The solar system: what do we know?

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 nine known planets, while the Sun is a very ordinary star about halfway through its lifetime with another 5000 million years to go. The only reason the Sun does not look like the other stars to us 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, Neptune and Pluto.

The inner planets, comparatively speaking, are very close to the Sun and are solid. Since most of the visible objects in the universe consist of the gases hydrogen and helium, these planets are oddities. Much farther out are the two giants of the solar system, the gaseous planets Jupiter and Saturn. Beyond them are the outer planets, Uranus, Neptune and Pluto. As far as we know, Uranus and Neptune have a rocky core covered by a liquid mantle of ammonia and a surrounding outer gaseous layer.

Currently the textbook view is that there are only nine planets in our solar system although 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 a tenth 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.