The Planets Today

Planets – The Basic Facts

What is the definition of a Planet?

The definition of planet set in 2006 by the International Astronomical Union (IAU) states that in the Solar System a planet is a celestial body that:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  3. has “cleared the neighbourhood” around its orbit.

So in layman’s terms:

1. It’s a body which is in orbit around the sun and not a satellite of a planet. e.g. the moon is ruled out as being a planet because it orbits the earth.
2. It’s shape is close to being the shape if it was made of water – e.g. close to a sphere. If the object is spinning then centrifugal force will cause the radius at the equator to be larger that the radius at the poles – so it’s never a perfect sphere.
3. It has a clear area around it. In other words it has sucked up or pushed away any other objects to become the dominant body in the area. It has been argued that Neptune should be a dwarf planet for the reason that objects like Pluto keep wandering through it’s orbit space. However Pluto has been locked into a phase in which it orbits the sun twice for every three orbits of Neptune. If this were not the case Pluto’s orbit would be unstable. Therefore Neptune’s much larger gravitational force is in control.

When is a satellite a satellite?

You may wonder how to define when one object is a satellite of another. When two objects are locked together by gravity then both objects can be said to be circling each other. The point about which they circle is called the barycentre. If one object is heavier than the other then the barycentre will be closer to that object. When one object is much much heavier then the barycentre can lie inside that object – which is an unofficial definition of a satellite. E.g. in the case of the earth and the moon, they orbit each other around a point about 1700km under the surface of the earth, hence the moon is a satellite of the earth.

It is known that Pluto and it’s moon Charon orbit about a barycentre outside of both objects. They are in fact a binary system – but currently the classification remains that Charon is a satellite of Pluto.

So if it’s not a planet… what is it?

What’s an SSSB?

If an object fulfils only the first criteria – e.g. it orbits the sun and is not a satellite of a planet – is not spherical and doesn’t dominate it’s orbit then its a “Small Solar System Body” (SSSB).

What’s a Dwarf Planet?

If an object fulfils the first and second criteria – e.g. it orbits the sun and is spherical, but doesn’t dominate the space around it then it’s a “Dwarf Planet”. This 1 minute video explains….

So… Which objects are planets, and which are dwarf planets?

The 8 planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

The International Astronomical Union (IAU) recognises 5 dwarf planets: Ceres, Pluto, Haumea, Makemake, Eris. However, it is estimated that there are hundreds to thousands of dwarf planets in the Solar System, mostly hiding the Kuiper Belt (a large region of space just outside Neptune’s Orbit). Some astronomers have put forward over 300 objects that are dwarf planet candidates. However the International Astronomical Union (IAU) are reluctant to classify these objects as planets until it can be proved that they meet all the requirements, and due to the problems of studying these small and remote objects it may take some time before reclassification occurs.

In 2008 it was decided to have a sub classification of Plutoids which means dwarf planets which orbit outside (mostly) of Neptune. There are 4 Plutoids : Pluto, Haumea, Makemake and Eris.

And SSSBs? Well there are loads of them, and they include all the asteroids and comets.

Names of all the Planets



the Planet Mercury


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 it’s 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. It’s 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. It’s 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 it’s 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 theMessenger 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 it’s surface.

Mercury is currently being studied by the Messenger Spacecraft. Messenger entered Mercury’s Orbit on 18th March 2011, the first man made object ever to do so. It is hoped that by studying Mercury, and it’s ancient surface, it will help to answer many questions about the formation of the solar system. Because of it’s inhospitable environment, mercury has been one of the least explored of the inner solar system planets.

The next mission to Mercury will be ESAs Bepicolumbo mission to launch in 2016.



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 it’s side) also has a clockwise spin. A day on Venus (sunrise to sunrise) lasts 117 earth days.

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. It’s 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 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 the 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 data from the surface of another planet. The Venera 13 analysing Luna soil samples and Venera 15 and 16 entering 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 the magellan spacecraft mapped the planet before deliberately crashing into the planet.

Currently Venus is being studied by Europe’s Venus Express Spacecraft which has been orbiting the planet since 2005. The Messenger probe also made measurements during two fly-pasts in 2006/7 on it’s way to mercury.


Earth and the Moon

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 moons 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 it’s core which is mainly comprised of molten iron.


Mars: Credit: NASA/JPL-Caltech/MSSS



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 it’s 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 it’s 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 -87 degrees C in winter up to a maximum of -5 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

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 it’s centre or 6000km above it’s 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 in 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 it’s 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 2010 and Opportunity is still performing (as of March 2015) and coming up to a marathon distance of over 42 kilometres travelled. NASA Mars Exploration Rover Site.

Mars is currently host to five functioning spacecraft: three in orbit – the Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter; and two on the surface – Mars Exploration Rover Opportunity and the Mars Science Laboratory Curiosity. 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

Asteroid Belt

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.

The Planet Jupiter


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 it’s 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 – it’s diameter around the equator is 9000km greater than the diameter measured at the poles.


Moons : Io, Europa, Ganymede and Callisto

Jupiter has many satellites (more than 60) but most of these are quite small (less than 10km diameter). The four largest moons (named above) 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 incredible 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. It’s 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 it’s 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 it’s surface.

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 it’s 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 (and only) 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 it’s 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 it’s 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, and will arrive in late 2016. The next planned mission to the Jovian system will be the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), due to launch in 2022.



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. It cools the further from the centre with only the top 350km being cooler than freezing – it’s upper atmosphere being around -180 degrees C.

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 in Silhouette

Saturn’s Rings

Saturn is of course best known for it’s 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 under 20m 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 it’s surface and may have a subsurface ocean of liquid water which occasionally erupts to the surface.

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 ‘reek 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, it’s rings and moons in 1980. Voyager 2 took more images in 1981 but a failure in it’s 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 it’s 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 to crashed into Saturn.

Cassini-Huygens Images of Saturnian System : The best images of Saturn and its Rings and Moons.




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 it’s moons and also it’s 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 and been measured in Uranus’s atmosphere – the coldest in the solar system.

Uranus’s Rings

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 Saturn and it’s 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 moon undergo gravitation interactions with each other which may in many millions of years lead to unstabilities and collisions.

Uranus and Man

Uranus, under clear dark skies, is actually visible to the naked eye. However it is very dim and it’s 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 namedGeorgium 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 it’s moons.

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’s 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 it’s 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’s Rings

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 only 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 volcano’s) 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 2oman 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.



Dwarf Planets




Video of Ceres

9 Dec 2015: Dwarf planet Ceres is shown in these false-color renderings, which highlight differences in surface materials. Images from NASA’s Dawn spacecraft were used to create a movie of Ceres rotating, followed by a flyover view of Occator Crater, home of Ceres’ brightest area.

Ceres is the closest dwarf planet to the sun and lies between Mars and Jupiter in the region of the asteroid belt. It is composed of rock and ice and is 950 km (590 mi) in diameter. It is the largest object in the asteroid belt and contains approximately a third of the total mass of the asteroid belt. Although classified as a dwarf planet, it is quite small with a diameter less than a third that of the moon, and a mass of about 1.2% that of the Moon.

It is thought to have a rocky core surrounded by a 100km crust of water ice. Such a volume of ice would be more that the total volume of fresh water on the earth. A thin atmosphere of water vapour sublimating from the ice is possible. The maximum temperature at the surface with the sun overhead is estimated to be a balmy -38 degrees C.

It’s orbit is over 4.5 years long and is inclined by 10 degrees to the plane of the ecliptic. At it’s extremes it reaches 0.5 Au (e.g. the half the distance as the earth is from the sun) away from the ecliptic plane.

It has no moons.

For the latest information on Ceres, watch this NASA video from 8th October 2015:

Like most NASA videos, there’s a very dull intro that you can skip of you jump to 3 minutes in. This video also has a lot of details about the asteroid Vesta.

Ceres and Man

Ceres was discovered by Giuseppe Piazzi at the Academy of Palermo, Sicily on 1 January 1801, which was half a century before the discovery of Neptune. It was the first object to be seen in the asteroid belt and was listed as one of the solar system planets for over 50 years. However as more and more asteroid belt objects were discovered Ceres became classed as the largest of the asteroids.

Piazzi named the planet after the goddess Ceres (Roman goddess of agriculture). As with all dwarf planets, it is invisible to the naked eye and was not known to the ancients.

Ceres is being observed by the Dawn spacecraft.

News: Water Detected on Ceres

Pluto and its Moons

For latest Images/Info, visit the New Horizons page.

Click for Live Display showing current positions of Pluto and its moons.

Click for Display showing a replay of New Horizons as it sped through the Plutonian System.


Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA’s New Horizons spacecraft, taken on July 13, 2015 when the spacecraft was 476,000 miles (768,000 kilometres) from the surface. Click for full story. Credits: NASA/APL/SwRI

Map of Pluto

Pluto is the second largest dwarf planet after Ceres, and is about 1/6 the mass of the moon. It has an diameter of 2370km and is made of rock and ice with a thin atmosphere of nitrogen, methane and carbon monoxide. It has a temperature of around -230 degrees C.

Its 248 year elliptical orbit occasionally takes it inside the orbit of Neptune. However the orbit remains stable because it orbits exactly 2 times for every 3 orbits of Neptune. The orbit is also inclined by 17 degrees to the plane of the ecliptic ensuring it remains distant from Neptune.


For more information on the moons see our Plutonian System page.

Pluto and it's moons

Pluto has five moons, the latest of which is Styx which was discovered in 2012 in a Hubble space telescope image. The largest Moon is Charon (1200km diameter). It is so large (12 percent the mass of Pluto) that it is forms a binary system with Pluto in which both objects orbit about each other every 6 days. The other moons orbit around both Pluto and Charon. The outer moons are quite small – between 10km to 100km diameter – and orbit with periods of (generally) less than a month.

Pluto and Man

Pluto is of course the most well known dwarf planet because of it’s previous classification as the outermost planet of the solar system. Rather like Ceres before it, it has suffered because of the number of similar objects now discovered in the same region mean that it can no longer be thought of as one of the major planets.

Pluto was discovered by Clyde W. Tombaugh on the 18th February 1930 after a long search for “Planet X” which was thought to exist due to perturbations in the orbit of Neptune. It was initially thought to be as large as Neptune but as better observations became possible it’s estimated mass was constantly reduced until , in the 1970s, it was found to weigh less that 1% that of the earth.

The name Pluto, after the god of the underworld, was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology. She suggested it in a conversation with her grandfather Falconer Madan who passed the name to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States. It was then chosen after a vote against other candidate names. Listen to Venetia here.

Pluto was visited for the first time ever by the New Horizons spacecraft which reached its closest approach on July 14, 2015. Scientific observations of Pluto began 5 months before closest approach and continued for a month after the encounter.


Haumea is one of the stranger dwarf planets in that it does not appear to be spherical. It has an extremely fast (for a planet) rotation of once every 4 hours and this has pulled it into a ellipsoidal shape – rather like a squashed rugby (or American football) ball, spinning on its side. Strange – but apparently still in hydrostatic equilibrium.

It orbits the sun once every 283 years and inhabits a similar area of space in the Kuiper Belt as Pluto. It’s orbit is inclined by 28 degrees to the ecliptic meaning it spends most of it’s time a long way above or below the ecliptic plane. It is thought to be comprised of rock with a thin covering of crystalline ice which is as bright as snow. However the low temperature (-220 degrees C) and high radiation should mean that the crystalline ice should have turned to a red coloured amorphous ice over the last 10 million years or so meaning that Haumea’s surface is possible much newer than expected.

It has a mass equal to 6% that of the moon.


Haumea has two moons, Hi’iaka and Namaka. These icy bodies have diameters of about 350km and 170km and orbit Haumea every 49 and 18 days respectively. Again the fact that they have crystalline ice means their surfaces are possibly relatively new.

Haumea and Man

Haumea was discovered in 2004/5 with some doubt over who actually gets the credit for it. A team at Caltech, USA and a team from the Instituto de Astrofísica de Andalucía in Spain both put in claims and which are being contested. However it seems that the Caltech team got the pleasure of naming it “Haumea” after the Hawain goddess of fertility and childbirth.

Because the team discovered Haumea on the 28th December, the team nicknamed it “Santa” and later the two moons “Rudolph” and “Blitzen”. However due to IAU guidelines that classical Kuiper belt objects be named after mythological beings associated with creation, the Hawain names were chosen (with Hi’iaka and Namaka being Haumea’s children) for the official names.

There have been no robotic missions to Haumea and none are currently underway.


Makemake’s orbit is similar to Haumea in that it resides in the Kuiper Belt with a 29 degree inclination to the ecliptic and with a 320 year period orbit. It has a diameter of around 1430km and appears to be reddish in colour. Like Pluto it has methane and possibly large amounts of ethane and tholins at its surface, and is covered by a very thin atmosphere at -240 degrees C. It’s thought to be spherical with a rotation of once every 8 hours.


Makemake has no (detected) moons which makes determining it’s mass difficult.

Makemake and Man

Makemake was discovered in 2005 by the same Caltech team who discovered Hameua and Eris, all of which were announced at the same time. Initially codenamed “Easter Bunny” since it was discovered near Easter, it was officially named “Makemake”, the creator of humanity and god of fertility in the mythos of the Rapanui, the native people of Easter Island. This name was chosen in part to preserve the object’s connection with Easter.

Just like Haumea, there have been no robotic missions to Makemake and none are currently underway.


Eris has an orbital period of 557 years and was at its furthest from Sun in 1977 at 97 AU (e.g. 97 earth orbit radius). It will come to it’s closest point to the Sun at sometime around 2257 (38 AU). With an orbit at almost 45 degrees to the plane it travels over 30 AU to the north and 50 AU to the south of the ecliptic. It is currently (2014) around 30 AU from the ecliptic plane on the southern side but getting closer.

Apart from long-period comets and space probes, Eris and its moon are currently the most distant known objects in the Solar System. Eris appears grey in colour and shows traces of methane in its atmosphere – rather like Pluto. The surface temperature varies between -217 and -240 degrees Celsius and its diameter, at 2400km, is thought to be similar to Pluto’s with a mass of around 1/5 that of the moon.


A moon “Dysnomia” orbits Eris every 15 days. Because of the problems with observing such a small object at such distances many details about Dysnomia are vague. The diameter is thought to be somewhere between 150 and 650km.

Eris and Man

Discovered by the same team as Haumea and Makemake in 2005, it was initially nicknamed “Xena” after the warrior princess TV character – and also because it began with X as in the fabled “Planet X”. It was eventually named after the Greek goddess Eris, a personification of strife and discord. The moon, Dysnomia, was named after the Greek goddess of lawlessness who was Eris’ daughter. Mike Brown (the team leader) says he picked it for similarity to his wife’s name, Diane. The name also retains an oblique reference to Xena who was portrayed on TV by Lucy Lawless.

As you may expect… there are currently no missions planned to visit Eris.

Original Source:

Share This: