For the homonymous articles, to see Planet (homonymy).

One planet is one celestial body spherical revolving around one and not emitting .


Etymology and first observations

The word planet the word originates in planeta, itself derived from the word Greek ???????? (planêtes) which in the expression planêtes asteres indicate "stars moving", in opposition to which appears motionless on the vault of heaven.

This movement - very apparent with the naked?il - was observed very early by the men of all civilizations, but its complexity a long time remained a mystery for the astronomers until the identification of this apparent movement to the resultant of the elliptic races of and other planets around .

So certain planets of is visible the night in the sky, it is because they reflect the light of , contrary to the stars which shine of their own fire.

Planets of the solar system

1-mercury, 2-Venus, 3-Ground, 4-March, 5-Jupiter, 6-Saturn, 7-Uranus, 8-Neptune, 9-Pluto

There exists in nine planets orbits about it around : , , , , Jupiter, , , Neptune, and . Recently, the discovery of several objects similar to Pluton, inter alia 2003 UB313 (called Xéna), on the carpet the question of the definition of the term "planet gave". Some want to take from Pluton its traditional planet title, others want to extend this quality to largest transneptuniens belonging to girdle external asteroids. International astronomical union did not slice yet. Let us note that it , NASA announced the discovery of (90377) Sedna, that it described as "10E planet ", beyond the orbit of Pluto. After review of its statute it was decided that it did not act of a planet. (lower cf) It 2005 it is the astronomer Michael E. Brown and its team of Caltech who announced by the way of the UAI the discovery of one "10E planet ", this time measuring 1.5 times the diameter of Pluto: 2003 UB313.

To remember the order of planets of the solar system, one can think of the following sentence (the first letter of each word corresponds to the first letter of each planet, the most brought closer of with furthest away from the Sun): " Mhas Vieille Thandle Marie Joue Sur UN Nouveau Piano "or" Mone Vieux TU Mehave Jeté Sur U Nouvelle Planète ". See Mnemonic for the order of planets.

Seek of a definition of a planet

What a planet? The National Academy of Sciences American defines a planet as being a body of less than 2 masses joviennes revolving around a star. And in fact, around the Sun ten planets revolve by holding account of Xéna (2003 UB313) discovered in 2005. But historically, in 2003 Sedna had already been issued as being the 10eme planet of the solar system... How a planet can thus see its usurped title?

There does not exist yet of scientific definition of the word "planet". For the dictionary it acts of a "compact heavenly object, deprived of thermonuclear reactions, revolving around the Sun or, by extension, of a star". One can thus conclude from it by extension that any celestial body lower than two masses joviennes and not producing its own light is a planet. If we take the example of the Earth, indeed it is 317 times lighter than Jupiter and makes only considered the light that it receives Sun. Jupiter on the other hand emits 2.5 times more radiation than it do not receive any but it does not act of light. It is thus well a planet. But what is it occurred by it of Sedna which usurped a time the title of 10eme planet? It however answers the same criteria but the astronomers excluded it from the closed club of planets. Our definition thus misses clearly precision.

Any astronomer needs to build a scientific definition which can prove sometimes rather far away from the commonly allowed definition.

Four definitions were proposed in 2005 by Michael E.Brown who allow to have a clearer idea on the question:

1. Not seen purely historical. Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluton are planets and no other moreover.

2. Updated history. One can consider historical reasons holding account of the last discoveries. In this case Mercure to Pluto are planets like all new object larger than Pluton.

3. The gravitational sphere. Any object rounded because of its gravitational force, which do not produce its own light and which revolves directly around the Sun, and by extension of a star, is a planet. Corpo de gravité permet de classer le do la do par do régi de Cette définition de l'aspect Cérès girdle asteroids among planets.

4. Classes of populations. This definition of the term "planet" is most complex but also most satisfactory from a scientific point of view. A population is a whole of individuals belonging to the same species. In our context it acts of a whole of solitary objects sharing the same properties.

Michael Brown and her team recognize that there is not scientific definition which marries at the same time the conditions met in the solar system and our culture. As he wrote "for once I decided to let gain the culture. We, scientific, can continue our debates, but I hope that we will be ignored overall ".

The question is thus heard: in 2005 there are thus 10 planets in the solar system and a string of other populations of small bodies.

Other planetary systems

Since , year of discovered first (or exoplanète), it is known that there are planets around other stars. It is even probable that their presence is very current being given the number of planets since then identified (more than one hundred). Even if those which were detected up to now are all of giant planets (at least of the size of Jupiter or Saturn), the astronomers do not despair to highlight similar planets at the Earth, which could justify certain research of a life extraterrestrial. Between 1995 and 2005, nearly 150 exoplanètes were thus discovered.

In 2005, for the first time, of the astronomers could distinguish the light emitted directly by two planets, in spite of the dazzling gleam and near to their stars. Hitherto, the discoveries were only indirect, by noting the disturbances exerted by planets on their stars or by measuring a fall of luminosity at the time of an eclipse.

This time, two almost simultaneous discoveries were made by two teams different observing from different planets. But as the two teams both have used it infra-red space telescope American Spitzer, NASA decided to benefit from the occasion to announce the two at the same time discovered ones.

2005, a team of scientists American the discovery of the 155 announcedE exoplanète discovered since 1995. The characteristics of this planet are:

  • outdistance with the Earth: 15 light-years
  • temperature: between 204 and 361 degrees Celsius
  • mass estimated: 5,8 to 7,5 times that of the Earth
  • its star is Gliese 876 (also known under the name of BD-15°6290, one dwarf red in Aquarius)

In the review Nature 2005, itastrophysicist Polish Maciej Konacki California Institute of Technology (Caltech) revealed that it had discovered a gas giantess, around HD 188753, a triple star (a binary system revolving around a primary star of solar type). The planet, HD 188753 Ab, revolves around principal star and is of the type Jupiter heat, i.e. a gas giantess like Jupiter, but much nearer to its star than is to it Jupiter of the Sun - more near to its star only is not to it Sun, in fact! The current models (July 2005) of formation of such planets supposed a formation at a suitable distance for a giant planet, followed of a bringing together towards central star, which is not possible in the particular case of HD 188753.

Typology and formation of planets

It is considered that the planets are formed at the same time as them , by accretion and condensation of an interstellar matter and gas cloud under the influence of the gravitation. The planetary bodies are born from disturbances in the accretion disc, and attract in their turn various objects in their gravitational field. This phase of creation is carried out by successive impacts of objects which circulate in space at rather important speeds relative (several kilometers with tens of kilometers a second), which releases from heat with each collision. The lately formed and bombarded planets remains are thus mainly made up of materials in fusion, of which heat can preserve several million (even billion) years even if surface cools notably (enough to harden in the form of rocks or of gas).

Two great types of planets can appear: gas planets and them telluric planets (or rock).

See too

The list of the exoplanètes is available on the site:

Some drawings of exoplanètes on:

Gate Astronomy - Reach the articles of Wikipédia concerning astronomy.


  > French to English > (Machine translated into English)