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Planets

Saturn and four of its moons. Three (Tethys, Dione and Rhea) are visible against the darkness of space, and another smaller moon...Read More >>Saturn and four of its moons. Three (Tethys, Dione and Rhea) are visible against the darkness of space, and another smaller moon (Mimas) is visible against Saturn's globe.A 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 System. In order of distance from the Sun on the left-hand edge of the diagram, they are: Mercury,...Read More >>The planets of the Solar System. In order of distance from the Sun on the left-hand edge of the diagram, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The main part of the Asteroid Belt, made up of millions of small, planet-like objects, lies between Mars and Jupiter. The Kuiper Belt lies beyond Neptune and includes the dwarf planet Pluto. It contains many thousands of small icy objects.
Astronomers believe that a rocky, Earth-like planet is forming around one of the stars in a two-star, or binary star, system...Read More >>Astronomers believe that a rocky, Earth-like planet is forming around one of the stars in a two-star, or binary star, system called HD 113766, 424 light years from Earth. The brown ring is a belt of dust and rock fragments, which will eventually clump together to form larger and larger objects, finally becoming a planet. The outer grey ring is a belt of ice particles.

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".

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