Planet

A planet in common parlance is a large object in orbit around a star that is not a star itself. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. The International Astronomical Union lists nine planets in our solar system, based on historical consensus. Since the term planet has no scientific definition, many astronomers contest that figure, with some saying it should be lowered to eight (removing Pluto from the list), and others claiming it should be raised to fifteen, about twenty, or even dozens, depending on the definition used.

Planetary formation

It is not known with certainty how planets are formed. The prevailing theory is that, when a protostar forms from a condensing nebula, the remnant of the nebula becomes a thin disc of gas and dust that revolves around the protostar. Localised mass concentrations within this disc form increasingly dense pockets of matter, which then collapse inward under gravity to form planets. When the star's core ignites its solar wind blows away the remaining material, leaving a solar system like our own.

Related Topics:
Protostar - Nebula - Gravity - Solar wind

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This theory is being greatly challenged with the discovery of extrasolar systems that vary greatly from our own.

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