Movements in the solar system

The solar system – what's in our locality?

We live on a planet called the Earth that orbits the Sun once every 365 days. The Earth is one of eight known planets, while the Sun is a very ordinary star about half way through its lifetime with another 5000 million years to go. The only reason the Sun does not look like the other stars is because it is much nearer to us. Even so, at 147 million kilometres (93 million miles) away, it still takes about 8 minutes for light to reach us from the Sun. All the planets orbit the Sun in more or less the same plane. This is called the plane of the ecliptic.

The planets are not evenly spaced but are in three groups: the inner planets, Mercury, Venus, the Earth and Mars ; the gas giants, Jupiter and Saturn; the outer planets, Uranus, and Neptune.

Pluto lost it status as a planet in 2006 and analysis of the orbits of comets (see later in this episode) has suggested that there may be another planet, between 1 and 10 times the size of Jupiter. This planet, if it exists, is about three trillion miles out from the Sun and is invisible to telescopes. Expect such claims and counter claims to arise from time to time.

Knowledge about the planets, ideas based on evidence

Much of the information about the planets in the solar system has been determined by observation. The planets Venus, Mars, Jupiter and Saturn can all be seen with the naked eye.

However much better information can be gathered with a telescope and, better still, by satellite and space probes.

The name planets comes from the Greek word planetos, which means wanderer. This is because, unlike the stars whose position relative to each other is fixed, the planets appears to wander across the sky, first going ahead of the fixed stars and then appearing to stop and fall behind. As you can imagine, trying to devise theories to explain why this happened was a major preoccupation for many astronomers for much of history.

Much of our knowledge about planets has come from simple observations with telescopes.

For example, telescopes have revealed the rings of Saturn, the great red spot of Jupiter, the polar ice caps of Mars, the meteorite craters on the Moon and the clouds on Venus.

In the field of astronomy, scientists are limited in the extent to which they can carry out direct experiments on planets (as they have done on Mars and Venus). They are mainly limited to observation, so they must develop theories that are consistent with observational data.

For example:

• The fact that Venus has clouds means that it must have an atmosphere.
• The fact that the Moon has large craters over its surface and no visible signs of water suggests that it does not have an atmosphere. The craters are produced by meteors hitting the surface. In the cases of the Earth and Venus, the meteors are burnt up as a result of friction with the atmosphere. Those that collide with the Moon (and with Mercury, which also has no atmosphere) go flying in with nothing to slow them down.
• The observation that Mars has polar caps, that shrink and grow with the seasons, suggests that it must have an atmosphere in which snow and ice can vaporise and then condense.

In the past 50 years, enormous advances have been made in our knowledge of the planets. The improvements began with radar observations but the big leap forward came with the ability to fly space probes to the planets and their moons or in the case of our own Moon, to visit it.

Most of our recent knowledge about Jupiter and Saturn has come from a succession of space probes sent to them in the 1970s and 1980s.

The outer planets, Uranus and Neptune, have been visited by a single space probe, Voyager 2, but Pluto has not. Consequently we know little about Pluto other than what we can calculate from its period of orbit and observe through telescopes.