Planet

A planet (from Greek πλανήτης, meaning "wanderer") is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.

The term planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Though the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits to be not circular, but elliptical. Astronomers saw that, like Earth, the planets rotated around tilted axes, and some share such features as ice-caps and seasons. Since 1992, the discovery of hundreds of planets around other stars, called extrasolar planets caused scientists to understand that Earth is not a unique planet: many of its features are shared with extrasolar planets. As of November 2009, there are 405 known extrasolar planets, ranging from the size of gas giants to that of terrestrial planets.

There are three general kinds of planets. The smallest are known as dwarf planets and range from 800 to 2500 kilometers across. Dwarf planets have been recently established as planets; until recently, smaller ones were regarded as asteroids, and larger ones were regarded as small terrestrial planets. Terrestrial planets, like Earth, Venus, Mercury, and Mars, are bigger than dwarf planets, and like dwarf planets, have a rocky crust. Terrestrial planets are the closest to their star. Finally, gas giants are balls of gas too small to be categorized to be stars, and differ from stars in that they do not generate their own heat.