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Particle Physics and Astronomy Research Council

Royal Greenwich Observatory

Information Leaflet No. 19: 'Planets'


The Earth is a planet and the main difference between planets, including the Earth, and stars is that stars shine with their own light, generated in their interiors by fusion of the elements whereas planets shine by reflected light from the star around which they orbit.

Astronomers have tried very hard to find planets around other stars. The evidence so far for planets at all like the Earth is not very convincing, although astronomers think that it is very likely that many stars do have planetary systems. Before we think about other stars let us consider our Solar system.

The Sun:

The Sun is more fully discussed in a separate pamphlet. Here we give a quick summary of its properties.
The Sun has the vast majority of the matter which comprises the Solar System. The most massive of the planets, Jupiter has only one thousandth of its mass and all the other planets add up to less than the mass of Jupiter.
The Sun consists of a massive ball of gas. The photosphere (the visible disk) is at a temperature of about 5000° C while the centre is at an incredible 15,000,000° C. In the centre the temperature and pressure are high enough for the hydrogen atoms to combine to give helium atoms. This process converts a small amount of mass into energy. In a second the Sun loses half a million tons to generate 10 million, million, million kWh of energy!

The Orbits of the Planets:

The Sun's mass acts through gravity to keep each of the planets in an orbit around it. These orbits are elliptical and the time each planet takes to go around the Sun depends on the distance of the planet from the Sun according to the law discovered by Kepler in the 17th century.
The orbits of the planets about the Sun are all close to the same plane. This is thought to be due to the way in which they were all formed from a disk of residual matter around the Sun following the collapse of the original gas cloud.

The Terrestrial Planets:

The planets closest to the Sun (Mercury, Venus, the Earth and Mars) are basically similar in that they all have solid surfaces. They all have metallic cores and silicate mantles with a crust near their surfaces.

All show signs of having been bombarded by large bodies during their early existence although those planets with atmospheres show weathering of these early features. The Earth shows the most weathering with very little early cratering remaining.

The larger, and more massive, of the group have atmospheres but these are very different from the Earth's. Active volcanoes have been 'seen' with radar echoes on Venus and huge extinct ones were discovered on Mars by the Mariner 9 probe in 1971. Our knowledge of the surfaces of all the planets rests almost entirely on the results from space probes.

More details on the planets are given in the individual sheets.

The Gaseous Planets:

The gaseous planets (Jupiter, Saturn, Uranus and Neptune) are far bigger than the terrestrial planets. They are also very different as they are enormous gas balls. Their interiors are believed to be composed of liquid forms of the gases, with a peculiar form of liquid hydrogen, which behaves like a metal, at their centres. The outer parts of these planets are composed of hydrogen, helium, methane and ammonia, with clouds of different molecules being visible as bands across the visible disk. All these planets have many satellites and they also all have ring systems, although the most extensive and beautiful is that of Saturn.

The Minor Planets and Comets:

Between the orbits of Mars and Jupiter there are very many small bodies orbiting the Sun. The largest of these is called Ceres, with a diameter of 1,003 km. They are called asteroids, or minor planets, and they fall into two different types. There are asteroids which are composed of silicate-type material and some which have carbonaceous composition. Most asteroids are small bodies with diameters of a few kilometres.

There are some asteroids which have orbits that depart markedly from the asteroid belt. Some have orbits that cross that of the Earth. These are called Apollo asteroids. It is possible that collisions between such asteroids and the Earth could occur but they are only likely on a time scale of tens of thousands of years. Even that low probability has worried some people because of the devastation that such a collision would cause, even possibly destroying life as we know it.

Comets are also small bodies with diameters of a few kilometres. Most comets are in highly eccentric orbits about the Sun and are only seen when they come close to the Sun. The radiation from the Sun evaporates some of the outer layers of ices which are blown away from the nucleus of the comet by the radiation pressure of the Sun's light and the particle stream from the Sun (called the solar wind). The stream of gas and dust can be a million kilometres long. It reflects the Sun's light and this is seen by us as the comet's tail.

Comets are believed to be left-over parts of the fragments of the original gas cloud that preceded the Sun and the planets. There is believed to be a ring of such bodies slowly circulating the Sun at a vast distance.
Occasionally one of the bodies in this cloud has, for one reason or another, its orbit disturbed and it starts to fall towards the Sun. When it is near the Sun it becomes visible for a short time before its orbit takes it back out to the outer reaches of the Solar System.
It is likely that these comets only have one journey into the Sun's vicinity.
Occasionally, however, during their path towards the Sun, comets pass close to one of the planets. The gravitational pull of the planet disturbs the comet's orbit and it can become a ellipse with a relatively short period.
Halley's comet is an example of such a comet.

Satellites of the Planets:

Details of the individual satellites of the planets are given on a separate leaflet.
Only Mercury and Venus have no satellites. The Earth has the Moon, Mars has two very small moons that are probably captured asteroids. Jupiter, Saturn, Uranus and Neptune have many, and Pluto, a peculiar planet about which we know very little, has one.

Planets Around Other Stars:

Astronomers have tried to identify planets around other stars for many years. The task is not an easy one because the light from the central star is so bright that it dazzles the small amount of reflected light coming from any planet. So the ways of detecting possible planets are indirect methods.

One method is to measure the positions of the nearby stars very accurately. If they have massive planets around them, then the effects of the gravitational force of the planets on the central star will be visible as very small deviations in its apparent motion. Some astronomers claim to have measured such deviations for some of the closest stars to the Sun, but these claims are not universally accepted. They could indicate the presence of planets of about Jupiter's size.

A variant of this method is to measure the radial velocity (the speed of approach or recession in the line of sight) of nearby stars. If they have planets then we would expect that the small deviations in velocity of the central star due to the gravitational forces of the planets would be seen. Such a programme will take years to complete and needs very exacting measurements.

Several stars similar to the Sun, have been investigated in this way. The star 51 Pegasi was the first to show variations indicating the likely presence of planets. It appears to have a planet of about Jupiter's mass going around it every four and a half days. This means that this system is very different from our Solar System.

The stars 70 Virginis and 47 Ursae Majoris were the first stars found to have planets more like those in the Solar System. Each star has a planet more massive than Jupiter with a period like that of a planet in the Solar System. Unfortunately, these planets are gas-giants and are unlikely to have any form of life.

A variant of this method uses the fact that 'pulsars' emit very accurately defined pulses. If we look at the pulses coming from a pulsar which has planets around it, then the pulse frequency will be altered. Planets have been found around one pulsar but they must be quite unlike planets in the Solar System and, almost certainly, can not sustain life.

Astronomers expect that many stars have planetary systems around them. One intriguing question, which has no answer, is whether there is life, and intelligent life, anywhere else. We think that, apart from the Earth, none of the planets in the Solar System have life on them.

The Planets Through a Small Telescope:

With a small telescope you should not expect to see all the fantastic detail that you can see in the space-probe pictures. There are interesting things to look for, however.

Mercury is difficult to see with the naked-eye but can be seen even in the daytime with a telescope, if you know where to look. A small disk can be seen when Mercury is close to the Earth, and phases (like the Moon's) can be seen.

Venus can be too bright for a telescope when the sky is dark. It is generally better to look in twilight or even in the day. The phase is easily seen but no surface details can be seen.

Mars is often disappointing because, except near close approaches to the Earth, it shows only a small disk. The polar caps can often be seen and some of the dark markings can be seen when Mars is close. Occasionally these are obscured by enormous Martian sandstorms which can take weeks to clear away.

Jupiter has definite atmospheric bands which can be seen in quite small telescopes. These change as the planet rotates and also can be very different from one season to another. The Great Red Spot can be seen.
Jupiter has four moons (the 'Galilean' satellites: Io, Europa, Ganymede and Callisto) which can be seen in binoculars and these have eclipses and occultations which are always interesting and were used for one of the first determinations of the speed of light.

Saturn is the most beautiful of the planets with its ring system. One of its moons (Titan) is easily seen.

Uranus and Neptune look like small greenish discs through a small telescope.
Pluto can only be seen in biggish telescopes.
Several of the minor planets can be seen, even in binoculars, but star charts and known positions will be necessary to identify them.

See also; Leaflets on each individual planet, 'Comets', 'Minor Planets', 'The Sun', and 'Pulsars'.

Produced by the Information Services Department of the Royal Greenwich Observatory.

PJA Thu Apr 18 10:42:05 GMT 1996

Pluto is now a "dwarf planet" by The IAU definition of "planet" and "dwarf planets" (August 24 '06).
See Observatorio ARVAL: Solar System Data.

On 13 September '06 the IAU Minor Planet Center assigned to Pluto the asteroid number 134340.
See IAU Minor Planet Center Circular 8747 (.pdf).

Updated: September 15 '06, June 23 '14

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