Planets Main Index

The Planets and the Solar System

The Solar System consists of the Sun and the planets and their moons, and a whole range of other objects, with diameters of from less than a metre to more than a hundred thousand kilometres, all in orbit round the Sun.

When we look at the night sky with just our eyes we may see some of the brightest fixed stars and some of the the naked-eye planets: Mercury, Venus, Mars, Jupiter and Saturn. Uranus and Neptune are also planets but we cannot see them without a telescope.

(Pluto was taken off the list of planets in August 2006 for reasons explained here.)

The best way to think of the Fixed Stars is to imagine the Earth as at the centre of a huge hollow sphere - the “celestial sphere”. The Fixed Stars are stuck to the inside surface of this sphere, with the Pole Star above the North Pole. As the Earth rotates on its axis the stars will appear to rotate round the Pole Star; as we walk around on the Earth's surface we shall see different parts of the celestial sphere and so different stars. But the stars are always in the same position relative to each other.

This analogy works because although the stars are not actually fixed in space or the same distance from Earth they are so far away that seen from Earth their movement is so slow as to be unnoticeable - Orion seems the same shape to us as it did to the ancient Egyptians more than five thousand years ago.

The Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn are not however always in the same position on the celestial sphere but move (wander) backwards and forwards against the background of the Fixed Stars. Our word planet comes from the Greek word for wandering star.

Originally the planets were the Sun, the Moon, and Mercury, Venus, Mars, Jupiter and Saturn. Now however we use the word planet to mean a certain type of body orbiting a star, so that Earth is a planet but the Sun and Moon are not.

With the naked eye Mercury, Venus, Mars, Saturn and Jupiter look like very bright stars and unless you watch them over several days so you can see them moving through the Fixed Stars you might think they are stars. But with even a small telescope or pair of binoculars you can see that a planet is a disc but the stars are still just points of light. The brightest star in the sky is Sirius (sometimes called the Dog Star), and if you can find Sirius you know that anything brighter than that must be a planet.

In the order of their mean distance from the Sun the planets are

Everything in the Solar System beyond the orbit of Neptune is called a Trans-Neptunian Object (TNO). TNOs include Pluto. Pluto was redefined as a Dwarf Planet rather than a Planet in August 2006, for reasons explained later on this Page.

To understand the sheer size and scale of the Solar System you really need to make a model - you can find the instructions here.

With the naked eye we can easily see Mercury, Venus, Mars, Jupiter and Saturn. On a very dark clear night we can also just see Uranus provided we know where to look, but it moves so slowly against the background of the Fixed Stars that it was not identified as a planet until 1781. We need a telescope or a good pair of binoculars to see Neptune (and a very powerful telescope to see Pluto).

Most school textbooks and many other books and web sites tell you that the first person to use a telescope to look at the sky was Galileo (1564 - 1642), and he was a very famous astronomer. Although his telescope was the best in the world at the time, by today’s standards it was about as powerful as a pair of cheap mail-orderbinoculars, and nothing like as sharp.

As described above, Pluto is no longer classified as a planet, but it will probably take several years for this to be reflected in popular thinking and school science teaching, so the next Section is still relevant to those studying science at school or taking part in tv and pub quizzes etc.

The orbits of the planets, and all other objects in orbit round the Sun such as asteroids, comets and Trans-Neptunian Objects, are not circles but ellipses. Most of the planets have orbits only slightly elliptical, but the orbit of Pluto is very elliptical. Its mean distance from the Sun is about 5900 million km, compared with about 4500 million km for Neptune, but its aphelion (furthest distance from the Sun) is about 7300 million km and its perihelion (nearest distance from the Sun) is about 4400 million km, so that at perihelion it is actually nearer the Sun than Neptune. It was at perihelion in 1989 so from 1979 to 1999 the planet furthest from the Sun was not Pluto but Neptune, and at the time this caused a lot of problems, for example in poorly worded astronomy questions in exams. If we draw the solar system on a sheet of paper, in two dimensions, the orbits of Pluto and Neptune seem to cross, but in three dimensions their orbits are at an angle to each other so they do not really cross, and Pluto and Neptune will never collide.

The time for each planet to go round the Sun depends upon how far it is from the Sun and varies from 88 days for Mercury to a year for the Earth and 165 years for Neptune (and 248 years for Pluto).

Because Mercury and Venus are nearer the Sun than the Earth we can only see them just before sunrise or just after sunset, we can never see them in the middle of the night. The planets further from the Sun than the Earth can be seen at night but as they go round the Sun sometimes they are the other side of the Sun from the Earth so not all of them can be seen every night.

Mercury is very close to the Sun, and so sets very soon after the Sun, and although you can see it with the naked eye you do need to know where to look. It is very dangerous to look for Mercury with a telescope or binoculars before the Sun has set because if you accidently point the telescope at the Sun you may blind yourself.

Mercury is about 58 million kilometres from the Sun and has a diameter of about 4880 km. It goes round the Sun once every 88 days.

Venus is nearer the Sun than the Earth and it is clearly visible in the evening or the morning sky - hence its name as the Evening (or Morning) Star. A very bright “star” seen in the evening just after sunset is almost certainly Venus. With even a very ordinary pair of binoculars you can see that Venus shows phases (seems to change its shape), like the Moon, and this was Galileo's first proof of the heliocentric universe (a universe with the Sun in the middle rather than a geocentric universe with the Earth in the middle).

Venus is about 108 million kilometres from the Sun with a diameter of 12 100 km. It goes round the Sun once every 224 days.

After Venus comes Earth (hence the title of the tv series Third Rock from the Sun). We call it Earth but a Martian or other extra-terrestial being would be more likely to call it Ocean, as seen from space it is mostly not land but water.

It is about 150 million kilometres from the Sun with a diameter of about 12 800 km and goes round the Sun once a year.

Mars is a red colour - hence its other name of the Red Planet.

It is about 228 million kilometres from the Sun and has a diameter of about 6800 km. It goes round the Sun once every 687 days.

Between the orbits of Mars and Jupiter is the Asteroid Belt. The asteroids are hundreds of thousands of lumps of rock ranging in diameter from less than 1 m to more than 100 km. At one time they were thought to be the remains of a planet which had broken up, but we now think they are some of the particles from which the solar system was formed which were prevented from condensing into a planet by the very powerful gravity of Jupiter. Sometimes Jupiter and the other planets line up in such a way that their combined gravity pulls an asteroid out of its normal orbit and it may pass much nearer or even hit the Earth. An asteroid hit the Earth about sixty five million years ago and many scientists believe that it was this which caused the extinction of the dinosaurs. The asteroids are very important to scientists because they can tell us a lot about the material from which the solar system was formed. Asteroids are discussed further in the Page on Shooting Stars.

The first asteroid to be discovered was Ceres, on 1st January 1801. It has a diameter of about 760 km. It was originally called a planet, but within a few years other bodies were discovered in the same orbit. So Ceres and the other objects in the same orbit were renamed minor planets. They are now called asteroids, and Ceres is now regarded as just a large asteroid.

Jupiter is one of the giant planets. It has several satellites (moons) and four of them (Io, Europa, Ganymede and Callisto) are easily visible with a small telescope or pair of binoculars. They are called the Galilean Moons because they were discovered by Galileo and were used by him as his second proof of the heliocentric universe.

Jupiter is about 780 million kilometers from the Sun, with a diameter of about 143 000 km. It goes round the Sun about once every 14 years.

Saturn is the second of the giant planets. It has a ring (actually made up of several rings) which is clearly visible with a small telescope or pair of binoculars. Galileo's telescope was not very good and the rings appeared so fuzzy that he did not know what they were and thought that Saturn had ears, like the Disney Mickey Mouse logo!

Saturn is about 1430 million kilometres from the Sun with a diameter of about 120 000 km. It goes round the Sun about once every 29 years.

Uranus is the third of the giant planets. On a very clear very dark night you can just see it with the naked eye if you know where to look.

It is about 2870 million kilometres from the Sun, and has a diameter of about 52 000 km. It goes round the Sun about once every 84 years. It moves so slowly across the fixed stars that it was not identified as a planet until 1781, after the invention of the telescope.

Beyond Uranus is Neptune. It cannot be seen without a telescope.

It is about 4500 million kilometres from the Sun, and has a diameter of about 50 000 km. It goes round the Sun about once every 165 years.

Beyond Neptune is the Kuiper Belt. (Kuiper rhymes with viper.) This stretches thousands of millions of kilometres further into space and, together with the Oort Cloud which goes out even further into space, contains the rest of the material from which the solar system was formed. Objects in this belt are called Kuiper Belt Objects (KBOs). So far more than a hundred KBOs have been found.

Pluto was discovered in 1930. At that time the Kuiper Belt had not been discovered, so Pluto was classified as a planet. The existence of the Kuiper Belt was first put forward on theoretical grounds in 1950, and the theory was confirmed by a computer simulation of the formation of the solar system in the early 1980s, although the first KBO (after Pluto) was not actually discovered until 1992. In August 2006 Pluto was officially reclassified as a dwarf planet rather than a planet.

The Oort Cloud and Comets

Beyond the Kuiper Belt is the Oort Cloud. This contains thousands of millions of lumps of ice and frozen methane and ammonia. It stretches out about thirty million million kilometres (about three light years) from the Sun.

Beyond the Oort Cloud the Sun’s gravity is so weak that it can no longer hold any body in orbit round it against the gravity of other stars. This is the beginning of interstellar space. The space between the galaxies is called intergalactic space.

Even inside the Oort Cloud the Sun's gravity is so weak that the tiniest disturbance, for example the gravity of one of the giant planets such as Jupiter, may send a lump of ice into a totally different orbit. Comets are lumps of ice, often described as “dirty snowballs,” going round the Sun in very elliptical orbits. At perihelion (from the Greek for nearest to the Sun) they may be closer to the Sun than Mercury, but at aphelion (furthest from the Sun) they may be in the Oort Cloud.

Why isn’t Pluto a planet any more?

To the Ancient Greeks and Romans a planet was a wanderer, a heavenly body (one visible without a telescope of course) which wandered through the sky against the background of the Fixed Stars. The planets were the Sun, the Moon, and Mercury, Venus, Mars, Jupiter and Saturn. Once it was realised that the Earth was not the centre of the Solar System the word planet came to mean a body moving round the Sun, that is, Mercury, Venus, the Earth, Mars, Jupiter and Saturn. Later, in 1781, Uranus was identified as a planet, and became the seventh planet.

Then Ceres was discovered, in 1801, and this became the eighth planet. But very soon lots of other objects were discovered in the same orbit as Ceres, so all the objects in this orbit were renamed, initially minor planets but later asteroids. Then, in 1846, Neptune was discovered, and this became the new eighth planet.

Finally, in 1930, Pluto was discovered and became the nineth planet. Pluto is very small and cannot be seen except with a very powerful telescope. It was actually discovered quite by accident. Some astronomers were doing some calculations on the orbit of Neptune. They thought that the orbit of Neptune was being affected by another planet further from the Sun. They worked out where this planet should be, and when they looked there with a very powerful telescope there it was! In fact their calculations were wrong, and the orbit of Neptune was not being affected by an undiscovered planet; it was pure chance that Pluto was where they had calculated this undiscovered planet should be. Pluto is so small that had it not been for this mistake it is unlikely that it would have been found for at least another sixty years, until after the Kuiper Belt had been discovered, and it would have been classed as a Kuiper Belt Object rather than a planet.

In 1978 astronomers discovered Charon. Charon was originally classed as Pluto’s moon, but it is now realised that it is big enough to form a binary system with Pluto, two bodies rotating not one round the other but both round their common centre of gravity. Then in June 2005 the Hubble Telescope found two more, very tiny, moons of Pluto and they have been given the names Nix and Hydra.

Since the 1950s developments in telescopes, spacecraft and computer technology have made it possible for astronomers to find out a lot more about the Solar System. Most astronomers agreed informally that lumps of rock only a few kilometres across should not be called planets. But the discovery in 2005 of a Kuiper Belt Object much bigger than Pluto, and given the name Eris after the Greek Goddess of Discord, made it necessary for astronomers to agree on a formal definition of a planet. In August 2006 an international conference of astronomers agreed on such a definition; under this definition Pluto is not a planet. The arguments are quite technical but, put simply, if we include Pluto as a planet then we must also include Eris and lots of other Kuiper Belt Objects, and even some asteroids. Astronomers had the choice of either increasing the list of planets to at least twenty but up to a hundred or taking Pluto off it, and in a triumph for common sense they chose to take Pluto off it. The new definition requires that to be a planet an object must be big enough to have enough gravity to attract everything else in the neighbourhood of its orbit, that is, there must be no other smaller objects close to it also going round the Sun in the same orbit, and Pluto is not big enough to do this.

Objects in orbit round the Sun are now divided into three main groups.

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© Barry Gray October 2018