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Learn about the eight planets and five dwarf planets in our solar system, their features, moons, and missions. Find out what is a planet, and why some planets are more mysterious than others.
- Overview
- Planets of the solar system
- Planets of other stars
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planet, (from Greek planētes, “wanderers”), broadly, any relatively large natural body that revolves in an orbit around the Sun or around some other star and that is not radiating energy from internal nuclear fusion reactions. In addition to the above description, some scientists impose additional constraints regarding characteristics such as size ...
The idea of what exactly constitutes a planet of the solar system has been traditionally the product of historical and cultural consensus. Ancient sky gazers applied the term planet to the seven celestial bodies that were observed to move appreciably against the background of the apparently fixed stars. These included the Sun and Earth’s Moon, as well as the five planets in the modern sense—Mercury, Venus, Mars, Jupiter, and Saturn—that were readily visible as celestial wanderers before the invention of the telescope. After the idea of an Earth-centred cosmos was dispelled (see Copernican system) and more distinctions were made about the nature and movement of objects in the sky, the term planet was reserved only for those larger bodies that orbited the Sun. When the giant bodies Uranus and Neptune were discovered in 1781 and 1846, respectively, their obvious kinship with the other known planets left little question regarding their addition to the planetary ranks. So also, at first, appeared to be the case for Pluto when, during a concerted search for a ninth planet, it was observed in 1930 as a seemingly lone object beyond the orbit of Neptune. In later decades, however, Pluto’s planetary status became increasingly questioned by astronomers who noted that its tiny size, unusual orbital characteristics, and composition of ice and rock made it an anomaly among the other recognized planets. After many more Pluto-sized and smaller icy objects were found orbiting beyond Neptune beginning in the 1990s, astronomers recognized that Pluto, far from being unique in its part of the solar system, is almost undoubtedly one of the larger and nearer pieces of this debris, known collectively as the Kuiper belt, that is left over from the formation of the planets. (See also planetesimal.)
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In August 2006, after intense debate over the question of Pluto’s planetary status, the general assembly of the IAU approved a definition for a solar system planet that excluded Pluto. At the same time, it defined a new distinct class of objects called dwarf planets, for which Pluto qualified. Following the IAU proclamations, many scientists protested the definitions, considering them flawed and unscientific and calling for their reconsideration.
According to the 2006 IAU decision, for a celestial body to be a planet of the solar system, it must meet three conditions: it must be in orbit around the Sun, have been molded by its own gravity into a round or nearly round shape, and have “cleared the neighbourhood around its orbit,” meaning that its mass must be large enough for its gravity to have removed rocky and icy debris from its orbital vicinity. Pluto failed on the third requirement because it orbits partially within, and is considered to be part of, the Kuiper belt.
To be a dwarf planet under the IAU definition, the object must meet the first two conditions described above; in addition, it must not have cleared its neighbourhood, and it must not be a moon of another body. Pluto falls into this category, as do the asteroid Ceres and the large Kuiper belt object Eris, which was discovered in 2005 beyond the orbit of Pluto. By contrast, Charon, by virtue of its being a moon of Pluto, is not a dwarf planet, even though its diameter is more than half that of Pluto. The ranks of dwarf planets will likely be expanded as other objects known or yet to be discovered are determined to meet the conditions of the definition.
The planets and other objects that circle the Sun are thought to have formed when part of an interstellar cloud of gas and dust collapsed under its own gravitational attraction and formed a disk-shaped nebula. Further compression of the disk’s central region formed the Sun, while the gas and dust left behind in the midplane of the surrounding disk ...
Learn about the eight planets of the solar system and their features, origins, and classifications. Find out how Pluto became a dwarf planet and what are the criteria for planethood.
- The Editors of Encyclopaedia Britannica
Over 99.86% of the Solar System's mass is in the Sun and nearly 90% of the remaining mass is in Jupiter and Saturn. There is a strong consensus among astronomers [d] that the Solar System has at least eight dwarf planets: Ceres, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna.
- 4.568 billion years
- Proxima Centauri system (4.2441 ly)
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For other uses, see Planet (disambiguation). The eight planets of the Solar System with size to scale (up to down, left to right): Saturn, Jupiter, Uranus, Neptune (outer planets), Earth, Venus, Mars, and Mercury (inner planets) A planet is a large, rounded astronomical body that is neither a star nor its remnant.
Our solar system consists of our star, the Sun, and everything bound to it by gravity – the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune; dwarf planets such as Pluto; dozens of moons; and millions of asteroids, comets, and meteoroids.
Learn how the definition of a planet has evolved over time and what criteria astronomers use to classify planets and dwarf planets in our solar system. Explore the history, debate, and discoveries of planets and their moons.
10 Things. Our solar system is made up of a star, eight planets, and countless smaller bodies such as dwarf planets, asteroids, and comets. Our solar system orbits the center of the Milky Way galaxy at about 515,000 mph (828,000 kph). We’re in one of the galaxy’s four spiral arms.
