Planet

This article is about the astronomical object. For other uses, see Planet (disambiguation).

Planetary-sized objects to scale: Top row: Uranus and Neptune; second row: Earth, white dwarf star Sirius B, Venus; bottom row (reproduced and enlarged in lower image) – above: Mars and Mercury; below: the Moon, dwarf planets Pluto and Haumea.

Planetary-sized objects to scale:
Top row: Uranus and Neptune; second row: Earth, white dwarf star Sirius B, Venus; bottom row (reproduced and enlarged in lower image) – above: Mars and Mercury; below: the Moon, dwarf planets Pluto and Haumea.

A planet (from Greek πλανήτης αστήρ planētēs astēr "wandering star") 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. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.

History

Further information: History of astronomy and Definition of planet

See also: Timeline of solar system astronomy

Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539

The idea of planets has evolved over its history, from the divine wandering stars of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.
The five classical planets, being visible to the naked eye, have been known since ancient times, and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights πλάνητες ἀστέρες (planetes asteres "wandering stars") or simply "πλανήτοι" (planētoi "wanderers"),^[5] from which today's word "planet" was derivedIn ancient Greece, China, Babylon and indeed all pre-modern civilisations, it was almost universally believed that Earth was in the center of the Universe and that all the "planets" circled the Earth. The reasons for this perception were that stars and planets appeared to revolve around the Earth each day,^[10] and the apparently common-sense perception that the Earth was solid and stable, and that it was not moving but at rest.
The name for planets in Chinese astronomy had the same motive as the Greek name, 行星 "moving star". In Japanese during the Edo period there were two competing terms, 惑星 "confused star" and 遊星 "wandering star". In modern Japan, terminology was unified in favour of 惑星, but in science fiction the alternative term 遊星 retains some currency.

Extrasolar planet definition

In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated the following working definition, mostly focused upon the boundary between planets and brown dwarves:^[2]

Artistic comparison of Eris, Pluto, File:EightTNOs.pngMakemake, Haumea, Sedna, Orcus, 2007 OR₁₀, Quaoar, and Earth (scales are outdated)

1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun^[39]) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

This definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals.^[40] Although temporary, it remains an effective working definition until a more permanent one is formally adopted. However, it does not address the dispute over the lower mass limit,^[41] and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b.

Solar System

Planets and dwarf planets of the Solar System. (Sizes to scale, distances not to scale)

The inner planets. From left to right: Mercury, Venus, Earth and Mars (sizes to scale).

The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale, distances not to scale)

Main article: Solar System

See also: List of gravitationally rounded objects of the Solar System

According to the IAU's current definitions, there are eight planets and five dwarf planets in the Solar System. In increasing distance from the Sun, the planets are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.
The planets of the Solar System can be divided into categories based on their composition:

• Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System, while Earth is the largest terrestrial planet.
• Gas giants (Jovians): Planets largely composed of gaseous material and significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, while Saturn is one third as big, at 95 Earth masses.
  • Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass (only 14 and 17 Earth masses), and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
• Dwarf planets: Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets

Extrasolar planets

Exoplanets, by year of discovery, through 2011-07-10.

The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the more than 573 extrasolar planets discovered by August 10, 2011,^[74] most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed.^[74] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.^[75]
There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,^[74] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.^[76] These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).^[77]
Another new category are the so-called "super-Earths", possibly terrestrial planets larger than Earth but smaller than Neptune or Uranus. To date, about twenty possible super-Earths (depending on mass limits) have been found, including OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,^[78][79] Kepler 10b, a planet with a diameter roughly 1.4 times that of Earth, (making it the smallest super-Earth yet measured)^[80] and five of the six planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass,^[81] while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone.^[82] However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient greenhouse gases.^[83]

Size comparison of HR 8799 c (gray) with Jupiter. Most exoplanets discovered thus far are larger than Jupiter, though discoveries of smaller planets are expected in the near future.

It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There may also exist a class of hot Jupiters, called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered

Magnetosphere

Schematic of Earth's magnetosphere

One important characteristic of the planets is their intrinsic magnetic moments which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.^[125]
Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.^[125] In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future manned missions to its moons). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.