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The Nine Planets is an encyclopedic overview with facts and information about mythology and current scientific knowledge of the planets, moons, and other objects in our solar system and beyond.
- Planets
The Planets of the Solar System Detailed information and...
- Solar System
The main asteroid belt (not shown) lies between the orbits...
- Objects
Latest Buyer’s Guides. How to choose your telescope...
- Questions
Frequently asked space questions to help you learn about...
- Buyer’s Guides
Space and Astronomy Product Reviews We review the best...
- Gas Giants
Latest Buyer’s Guides. How to choose your telescope...
- Moon Phases
The darkest part of the Moon is the one always facing away...
- Constellations
A constellation is an area on the celestial sphere in which...
- Mercury Mercury facts →
- Venus Venus facts →
- Earth Earth facts →
- Mars Mars facts →
- Jupiter Jupiter facts →
- Saturn Saturn facts →
- Uranus Uranus facts →
- Neptune Neptune facts →
- Ceres Ceres facts →
- Pluto Pluto facts →
- Haumea Haumea facts →
- Makemake Makemake facts →
- Eris Eris facts →
- The Sun Sun facts →
- The Moon Moon facts →
- Comets Comet facts →
- Asteroids Asteroid facts →
- The Asteroid Belt Asteroid belt facts →
- Planets
The solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. There are five officially recognized dwarf planets in our solar system: Ceres, Pluto, Haumea, Makemake, and Eris. What is a Planet? Inner Planets. The first four planets from the Sun are Mercury, Venus, Earth, and Mars.
- Overview
- Planets of the solar system
- Planets of other stars
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.)
Britannica Quiz
Science Quiz
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 ...
- The Editors of Encyclopaedia Britannica
A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself.
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. Beyond our own solar system, there are more planets than stars in the night sky.
Jun 27, 2019 · What Is a Planet? The Short Answer: A planet must do three things: it must orbit a star, it must be big enough to have enough gravity to force a spherical shape, and it must be big enough that its gravity cleared away any objects of a similar size near its orbit.
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A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
