the Sun and the second of the gas giants. At a mean distance of 1,427 million kilometres (9.5AU), and travelling at about 10km/s, it takes Saturn          about 29.46 years to orbit the Sun once. Saturn rotates on          its axis once every 10 hours 15 minutes. With an equatorial          diameter of about 120,540km (10.4% polar flattening), Saturn          is about 9.5 times the size of the Earth. Like Jupiter, Saturn          has no real surface. Its atmospheric cloud layers are          dominated by hydrogen and helium with traces of methane,          ammonia and water vapour. The cloud layers are thick and          show only faint banding. The temperature at Saturnï¿½s cloud          tops is about - 180C. Saturn is best known for its prominent          and spectacular ice and dust dominated ring system (gravitationally          trapped debris or a failed moon). Saturnï¿½s rings vary in thickness          up to only 200m but extend into space around the planet for          about 1 million kilometres (diameter). Saturn has at least          18 confirmed moons (possibly as many as 24), the largest of          which is Titan (about 5,000km across) with a thick, nitrogen          dominated atmosphere. Many moons are co-orbital.
 
Uranus
Discovered in 1781, Uranus is the seventh planet from the Sun and the third of the gas giants. At a mean distance of 2,871 million kilometres (19.2AU), and travelling at about 7km/s,          it takes Uranus about 84.01 years to orbit the Sun once. Uranus rotates on its ï¿½sideï¿½ once every 17 hours 14 minutes. With          an equatorial diameter of about 51,118km (2.3% polar flattening), Uranus is about 4.0 times the size of the Earth. Like the          other gas giants, Uranus has no real surface. Its almost featureless atmospheric cloud layers of hydrogen, helium and methane are bluish in colour. The temperature at Uranusï¿½s cloud tops is about -216C. Uranus possesses a faint ring system made out of the darkest material known in the Solar System and 15 known          moons, all named after Shakespearean characters. The most          unusual moon is Miranda with its patchwork surface features.
 
Neptune
Discovered in 1846, Neptune is normally the eighth planet from          the Sun and the last of the gas giants. At a mean distance of          4,497 million kilometres (30.0AU), and travelling at a little over          5km/s, it takes Neptune about 164.80 years to orbit the Sun          once. Neptune rotates on its axis once every 16 hours 6 minutes.          With an equatorial diameter of about 49,528km (1.8% polar          flattening), Neptune is about 3.9 times the size of the Earth.          Like Uranus, its twin, Neptune has no real surface. Its atmospheric          cloud layers, dominated by hydrogen, helium and methane, are          blue in colour and faintly banded. The temperature          at Neptunes cloud tops is about -216C. Neptunes one time          most prominent feature, the Great Dark Spot, a rotating, anticyclonic          storm over 20,000km across, no longer exists. Winds sweep          around Neptunes equator at up to 2,000km/h making it the windiest          planet in the Solar System. Neptune possesses a faint ring          system and 8 known moons, the largest of which is Triton (2,700km          across, retrograde orbit). Triton is consistently the coldest          place in the Solar System with estimated surface temperatures          as low as -235C
 
Pluto
Discovered as recently as 1930, Pluto is normally the        furthest and ninth planet from the Sun. At a mean distance of        5914 million miles (39.5AU), and travelling at almost 5km/s,        it takes Pluto about 248.5 years to orbit the Sun once. Plutoï¿½s        orbit around the Sun is so elliptical that for about 20 years        of its orbital period it is closer to the Sun than Neptune        (e.g. between 1979 and 1999). Pluto rotates on its axis once        every 6.4 days. With a diameter of only 2,300km (no polar flattening),        Pluto, at 0.2 times the size of the Earth, is the smallest planet        in the Solar System. Pluto may possess a very thin, temporary atmosphere of methane and nitrogen. Pluto probably has a surface temperature of about -220C. Pluto has one moon, Charon. Many astronomers consider Pluto together with its single moon to be a binary system.