Planet
A planet is a celestial body that orbits a star, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, and has cleared the neighbourhood around its orbit of other debris. The International Astronomical Union (IAU) formalised this definition in 2006, which reclassified Pluto as a dwarf planet. There are eight recognised planets in our Solar System, divided into terrestrial (rocky) planets and giant (gaseous or icy) planets.
The Eight Planets of the Solar System
| Planet | Type | Distance from Sun (AU) | Orbital Period (Years) |
|---|---|---|---|
| Mercury | Terrestrial | 0.39 | 0.24 |
| Venus | Terrestrial | 0.72 | 0.62 |
| Earth | Terrestrial | 1.00 | 1.00 |
| Mars | Terrestrial | 1.52 | 1.88 |
| Jupiter | Gas Giant | 5.20 | 11.86 |
| Saturn | Gas Giant | 9.58 | 29.46 |
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Related Terms
Solar System
The Solar System is the gravitationally bound system comprising the Sun and all the objects that orbit it, including eight planets, their moons, dwarf planets, asteroids, comets, and interplanetary dust. It formed approximately 4.6 billion years ago from the gravitational collapse of a giant molecular cloud. The Solar System extends from the Sun to the Oort Cloud, spanning distances of up to 100,000 astronomical units.
Planetary Orbit
A planetary orbit is the curved path followed by a planet as it moves around the Sun (or another star) under the influence of gravitational attraction. According to Kepler's First Law, planetary orbits are ellipses with the Sun at one of the two foci. The shape of an orbit is described by its eccentricity, where 0 represents a perfect circle and values approaching 1 represent highly elongated ellipses.
Natural Satellite
A natural satellite is a celestial body that orbits a planet or other non-stellar body under the influence of gravity, as opposed to an artificial satellite launched by humans. Natural satellites, commonly called moons, are held in orbit by the gravitational attraction of their parent body. Earth's Moon is the most familiar example, but the Solar System contains over 200 known natural satellites, with Jupiter and Saturn each hosting more than 80.
From Latin "planeta" and Greek "planetes" (wanderer, from "planasthai" meaning to wander), because planets appear to move against the fixed background of stars. Ancient astronomers called them "wandering stars".