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 480°C, 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 Earth’s 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 60°C to -90°C. 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 ‘disappeared’. 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 Jupiter’s 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 Saturn’s 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). Saturn’s 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 ‘side’ 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 Uranus’s 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. Pluto’s 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.

 

Contents

The Universe
Galaxies
The Milky Way Galaxy
Stars
Constellations, Surveying the Solar System
The Sun
Asteroids, comets and meterorites, Exploring the Earth-Sun-Moon System
Day and night
Seasonal change
The Moon's phases and eclipses
Further Reading
Self assessment(1)
Self assessment(2)
Self assessment(3)