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Planets

Kepler 22b, an exoplanet close to a star in the constellation of Cygnus, 600 light years away from EarthA 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.



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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