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Planets of the Solar System
This page provides a brief description of each of the planets (and links to dwarf planets) of our solar system. You can also find out about the difference between planets, dwarf planets and small solar system bodies (SSSBs) here.
Solar System Map - showing size, mass and orbital period, and orbit scale of planets & dwarf planets Available as a poster here.
Mercury is the closest planet to the Sun. It orbits in a highly elliptical orbit ranging from 46 million km (29 million miles) from the Sun out to 70 million km (43.5 million miles).
It takes about 88 Earth days to orbit the Sun but rotates on its axis once every 59 Earth days. Because of the slow rotation, a single day on Mercury (mid day to mid day) takes 176 Earth days. This means that Mercury has the longest day of any planet in the solar system (and not Venus which is the answer given by many other websites). For a full explanation of this, visit our "Which planet has the longest day?" page.
Its axial tilt is remarkably small at 3/100ths of a degree. Much smaller than any other planet.
Mercury is quite small with a diameter of 4,878km, (2/5ths that of Earth) and only 5% of Earth’s mass. Its gravity on the surface is 1/3rd of Earth’s.
Mercury has almost no atmosphere and is blasted by the Sun during the day and exposed to cold space during the night. This means that it undergoes some of the widest temperature swings of any body in the Solar System with temperatures reaching +430 C and dipping down to -180 C.
It has a highly cratered rocky surface and is known to have an iron core. However its magnetic field is much weaker than the Earth’s (1% as strong). Initially RADAR waves reflected from the surface of Mercury indicated that water ice might be present at the poles. It has recently been confirmed by the Messenger Spacecraft that ice water does indeed exist in deep craters at the poles the interiors of which are permanently shrouded in shadow.
Because Mercury is so close to the Sun, it is only ever seen (with the naked eye) just before sunrise and just after sunset. At all other times it is masked by the brightness of the Sun.
Mercury and Man
The Greeks had two names for Mercury, "Apollo" when it appeared in the morning and "Hermes" when it appeared in the evening.
In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky.
Before 2011 it had only been visited by 1 spacecraft - the Mariner 10 spacecraft which performed 3 fly-pasts in 1974/75 mapping about 45% of its surface.
Mercury was recently being studied by the Messenger Spacecraft. Messenger entered Mercury's Orbit on 18th March 2011, the first man made object ever to do so. Messenger was impacted into the surface of Mercury on April 30, 2015, at a speed of more than 3.91 kilometres per second (8,750 miles per hour), marking the end of operations for the hugely successful Mercury orbiter.
Amongst other things, Messenger discovered that:
1. Mercury's weak magnetic field is not symmetrical which allows more solar radiation to hit the south pole than the north.
2. Water ice exists at the poles where it is hidden from the Sun in deep craters.
3. The iron core of Mercury is much larger than expected meaning it has a much thinner rocky crust.
4. There is a lot of sulphur on Mercury, 10 times as much as we see on Earth or Mars, giving rise to suggestions of past volcanic activity.
5. Tectonic features on the surface suggest the core has shrunk as it cooled reducing the planets diameter by 7km in radius - this is much more than expected.
All of the discoveries help us to understand the formation processes of Mercury and thus the formation of the solar system.
Because of its inhospitable environment, Mercury has been one of the least explored of the inner solar system planets.
The next mission at Mercury will be ESAs Bepicolumbo mission launched on October 20th, 2018 to arrive in late 2025.
Venus is the second closest planet to the Sun and orbits in an almost circular orbit at 108 million km. As it orbits, Venus comes closer to Earth than any other planet in the solar system and can come to within about 40 million km.
Venus takes about 225 Earth days to orbit the Sun and rotates at the incredibly slow rate of once every 243 days - and in a clockwise direction (as seen from looking down on the Suns north pole). Only Uranus (which almost spins on its side) also has a clockwise spin. Because of the direction of spin and orbital motion, a day on Venus (mid day to mid day) lasts 117 Earth days. So even though Venus has the slowest rotational rate of any planet, a day on spent Venus is shorter than a day spent on Mecury.
Venus has a gentle axial tilt of 3 degrees.
Venus, with a diameter of 12100 km, it is very nearly the same size as Earth (1000km smaller), and has 80% of Earth’s mass. its gravity on the surface is 90% that of Earth’s.
Venus has a very dense atmosphere with pressures at the surface over 90 times that of Earth’s. The atmosphere is comprised of carbon dioxide with thick clouds of sulphur dioxide. This atmosphere has the strongest greenhouse effect known in the solar system which keeps the planet at a reasonably constant temperature of 460 degrees C. This makes Venus the hottest planet in the solar system, far hotter even then mercury which is twice as close to the Sun.
The surface of Venus, although hidden from view by thick clouds, has been mapped using radar and it is known that is is covered by large flat volcanic plains with two higher areas of land (continents) with mountains and valleys. The surface also shows impact craters and volcano like structures. Venus has a very weak magnetic field.
Venus and Man
Because Venus is so close to the Sun, it is often the first star to appear in the evening and the last to disappear in the morning. Hence it has long been known as the "evening star" and the "morning star".
The early Greeks named these two aspects of Venus "Phosphorus" and "Hesperus" and the Romans "Lucifer", (literally "Light-Bringer"), and "Vesper".
Attempts to send probes to the planet started in the infancy of space flight. In 1961, the Russian probe Venera 1 was sent to impact with the planet, but communications were lost in transit. The USA then attempted to launch mariner 1 in 1962, but it was destroyed by a command from the control centre a few minutes after launch as it veered of course. Mariner 2 was launched a month later and made a fly past of Venus in December of 1962 and was the first successful robotic interplanetary mission. It successfully measured the atmosphere, surface temperature magnetic field and radiation levels.
The Russians sent the probe Venera 3 in 1966 which became the first probe to enter the atmosphere of another planet. Unfortunately it returned no planetary data. It was followed by Venera 4 which did return data which showed among other things, that the atmosphere was much denser than expected. Using the data from Venera 4, improved probes Venera 5 and 6 were also sent to the planet in the 60's - neither surviving long enough to reach the surface. Mariner 5 also performed a fly-by and data was shared between Russian and American scientists.
During the 70's and 80's many more missions were sent to the planet with Venera 7 being the first probe to send scientific data from the surface (rather than just the atmosphere) of another planet. Venera 9 and 10 were the first missions to send back images of the surface of Venus. These were the first images received from the surface of another planet. Venera 11 and 12 both suffered lens cap release failures preventing images from being returned.
Venera 13 followed and analysed soil samples and Venera 15 and 16 enteried orbit to map the surface using radar. The Americans sent Mariner 10 to photograph the planet in detail and the Pioneer Venus project sent an Orbiters and 4 atmospheric probes to the surface. In the 90's NASA's Magellan Spacecraft orbited the planet from 1990 to 1994 before deliberately crashing into the planet.
Venus was last studied by Europe's Venus Express Spacecraft which orbited the planet from April 2006 until crashing into it some time in January, 2015.
1. There are unstable vortices in the atmosphere at the poles.
2. There appears to be ongoing volcanic activity beneath the clouds detected as heat sources and sulphur clouds.
3. Venus seems to be spinning slower (by 6.5 minutes a day) than measured by the Magellan spacecraft giving rise to speculation on how molten its core is.
4. Wind speeds in the upper atmosphere increased from 300kmph to 400kmph throughout the mission.
5. There is a very cold layer (-175 degrees C) 125km up in the atmosphere that has much hotter layers above and below it. This layer might contain frozen carbon dioxide snow.
The Messenger probe also made measurements during two fly-pasts in 2006/7 on its way to mercury.
Venus is currently being studied by the Japanese spacecraft Akatsuki. A rocket motor failure prevented Akatsuki from entering orbit in 2010 causing it to orbit the Sun for 5 years before being inserted into orbit in December 2015 using its thrusters. Since May 2016 it has been carrying out scientific studies of the Venusian atmosphere.
Among other observations, Akatsuki has discovered a bow wave in the Venusian atmopshere which seems to be caused as the winds blow over the Venusian continent called Aphrodite Terra, which reaches an altitude of approximately 5 km. Article.
The third closest planet to the Sun is Earth and is the largest and densest of the inner planets. Earth orbits in a reasonably circular at 150 million km and is the first of the planets to have a moon. Earth is of course the only place that we know of that has life.
Earth takes 365.25 Earth days to orbit the Sun and rotates once every 23 hours, 56 minutes and 4 seconds. Because it rotates around the Sun the length of a day on Earth (sunrise to sunrise) takes 24 hours.
The Earth has an axial tilt of 23.4 degrees and a diameter of 12742km.
The Earth is thought to be 4.54 billion years old and has been accompanied by the Moon for most of that time. It is believed that the Moon was formed when a large Mars sized body impacted the Earth causing enough material to be ejected which eventually coalesced into the Moon. The Moon has had the effect of stabilising Earth’s axial tilt and is the source of the Earth’s ocean tides.
The Moon is 3,474km in diameter (27% that of Earth) and orbits at a distance of between about 362,000 to 405,000 km. It has also been affected by the gravitational pull of the Earth which has over time caused the Moon's rotation to be slowed until it matches the time it takes to orbit the Earth. This is why the same side of the Moon always faces the Earth.
Earth is protected from solar radiation by a strong magnetic field generated by movement of its core which is mainly comprised of molten iron.
Click for Live Display showing the current positions of the moons of Mars.
Mars is the fourth closest planet to the Sun and orbits in an fairly eccentric orbit at around 230 (+-20) million km.
Mars takes about 686 Earth days to orbit the Sun. It has a tilt (25.1 degrees) and rotational period (24 hour 37 minutes) which are both similar to the Earth with a day (sunrise to sunrise) lasting 24 hours, 39 mins. Because of the tilt it also has seasons in the same way as the Earth does.
Mars is about half the size of the Earth with a diameter of 6,792km. However its mass is only a tenth of Earth’s with gravity on the surface being around 37% that of Earth’s.
Because Mars no longer has a magnetic field to protect it, Mars has lost its original atmosphere due to the effects of the solar wind interacting with the atmosphere causing atoms to be lost into space. Spacecraft have detected streams of atoms trailing off into space behind Mars. As a result the atmospheric pressure on Mars is 1% that on Earth. It is comprised of mostly (95%) carbon dioxide. Mars is very cold. Not only is it about 1.5 times further from the Sun than Earth, it also has a thin atmosphere which cannot store much heat. Because of this the temperature ranges from about -143 degrees C in winter up to a maximum of 35 degrees C in summer.
Mars is very dusty and prone to huge dust storms which can envelop the entire planet. These are more likely to occur when the planet is closest to the Sun.
Map of Mars - Click to go to NASA's interactive map
Evidence has been increasing that liquid water has sculpted the landscape of Mars in the past and is probably flowing, or rather seeping through the rocks during periods within salts called perchlorate salts. The video below talks about the details of this discovery.
Skip the first ten minutes to avoid the NASA hard sell, and hang in there with the poor quality sound on Luju's report from France.
Moons : Phobos and Deimos
Mars is lucky enough to have 2 small moons - both discovered in August 1877 by Asaph Hall. Phobos is tiny - only about 22km across- orbiting very close to Mars (9300km from its centre or 6000km above its surface) every 7 hours. It can be described as a non symmetrical, heavily cratered, dirty rock. Deimos however, is even smaller. It is only 12km across and orbits at 23,000 km every 30 hours. The origins of the moons are disputed but it is likely they are captured asteroids. However their near perfect circular orbits which align with the planets equator could point toward them originating on or with Mars. For more information on the moons, click here.
Mars and Man
Mars is named after the Roman god of war and has been known since before Babylonian times where it was associated with Nergul, a god of war, fire and destruction - possibly inspired by its red colour.
Possibly because Mars has a more benign environment than that of any of the other inner planets (other than Earth of course) it has received quite a few robotic explorers.
The first successful fly-by of Mars was performed by Mariner 4 in 1965. Mariner 9 in 1971 became the first probe to orbit another planet when it entered Mars orbit. Shortly after 2 Soviet probes Mars2 and Mars3 became the first to successfully land on another planet - even though they ceased functioning very shortly after. 1976 saw the US Viking mission with two orbiters and two landers. The landers successfully relayed images of the Mars surface and other measurements and continued working for up to 6 years.
In 1988 the soviets sent 2 probes (phobos 1 and 2) to photograph and land on the moons. One lost communications in transit and the other successfully photographed the phobos but failed before deploying its landers. Mars Global Surveyor entered the Mars orbit in 1997 and spent 4 years mapping Mars in detail. Also in 1997 Mars Pathfinder landed on the surface with its robotic vehicle Sojourner which was able to wander up to 0.5 km from the lander and took many photographs and measurements from the rocks and soil. Another lander, Phoenix, landed in the polar regions of Mars and confirmed the presence of water on Mars.
Since the year 2000, many additional probes have reached Mars and now provide detailed monitoring of the planets atmosphere and geography. The Mars Exploration Rovers, Spirit and Opportunity landed in 2004 for their 90 day mission. They both exceeded their mission objectives with Spirit eventually failing in March 2010 and Opportunity being declared dead in February 2019 after contact was lost in July 2018 when a huge planet wide dust storm blocked it's energy source - the Sun. End of Opportunity Mission Article.
Near future missions include a whole host of spacecraft to arrive 2020 to 2021:
NASA's Mars 2020 Mission - a lander, and surprisingly (because the atmosphere is so thin) a solar powered drone to help with navigation.(Mars Helicopter Links: Mars Helicopter Article, NASA Video, JPL Video). ExoMars 2020 - the second part of ESA's ExoMars mission to search for life. It comprises a rover to land in 2021. A public competition is open to name the rover (link), 2020 Chinese Mars Mission - An orbiter and lander to arrive in 2021. Hope Mars Mission or 'Al-Amal' - A United Arab Emirates orbiter to arrive in 2021. India's Mars Orbiter Mission 2 - from the Indian Space Research Organisation. An orbiter and possible lander (2021-2022).
Defunct spacecraft on the surface include MER-A Spirit and several other inert landers and rovers such as the Phoenix lander, which completed its mission in 2008. Observations by the Mars Reconnaissance Orbiter have revealed possible flowing water during the warmest months on Mars.
Above is one of those most rarest of media - live video (even only at four frames a second) from another planet. This video follows the landing of Curiosity - from the ejection of it's heat shield, falling (on parachute) and then being lowered to the surface from the sky crane (a hovering rocket powered structure that kicks up a lot of dust in the video).
Above is a remarkable animation (made before Curiosity even left earth) showing Curiosity's exciting arrival and then the rover going about it's business. Strangely, it brings home the loneliness of the life of a Mars Rover.
Between Mars and Jupiter lies the Asteroid belt which is comprised of thousands of rocks left over from the formation of the solar system. These rocks vary in size from microscopic up to Ceres (950km diameter) which is classified as a dwarf planet. It is thought that Jupiter’s strong gravitational influence may have prevented the asteroids coalescing into larger objects such as a planet. Outside of the asteroid belt itself there are also three clusters of asteroids which are very much in Jupiter’s influence. These are the Trojan asteroids which orbit ahead and behind Jupiter and the Hilda Asteroids which orbit twice for every 3 Jupiter orbits. These asteroids are in Jupiter’s Lagrange points - areas of gravitational stability.
Although the asteroid belt has many bodies within it, it is still very thinly populated with many unmanned spacecraft having successfully passed through it without incident.
Jupiter is the fifth closest planet to the Sun and is the first of what are called the outer planets (being outside the asteroid belt). It is by far the largest planet in the solar system having two and a half times as much mass as all the other planets put together and one thousandth the mass of the Sun. This is so large that the Sun and Jupiter actually orbit each other about a point just outside of the Suns surface.
Jupiter orbits the Sun once very 12 years (at about 780 million km) and is comprised of gas (75% hydrogen and 24% helium) and is presumed to have a rocky core surrounded by a sea of liquid metallic hydrogen which forms a ball 110,000km in diameter. Jupiter’s total diameter is 142,984 km.
In the upper atmosphere is a cloud layer 50km thick. The clouds are comprised of ammonia crystals and other compounds which are arranged into bands moving at different speeds at different latitudes. The Great Red Spot is a large stable storm vortex laying between two layers.
Considering its size, Jupiter rotates very quickly at one rotation at just under once every 10 hours. This means that at the equator there is quite a large centrifugal force which means the planet has a pronounced bulge - its diameter around the equator is 9000km greater than the diameter measured at the poles.
NASA's Juno spacecraft soared directly over Jupiter's south pole when JunoCam acquired this image on Feb. 2, 2017, from an altitude of about 62,800 miles (101,000 kilometres) above the cloud tops. This image was processed by citizen scientist John Landino. This enhanced color version highlights the bright high clouds and numerous meandering oval storms. Credits: NASA/JPL-Caltech/SwRI/MSSS/John Landino
NASA's Juno spacecraft skimmed the upper wisps of Jupiter's atmosphere when JunoCam snapped this image on Feb. 2 at 5:13 a.m. PT (8:13 a.m. ET), from an altitude of about 9,000 miles (14,500 kilometres) above the giant planet's swirling cloud-tops. Credits: NASA/JPL-Caltech/SwRI/MSSS/Roman Tkachenko
Landing on Jupiter.... Not a good idea!
Moons : Io, Europa, Ganymede and Callisto
Jupiter has many satellites (as of July 2018, 79) but most of these are quite small (less than 10km diameter). The four largest moons (Io, Europa, Ganymede and Callisto) which were discovered by Galileo Galilei in 1610 are named after the lovers of Zeus. These moons are generally larger than the Earth’s moon with diameters ranging from 3100km to 5200km. Three of the moons are locked together in an orbital resonance in which for every orbit Ganymede takes, Europa takes exactly two orbits and Io exactly four orbits.
The closest to Jupiter, Io, has over 400 volcano's and is incredibly geologically active. This is thought to be due to Jupiter’s strong gravitational field constantly squeezing the moon as it orbits which warms the moons interior.
The next of the Galilean moons is Europa. Its surface is very smooth and comprised of water ice, possibly floating on a sea of liquid water. It's thought to have a rocky centre and has a thin oxygen atmosphere. Because of the presence of water it is thought to be a good candidate to find life outside of the Earth.
Ganymede is the largest satellite in the solar system and is larger than the planet Mercury. It is also covered in ice but is less geologically active with its surface marked by craters and ridges.
Callisto, the last of the Galilean moons is comprised of even quantities of rock and ice and a thin atmosphere of carbon dioxide and oxygen. It is possible it has liquid water buried 100km below its surface.
The newest batch of 12 moons to be discovered (in 2018) were found a large distance out from the plant with most orbiting in a retrograde direction (e.g. opposite Jupiters spin) as described below. Article
Jupiter and Man
Jupiter is named after the Roman king of the gods also known as Jove who was based on the Greek god Zeus.
Jupiter was first visited by the Pioneer 10 spacecraft in 1973 closely followed by Pioneer 11 in 1974. These spacecraft obtained the first close-up images of Jupiter and its red spot and moons and also measured Jupiter’s massive magnetic field. They are still traveling out of the solar system, but have lost communications with Earth.
The next visitors were Voyager 1 and 2 in 1979 and discovered, among other things, the faint Jovian ring system, several new natural satellites, volcanic activity on Io.
The Ulysses spacecraft which was designed to study the Sun used Jupiter’s gravitational field (1992) to swing it out of the plane of the ecliptic to allow it to orbit over the Suns poles.
Galileo became the first spacecraft to orbit Jupiter in 1995, orbiting the planet for 7 years before being deliberately crashed into the planet in order to ensure that it did not crash into, and contaminate, Europa. During its mission it collected a huge amount of data on the entire Jovian system and even witnessed the Shoemaker-Levy 9 comet impact in Jupiter’s southern hemisphere.
The Cassini probe flew past in 2000 and imaged Jupiter’s atmosphere revealing many unknown features.
The New Horizons probe flew past in 2007 on its way to Pluto and studied the Jovian moons, magnetic field and ring system.
NASA currently has a mission underway to study Jupiter in detail from a polar orbit. Named Juno, the spacecraft launched in August 2011, arrived in July 2016 and will orbit the planet until July 2021 when it will be de-orbited into Jupiter. It passes very close to Jupiter on each orbit and therefore has to survive Jupiter's intense radiation belts.
Saturn is the sixth closest planet to the Sun. It is the second largest planet in the solar system having a radius 9 times that of Earth (57,000 km) and a mass 95 times that of Earth.
Saturn orbits the Sun once very 29 years (at about 1400 million km) and is mainly comprised of gas (96% hydrogen and 3% helium) and is presumed to have a rocky core surrounded by a sea of liquid metallic hydrogen which forms a ball some 56,000km in diameter. The upper layers are thought to comprise of liquid water, ammonium hydrosulfide, hydrogen and helium.
Saturn's core is quite hot (11,700 degrees C) and it generates more heat than it receives from the Sun. The further from the planets centre, the less the temperature with the temperature reaching around -180 degrees in the upper atmosphere, and 0 degrees C at around 350km deep.
The cloud layers of Saturn are similar to those of Jupiter except that the banding is weaker and wider. Saturn also has a short lived but periodical storm called the great white spot which seems to occur every Saturnian year.
Saturn rotates at around 10 hours 39 minutes. The precise figure (like that of all gas giants) is not certain since there are several ways of measuring the rotation of a body which has no fixed visual references.
Saturn is of course best known for its ring system. These were first seen by Galileo Galilei in 1610 who, quite understandably, was confused by them and thought Saturn was being accompanied by two other planets which sat either side of it. In 1655 Christian Huygens using an improved telescope was able to see enough detail to suggest that there was a ring around Saturn.
The rings extend from 7000km to 120,000km above the surface of Saturn. Incredibly, they are estimated at being between 1 km to 10 meters thick comprising of mainly ice particles ranging in size from dust to boulders a few meters across. The gaps in the rings are caused by the gravitational effects of Saturn's moons, and also by larger "moonlets" which inhabit the rings causing particles to be nudged into banded orbits. Recent observations have discovered that there are some distortions in the rings causing particles to rise some 4km out of the normal ring plane due to the tilted orbit of certain moons. The rings are thought to be either the remnants of a moon destroyed in orbit or simply by material left over by the formation of the solar system.
Moons : Titan... and the others
Saturn has 62 moons with only seven being large enough to become spherical in shape. By far the largest of all of Saturn's moons is Titan which is larger than the planet Mercury, and the second largest moon in the solar system (Jupiter’s Ganymede being the largest). Titan is 5,150 km in diameter and has a dense atmosphere of nitrogen with traces of methane. It has large lakes of liquid methane/ethane on its surface and may have a subsurface ocean of liquid water which occasionally erupts to the surface.
Great video that explores Titan.
The next largest moon to Titan is Rhea which has a diameter of 1,530km (30% Titans diameter). It is an icy body (75% ice, 25% rock) with a heavily cratered surface. The other moons of Saturn tend to have similar characteristics being comprised mainly of ice and rock and all being heavily cratered. Two notable moons include Mimas which shows an impact crater with a radius 1/3rd that of the moon, Iapetus which has remarkable colouring with one side being black as soot and the other as white as snow. This colouring is thought to be the result particles being kicked up from impacts on the moon Phoebe which lay in Iapetus's orbit. As Iapetus passed through these particles they were deposited on Iapetus's leading hemisphere causing it to darken.
Saturn and Man
Saturn is named after the Roman gods Saturnus (equivalent to the Greek god Cronus) which was the god of agricultural and harvest.
Saturn was first visited by the Pioneer 11 spacecraft in 1979 which discovered among other things additional rings in the ring system, and the moon Epimetheus which it almost (within 4000km) collided with.
The Voyager probes were the next to study Saturn with Voyager 1 taking the first high resolution images of Saturn, its rings and moons in 1980. Voyager 2 took more images in 1981 but a failure in its camera pointing ability resulted in losing some expected imagery.
In 2004 the Cassini spacecraft became the first probe to enter into orbit around Saturn releasing the Huygens probe which entered Titans atmosphere early in January 2005. The Huygens probe successfully landed on the surface of Titan sending back images and data during its descent and from the surface. Cassini has made many fly-pasts of Saturn's moons and ring systems making many new discoveries including new rings and weather systems. The Cassini spacecraft is intended to continue to study the Saturnian system until 2017 when it will be deliberately crashed into Saturn.
Uranus is the seventh closest planet to the Sun and the third largest and fourth heaviest of the planets. It diameter (50,000km) is four times that of Earth with a mass over 14 times that of Earth.
Uranus orbits the Sun once very 84 years (at about 2900 million km) but is unusual in that it spins on its side (with an axial tilt of 97 degrees). This means that its moons and also its faint ring system also orbit in plane perpendicular to the plane of the ecliptic.
It is believed to be comprised of a small rocky core surrounded by a deep mantle of water, ammonia and methane. This is in turn surrounded by an atmosphere of hydrogen, helium and methane with an upper cloud layer. Another oddity in Uranus is the fact that it is very cold. All the other gas giant planets emit more heat radiation than they receive due to very hot cores, but Uranus does not. A temperature of -224 degrees C has been measured in Uranus' atmosphere - the coldest in the solar system.
Uranus has the second most extensive ring system of the solar system after Saturn. The rings, which are very difficult to see from ground based observations, were first discovered in 1977 by measuring the intensity of a star as Uranus and its rings passed in front of it. There are 13 known rings with radii of 38,000km to 98,000km. They are comprised of ice and some darker material which results in them being much darker than the rings of Saturn.
Uranus has 27 known moons with sizes ranging from over 1500 km diameter down to under 20km. The moons consist of ice, rock and other trace elements. Some of the inner moons undergo gravitation interactions with each other which may in many millions of years lead to instabilities and collisions.
Uranus and Man
Uranus, under clear dark skies, is actually visible to the naked eye. However it is very dim and its 84 year periodicity means that it moves slowly across the sky. However it is interesting that it was not noticed by the ancients and was only observed for the first time by Sir William Herschel in 1781 using a telescope. It was initially named Georgium Sidus (George's Star) by Herschel after King George III. However this unpopular name was eventually discarded and it was renamed Uranus after the Greek god of the sky. Uranus is the only planet to be named after a Greek god, rather than a Roman deity.
To date Uranus has been visited only once - by the Voyager 2 spacecraft. The fly-by occurred in 1986 and resulted in the discovery of 10 new moons and 2 rings. It also measured the chemical composition of the atmosphere and photographed the planet and its moons. This data is still being studied and In 2016 researchers claimed to have discovered evidence for two new moons which may be causing disturbances in its inner most rings.
A "Uranus Orbiter and Probe" mission is in the study stages.
Neptune is the eighth closest planet to the Sun and is (since the relegation of Pluto) the last Planet in the Solar System. It is similar in size and composition to Uranus with a diameter of 49,000km and a mass of over 17 times that of Earth.
Uranus orbits the Sun once very 165 years at about 4500 million km. This is 30 times the distance from the Sun to the Earth which means that the strength of sunlight at Neptune is 1% of its strength at Earth. Neptune spins on an axis with a tilt of 28 degrees, which is quite similar to that of the Earth, every 16 hours.
Unlike Uranus' almost completely bland cloud layer, Neptune's weather systems are more pronounced with great dark spot storm systems being seen in the southern and northern hemispheres and other visible banding. Wind speeds of up to 600m/s have also been observed. Due to its great distance from the Sun, it is not surprising that the planet is one of the coldest with temperatures of -220 degrees C in the upper atmosphere. The core is thought to be at around 5200 degrees C.
Neptune has a ring system comprised of three main rings with radii between 63,000 km and 42,000km which appear to have a clumpy structure in which they form arcs rather than complete rings. It is believed that gravitational effects from Neptune's moons may cause the clumping.
Moons - Triton
Uranus has 13 known moons of which Triton (2700km diameter) is the most significant. The next largest moon, Proteus, is only 420km in diameter. Triton is the only large moon to orbit in retrograde (e.g. in the opposite direction to most orbits) and is thought to be a captured object rather than created in orbit. It has a very thin atmosphere of Nitrogen, Carbon Dioxide and methane. Although it has a surface temperature of around -230 degrees C, it is thought to be geologically active with cryovolcanism (low temperature volcanoes) producing eruptions of water and ammonia which freeze to form complex valleys and ridges.
Neptune and Man
Neptune is invisible to the naked eye and was unknown to the ancients. It was first discovered when astronomers noticed anomalies in the orbit of Uranus and suggested that an as yet undiscovered planet could be the cause. In 1846 Neptune was observed near the position predicted by Urbain Le Verrier. This was the first planet to be discovered by mathematical prediction. Le Verrier proposed the name Neptune (Roman god of the sea) for the planet since it was in keeping with naming planets after Roman deities (other than Uranus which is Greek).
To date Neptune (like Uranus) has been visited only once - again by the Voyager 2 spacecraft. The fly-by occurred in 1989 and resulted in many discoveries including Neptune's weather systems, rings and 6 more moons. It also provided an accurate measurement of Neptune's mass which allowed discrepancies in the orbits of Uranus and Neptune to be solved.
There are currently no missions planned to visit Neptune, since the Neptune Orbiter and Probe mission was dropped in favour of the Uranus Orbiter and Probe mission.
Planet Nine is a hypothetical large planet in the far outer Solar System, the gravitational effects of which would explain the improbable orbital configuration of a group of trans-Neptunian objects (TNOs) that orbit mostly beyond the Kuiper belt.
In a 2014 letter to the journal Nature, astronomers Chad Trujillo and Scott S. Sheppard inferred the possible existence of a massive trans-Neptunian planet from similarities in the orbits of the distant trans-Neptunian objects Sedna and 2012 VP113. On 20 January 2016, researchers Konstantin Batygin and Michael E. Brown at Caltech explained how a massive outer planet would be the likeliest explanation for the similarities in orbits of six distant objects, and they proposed specific orbital parameters. The predicted planet could be a super-Earth, with an estimated mass of 10 Earths (approximately 5,000 times the mass of Pluto), a diameter two to four times that of Earth, and a highly elliptical orbit with an orbital period of approximately 15,000 years.
This planet has not been discovered yet, and if it exists, it will be hard to spot because it is far away and therefore will reflect very little light and have a very low temperature. Although this theory has gained some support over the years, there are still sceptics who offer other theories to try to explain the observed features in the movements of some trans-Neptunian objects.