Astronomy - The Planets

The Planets

The Solar System


Terrestrial Planets


Mercury  Venus  Earth  Mars

Gas Giants


Jupiter  Saturn

Ice Giants

Uranus  Neptune




Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth It is the smallest planet in the Solar System and the one closest to the Sun, with an orbital period of about 88 Earth days, which is much faster than any other planet in the Solar System.

Seen from the Earth, it appears to move around its orbit in about 116 days. It has no known natural satellites. It is named after the Roman deity Mercury, the messenger to the gods.

A year on Mercury is just 88 days long. One solar day (the time from noon to noon on the planet’s surface) on Mercury lasts the equivalent of 176 Earth days.

Mercury is nearly tidally locked to the Sun and over time this has slowed the rotation of the planet to almost match its orbit around the Sun. Mercury also has the highest orbital eccentricity of all the planets with its distance from the Sun ranging from 46 to 70 million km.

Partly because it has almost no atmosphere to retain heat, Mercury's surface temperature varies each day more than any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day in some equatorial regions.

The poles are constantly below 180 K (−93 °C; −136 °F). Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 of a degree)

Mercury consists of approximately 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm3, only slightly less than Earth's density of 5.515 g/cm3.

Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron Mercury's core has a higher iron content than that of any other major planet in the Solar System.

Early in the Solar System's history, Mercury may have been struck by a planetesimalof approximately 1/6 that mass and several thousand kilometre’s across.

The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.

Mercury may have a molten core. In recent years scientists from NASA have come to believe the solid iron core of Mercury could in fact be molten. Normally the core of smaller planets cools rapidly, but after extensive research, the results were not in line with those expected from a solid core.

Scientists now believe the core to contain a lighter element such as sulphur, which would lower the melting temperature of the core material. It is estimated Mercury’s core makes up 42% of its volume, while the Earth’s core makes up 17%.

Surrounding the core is a 500–700 km mantle consisting of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury's crust is estimated to be 100–300 km thick.

One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometersin length.

It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified.

Only two spacecraft have ever visited Mercury. Owing to its proximity to the Sun, Mercury is a difficult planet to visit. During 1974 and 1975 Mariner 10 flew by Mercury three times, during this time they mapped just under half of the planet’s surface.

On August 3rd 2004, the Messenger probe was launched from Cape Canaveral Air Force Station, this was the first spacecraft to visit since the mid 1970’s.


Venus is the second planet from the Sun and is the second brightest object in the night sky after the Moon.

Named after the Roman goddess of love and beauty, Venus is the second largest terrestrial planet and is sometimes referred to as the Earth’s sister planet due the their similar size and mass.

The surface of the planet is obscured by an opaque layer of clouds made up of sulphuric acid.

It takes 243 Earth days to rotate once on its axis. The planet’s orbit around the Sun takes 225 Earth days, compared to the Earth’s 365. A day on the surface of Venus (solar day) takes 117 Earth days.

Venus rotates in the opposite direction to most other planets. This means that Venus is rotating in the opposite direction to the Sun, this is also know as a retrograde rotation.

A possible reason might be a collision in the past with an asteroid or other object that caused the planet to alter its rotational path. It also differs from most other planets in our solar system by having no natural satellites.

Atmospheric pressure on Venus is 92 times greater than the Earth’s. While its size and mass are similar to Earth, small asteroids are crushed when entering its atmosphere, meaning no small craters lie on the surface of the planet.

The pressure felt by a human on the surface would be equivalent to that experienced deep beneath the sea on Earth.

The Earth and Venus are very similar in size with only a 638 km difference in diameter, Venus having 81.5% of the Earth’s mass. Both also have a central core, a molten mantle and a crust.

Venus is the hottest planet in our solar system. The average surface temperature is 462 °C, and because Venus does not tilt on its axis, there is no seasonal variation.

The dense atmosphere of around 96.5 percent carbon dioxide traps heat and causes a greenhouse effect.

It was only when radio mapping was developed in the 1960s that scientists were able to observe and measure the extreme temperatures and hostile environment.

It is thought Venus did once have oceans but these evaporated as the planets temperature increased.

A detailed study of Venus is currently underway. In 2006, the Venus Express space shuttle was sent into orbit around Venus by the European Space Agency, and is sending back information about the planet.

Originally planned to last five hundred Earth days, the mission has been extended several times. More than 1,000 volcanoes or volcanic centres larger than 20 km have been found on the surface of Venus.

The Russians sent the first mission to Venus. The Venera1 space probe was launched in 1961, but lost contact with base.

The USA also lost their first probe to Venus, Mariner 1, although Mariner 2 was able to take measurements of the planet in 1962.

The Soviet Union’s Venera3 was the first man-made craft to land on Venus in 1966.


Jupiter is the fourth brightest object in the solar system.

Only the Sun, Moon and Venus are brighter. It is one of five planets visible to the naked eye from Earth.

Jupiter is named after the king of the Roman gods. To the Greeks, it represented Zeus, the god of thunder.

The Mesopotamians saw Jupiter as the god Mardukand patron of the city of Babylon. Germanic tribes saw this planet as Donar, or Thor.

Jupiter has the shortest day of all the planets. It turns on its axis once every 9 hours and 55 minutes. Jupiter orbits the Sun once every 11.8 Earth years.

From our point of view on Earth, it appears to move slowly in the sky, taking months to move from one constellation to another.

It is the largest planet with a mass about three hundred times that of the Earth.

Jupiter’s interior is made of rock, metal, and hydrogen compounds.

There are layers of compressed hydrogen gas, liquid metallic hydrogen, and a core of ice, rock, and metals.

Jupiter has unique cloud features of colourful bands and swirling clouds. Which are the top of a dynamic gaseous atmosphere extending deep into the planet.

The atmosphere at the poles and the equator rotate at different rates giving rise to high winds. The largest feature is the Great Red Spot which is a huge storm rotating anti-clockwise. It is so large that three Earths could fit inside it, which has been observed for over three hundred years.

Jupiter’s interior is made of rock, metal, and hydrogen compounds. Below Jupiter’s massive atmosphere (which is made primarily of hydrogen), there are layers of compressed hydrogen gas, liquid metallic hydrogen, and a core of ice, rock, and metals.

Jupiter has a thin ring system.

Its rings are composed mainly of dust particles ejected from some of Jupiter’s smaller worlds during impacts from incoming comets and asteroids. The ring system begins some 92,000 kilometres above Jupiter’s cloud tops and stretches out to more than 225,000 km from the planet. They are between 2,000 to 12,500 kilometres thick.

Eight spacecraft have visited Jupiter.

Pioneer 10 and 11, Voyager 1 and 2, Galileo, Cassini, Ulysses, and New Horizons missions. The Juno mission is its way to Jupiter and will arrive in July 2016.


Saturn was known to the ancients, including the Babylonians and Far Eastern observers. It is named for the Roman god Saturnus, and was known to the Greeks as Cronus.

Saturn is the sixth planet from the Sun and the most distant that can be seen with the naked eye.

Saturn is the second largest planet and is best known for its fabulous ring system that was first observed in 1610 by the astronomer Galileo Galilei.

Like Jupiter, Saturn is a gas giant and is composed of similar gasses including hydrogen, helium and methane

Its polar diameter is 90% of its equatorial diameter, this is due to its low density and fast rotation.

Saturn turns on its axis once every 10 hours and 34 minutes giving it the second-shortest day of any of the solar system’s planets. Saturn orbits the Sun once every 29.4 Earth years.

.Saturn’s upper atmosphere is divided into bands of clouds. The top layers are mostly ammonia ice. Below them, the clouds are largely water ice.

Below that are layers of cold hydrogen and sulphur ice mixtures. Saturn has oval-shaped storms similar to Jupiter’s.

The region around its north pole has a hexagonal-shaped pattern of clouds. Scientists think this may be a wave pattern in the upper clouds. The planet also has a vortex over its south pole that resembles a hurricane-like storm.

Eventually, deep inside, the hydrogen becomes metallic. At the core lies a hot interior.

Saturn has the most extensive rings in the solar system. The rings are made mostly of chunks of ice and small amounts of dust. The rings stretch out more than 120,700 km from the planet, but are amazingly thin: only about 20 meters thick.

Four spacecraft have visited Saturn.

Pioneer 11, Voyager 1 and 2, and the Cassini-Huygens mission have all studied the planet.

Cassini continues to orbit Saturn, sending back a wealth of data about the planet, its moons, and rings.




Uranus was officially discovered by Sir William Herschel in 1781. At first Herschel thought it was a comet, but several years later it was confirmed as a planet. Herschel tried to have his discovery named “Georgian Sidus” after King George III.

The name Uranus was suggested by astronomer Johann Bode. The name comes from the ancient Greek deity Ouranos.

Uranus turns on its axis once every 17 hours, 14 minutes. The planet rotates in a retrograde direction, opposite to the way Earth and most other planets turn. Uranus makes one trip around the Sun every 84 Earth years.

Like the other gas giants, it has a hydrogen upper layer, which has helium mixed in. Below that is an icy “mantle, which surrounds a rock and ice core.

The upper atmosphere is made of water, ammonia and the methane ice crystals that give the planet its pale blue colour.

With minimum atmospheric temperature of -224°C Uranus is nearly coldest planet in the solar system.

The upper atmosphere of Uranus is covered by a methane haze which hides the storms that take place in the cloud decks.

Uranus has two sets of very thin dark coloured rings.

The ring particles are small, ranging from a dust-sized particles to small boulders. There are eleven inner rings and two outer rings. They probably formed when one or more of Uranus’s moons were broken up in an impact.

The first rings were discovered in 1977 with the two outer rings being discovered in Hubble Space Telescope images between 2003 and 2005.

Only one spacecraft has flown by Uranus.

In 1986, the Voyager 2 spacecraft swept past the planet at a distance of 81,500 km. It returned the first close-up images of the planet, its moons, and rings.


It is not visible to the naked eye and was first observed in 1846. Its position was determined using mathematical predictions.

It was named after the Roman god of the sea.

The atmosphere of Neptune is made of hydrogen and helium, with some methane. The methane absorbs red light, which makes the planet appear a lovely blue. High, thin clouds drift in the upper atmosphere.

Large storms whirl through its upper atmosphere, and high-speed winds track around the planet at up 600 meters per second. One of the largest storms ever seen was recorded in 1989. It was called the Great Dark Spot. It lasted about five years.

Despite being smaller than Uranus, Neptune has a greater mass.

Below its heavy atmosphere, Uranus is made of layers of hydrogen, helium, and methane gases. They enclose a layer of water, ammonia and methane ice. The inner core of the planet is made of rock.

Neptune has a very thin collection of rings.

They are likely made up of ice particles mixed with dust grains and possibly coated with a carbon-based substance.

Only one spacecraft has flown by Neptune.

In 1989, the Voyager 2 spacecraft swept past the planet. It returned the first close-up images of the Neptune system.

The NASA/ESA Hubble Space Telescope has also studied this planet, as have a number of ground-based telescopes.