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What is a Planet? The problem: the term “planet” has been around for thousands of years. Until recently, no one had ever bothered to formally define what makes a planet a planet. The International Astronomical Union (IAU) decided in 2006 that a planet was a celestial body that: 1. Is in orbit around the Sun, 2. Has sufficient mass for its self-gravity to overcome rigid bodily forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and 3. Has cleared the neighborhood around its orbit. The planets of our solar system can be divides into two groups: I. The telluric planets are spherical bodies with a crust of rock (Mercury, Venus, Earth and Mars). II. The gas giant (or “Jovian”) planets are spheres composed of gas and ice (Jupiter, Saturn, Uranus and Neptune). * Pluto is a solid body, whose composition consists of a variety of rocks & ices. A dwarf planet is a celestial body that: 1. Is in orbit around the Sun, 2. Has sufficient mass for its self-gravity to overcome rigid bodily forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and 3. Has not cleared the neighborhood around its orbit, and 4. Is not a satellite. Dwarf planets include (but are not limited to) Pluto, Ceres and Eris, others may be added after further study. Satellites (moons) are celestial bodies orbiting another celestial bodies of larger size. Exoplanets (extrasolar planets) are planets that are orbiting a star different from our Sun or a stellar remnant.