The Online Library of Knowledge

Planets and moons


Saturn and four of its moonsSaturn and four of its moonsA planet is an object orbiting a star. It can be made of rock, metal, liquid, gas or a combination of these. It does not share its orbit with any other significant objects. It is massive enough to have a rounded (rather than irregular) shape, as a result of its own gravitational pull. At the same time, it is not massive enough for nuclear fusion to take place inside it, as occurs inside stars like the Sun. In our own Solar System, there are eight true planets orbiting the Sun, our parent star. They are, in order from their distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (Pluto, once considered to be the ninth planet, has been reclassified as a dwarf planet). Observations of other stars made by astronomers using powerful telescopes indicate that many of these stars, too, have their own families of orbiting planets, called extrasolar planets or exoplanets. There could be billions of exoplanets in the Universe.

The planets of the Solar SystemThe planets of the Solar System
The formation of a planetThe formation of a planet

Origin of the planets

By studying meteorites, scientists have been able to work out the age of the Solar System itself: 4.6 billion years. At that time, a cloud of dust and gas drifted through space. The cloud became a swirling disc of matter, with a centre that became hotter and denser, eventually becoming the Sun. Particles of remaining dust clumped together and became boulders. These built up like snowballs into large balls of rock, called planetesimals, finally becoming planets.


The word "planet" comes from the Greek word planetes, meaning "wandering star".
