Planets and moons


Kepler-22b, an exoplanetKepler-22b, an exoplanetA 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 planets, including Earth, orbiting the Sun, our parent star. Observations of other stars made by astronomers using powerful telescopes indicate that some of these stars, too, have planets, called extrasolar planets or exoplanets. There could be billions of exoplanets in the Universe.

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".

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