According to the last definition of the international astronomical Union (UAI), “a planet is a Celestial body:
- which is in Orbite around the Sun;
- which has a sufficient Masse so that its Gravité overrides the forces of cohesion of the solid body and maintains it in hydrostatic balance (form Sphérique);
- which eliminated any body moving on a Orbite near”.
This definition was approved the August 24th 2006, in fence of the 26e General meeting of the international astronomical Union by a vote by show of hands of approximately 400 scientists and Astronome S after ten days of discussions. In complement, the UAI created a new class of objects: the dwarf planets whose first members are (134340) Pluto, (1) Cérès and (136199) Éris.
By extension, any star answering these criteria and revolving around an other star which the Sun is described as Exoplanète.
EtymologyThe word planet originates in the Latin word planeta , itself derived from the Greek word πλανήτης ( planêtês ) which in the expression πλανήτης αστήρης planêtês astêrês indicates “stars moving” (or “wandering star”), in opposition to the star S which appear motionless on the Vault of heaven.
This movement - apparent if one follows planet in the one night old sky to the other - 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 the Ground and another planets around the Sun.
If the planets of the Solar system are visible the night in the sky, it is because they reflect the sunlight, contrary to the stars which shine of their own fire.
Planets of the solar system
See also: Solar system
There exist eight planets in the solar system, in the order starting from the Sun:
Before the exclusion of Pluto like ninth planet of the solar system, one could remember about planets thanks to the famous following mnemonic sentences: Hello my old Ground! Me, I am a new planet , Me here very nice: I am a new planet , My old theater often plays me a new part or My old man you threw me on a new planet , whose initial ones follow the order of the stars of our solar system (of the Sun in Pluto for the first, of Mercury in Pluto for the following ones).
In the Years 2000, the discovery of several objects similar to Pluto, inter alia (136199) Éris, raised the question of the definition of the term “planet”. Some did not want to regard Pluton any more as a planet, others wanted to extend this quality to the largest objects transneptuniens pertaining to the girdles external asteroids. The international astronomical Union sliced the August 24th 2006: following the development of a new definition, Pluton is a dwarf Planet and the solar system counts nothing any more but 8 planets.
Let us note that the March 15th 2004, NASA announced the discovery of (90377) Sedna, that it called “10th planet”, beyond the orbit of Pluto. After review of its statute it was decided that it was not about a planet. The July 30th 2005, it is the astronomer Michael E. Brown and her team of the Caltech who announced by the way of the UAI the discovery of one “10th planet”, measuring this time once and half the diameter of Pluto: who will become (136199) Éris.
DefinitionThe definition of a planet such as resumption above known as in substance which a body must present a mass from at least 5 × 1020 kg and a diameter of at least 800 km to be regarded as a planet.
For the dictionary, whose definitions have only one academic and nonscientific value, a planet is one celestial object compact, deprived of reactions Thermonuclear S (or in the past: without clean light), revolving around of the Sun or, by extension, of a star .
In 2003, Sedna already had been issued as being it tenth planet of the Solar system, but much of astronomers were reticent to grant this statute to him. In fact, the astronomers were not unanimous on the definition of a planet and the UAI thus solved the question.
Until 2006, the American National Academy off Sciences defined a planet as being a body of less than two masses joviennes revolving around a star. But this definition did not take account of recent discovered, of which those of (136199) Éris (in 2005), of (90377) Sedna and other objects of the Ceinture of Kuiper.
Planet vs starClassically, the term “planet” is opposed to that of “star”. Planet and star differ in this that the luminous energy radiated by a planet does not come from its own center but from the star around which it revolves (any planet emits electromagnetic radiations, generally in the Infrarouge because of its weak Température). Even if this opposition between production and reflection of light keeps an essential share of its relevance, it poses some conceptual problems of definition.
What today the most usefully distinguishes concept of planet and that of star is the mode of formation:
- the formation of a star results from the collapse of a Sphère of Gaz;
- the formation of a planet results from the aggregation of Poussière S in a disc, followed or not gas accretion, according to the mass of the core.
PlanetsThe planets produce in spite of very a little energy, detectable in infra-red. For the Ground, it unimportant is seen space (: 4000 times less than what is received Sun) but it are more sensitive for Jupiter, Saturn or Neptune: in IR, they return 2 to 2,5 times more energy than they do not receive Sun from it. This property can be made profitable for the search for exoplanètes, those becoming proportionally more emissive in the infra-red than stars.
In another order of idea one can imagine planets wandering, formed around stars but then released from their gravitational bond by ejection in a system with NR body and not reflecting more this fact no energy stellar.
StarsThe brown dwarf smallest stars were never enough massive to generate a process of fusion thermonuclear in their center, with share most massive which burn the Deutérium of their envelope during a few tens of million years before cooling. The dwarf brown ones radiate a great number of billion years but not according to the traditional process (proton/proton or CNO); they do not belong of this fact to the principal Séquence.
Recent proposals for a definitionAny astronomer needs to build a scientific definition which can prove sometimes rather far away from the commonly allowed definition.
- purely historical Point of view. Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluton are planets and no other moreover. Updated
- Historical. One can consider historical reasons taking account of the last discoveries. In this case Mercure to Pluto are planets like all new object larger than Pluton.
- 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. This definition of the aspect governed by gravity makes it possible to classify the body Cérès of the Ceinture of asteroids among planets.
- 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 is about a whole of solitary objects sharing the same properties.
Michael Brown and her team recognize that there does not exist 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 . For him, the question is thus heard: in 2005 there thus exist ten planets in the solar system and a string of other populations of small bodies.
Contrary, much of astronomers prefer to consider that there exist eight planets (of Mercury with Neptune), and besides that because of their characteristics, Pluton and the other bodies of the Ceinture of Kuiper, that they are small or large, are of the objects of another type (which one indicates under the generic term of transneptuniens).
Other planetary systemsSince 1995, year of discovered first planet extrasolaire, one knows that there exist planets around other stars. It is even probable that their presence is very current being given the number of planets since then identified (254 with the October 15th 2007, to see '' The Extrasolar Planets Encyclopedia '' of the Observatory of Paris for an up to date figure), whereas the techniques which one has for the moment make it possible to detect only planets massive and close to their star. Even if those which were detected up to now are all of the giant planets (at least of the size of Jupiter or Saturn), the astronomers do not despair to highlight planets similar to the Earth, which could justify certain searchs for a life Extraterrestre. Between 1995 and 2005, nearly 170 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 have both used the infra-red space telescope American Spitzer, NASA decided to benefit from the occasion to announce the two at the same time discovered ones.
- Distance to the Earth: 15 light-years;
- Temperature: between 204 and 361 degrees Celsius;
- Mass estimated: 5,80 to 7,50 times that of the Earth;
- Its star is Gliese 876 (also known under the name of BD-15°6290, a Dwarf red in the Aquarius).
In the Natural review of the July 14th 2005, the astrophysicist Polish Maciej Konacki of the California Institute off Technology (Caltech) revealed that he 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 Mercure 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 by a bringing together towards central star, which is not possible in the particular case of HD 188753.
Formation of planetsOne considers that the planets are formed at the same time as them star, by Accrétion and Condensation of a gas cloud and dust under the influence of the gravitation. All the models of planetary formation thus start with the formation of one, even of two or more, stars within a collapse, followed by accretion of dust in the residual disc circumstellaire.
It is thus necessary to start by saying two words of the stellar formation in the galactic atmosphere.
A Galaxie is a formed flattened autogravitant more or less ionized gas body (more or less hot in other words) which is laminated according to the thickness by gravity. The median plane, called the galactic floor , densest, corresponds could one say to terrestrial troposphere and it is in its center that is held the star formation, comparable to gas precipitates, followed of a partial restitution under the mode Nébuleuse planet gear or Supernova, according to the mass of star. The restored gas is enriched in heavy elements (C, NR, O, If, Al, Mg, Fe, etc) which condense in dust, whose later role is essential for the phenomenon which occupies us.
The stars are born in group within vast molecular complexes which strew the galactic floor. These complexes (or clouds) molecular are thus named in reference to the fact that hydrogen is presented to it in the form of molecule of dihydrogene H-H. These “areas H2” (not to be confused with less dense but formed strongly emissive hydrogen area HII ionized under the effect of a close radiation) are particularly dense (more than 10.000 atoms/Cm3 against 10 or less in the neighborhoods, constituting areas HII) and cold (typically 10 to 100 K against typically 10.000 K neighborhood). The formation of these areas introduces us with the central phenomenon of the stellar formation (which will reproduce a little differently for gas planets, at the time of accréter: gravitational collapse).
There is collapse when the force of gravity created by the cloud exceeds the thermal pressure resulting from the couple temperature-density. Collapse is typically a self-sustained phenomenon: as the molecules of the cloud move towards the center, its density increases and with it the gravity which it generates.
But the process cannot however be continued that if there is average to evacuate thermal energy. While contracting, i.e. while falling freely on itself, the cloud converts its gravitational energy into kinetic energy and this one generates a thermal pressure, at the time of many shocks. It is thus necessary that the cloud radiates, phenomenon facilitated by the increasing density, which increases the probability of the molecular shocks, on the occasion inelastic.
It is thus formed in the center a gas core, called for the time being protostar on which fall a flow from gas at a speed which believes with the gravity of the star, i.e. with its mass. A body in freefall strikes the surface of the star with a speed equal to the escape velocity of this star. It quickly measures of ten km/s for protostar. With the assessment, the gravitational energy of the cloud (Eg = GM ² /r) is converted into heat on the surface of the young star and this represents a considerable radiated quantity. The star incipient, before even starting the process of fusion of hydrogen has a temperature of surface 10 times higher than that which it will adopt in principal sequence (either for the Sun of about 60.000 K against 6.000 K thereafter). The intense radiation of the proto star, located in UV, thus allows the continuation of the process, as long as the cloud which overhangs it transparent remainder.
This transparency is contrecarée by the presence of dust of increasing density with the collapse and which opacifies it. However at the same time as the cloud contracts, it increases its angular velocity of rotation in order to preserve its moment M of rotation.
In any point, M ~ w.r with W angular velocity, in rad.s-1 and R the distance to the center of gravity. If the R average decreases, W increases: the poles are depopulated consequently in favor of the equator and this accelerated whirling flattens the cloud.
The poles discharged from matter, the star can radiate freely on a half of its solid angle. On the other hand, the rotation of this disc (where will be held the planetary formation) prevents it from crumbling front what blocks the process in the absence of capable mechanisms to dissipate its energy of rotation.
This disc is extraordinarily thin, compared to any shape of state of the matter which can be conceived on Earth. It is however about a dense oasis of gas and dust, on an interstellar scale. A metric body of size in orbit in its center puts less than 10 My to fall on protostar, by dissipating its gravitational energy by frictions.
It is in this interval that will be able to be formed planets.
Phase a: formation of flocculate centimetricAt the beginning, the cloud has an opacity on a considerable thickness (about 10 to 30 UA). Dust responsible for this opacity falls gently, at one to ten meters a second, within the thin Gaz, towards the Plan of rotation. In: approximately 10000 years, the Protostar obtains a fine disc of dust (a few kilometers thickness) enclosed in a gas wafer which keeps its initial thickness, or little is necessary oneself some. Dust, during its fall within a turbulent gas forms randomly flocculate which can reach centimetric sizes (either a profit of four orders of magnitude). Aggregation results from the simple forces of contact between grains.
Phase b: formation of the planétésimaux oneBefore these grumeaux dusty reached a kilometric size, they generate a sufficient hydronynamic trail to make them plunge towards the surface of young star in less one century (for a body of one meter located at an astronomical unit). It is thus about a critical stage. The phase of formation going the centimetre to the kilometer (that is to say a profit of five orders of magnitude) is one of with difficulty the modélisable, the meetings randomly at high speed (several kilometers with tens of kilometers a second) being as much likely to pulverize the aggregate than to form a more massive body able to box the later shocks.
Because of its higher Mass, one of the bodies manages to attract Gravitation nellement Poussière S of the planetary furrow in a Périmètre which exceeds its Diamètre. At the conclusion of this stage, it can reach the kilometer and is at the same time gravitational for what surrounds and resistant in term of trail. It was then formed a Planétésimal, whose diameter can reach five to ten kilometers and the mass is about thousand billion tons. It will become a small body (Astéroïde or Comet) or a planet.
At this stage, the system is populated billion comets coexisting with solid bodies energy of the micrometer to the kilometer.
Phase C: formation of the planetary hearts
The planet formation from the planétésimaux ones lasts approximately: 100000 years and was the subject of digital simulations which give the following image of it:
- at the beginning, of the random collisions within a whole of billion planétésimaux generates the growth of some at the expense of the others;
- as soon as a planétésimal gained a mass largely higher than the average mass of the planétésimaux neighbors, it can absorb all that is in its zone of gravitational influence;
- once the vacuum made around him, its growth stops material fault: there is then business in a planetary heart which one says that it reached its “mass of insulation”. With a UA, this mass of insulation represents approximately the tenth of the terrestrial mass and corresponds to the agglomeration of approximately a billion planétésimaux.
Phase D: formation of the telluric coresThe digital simulations show that the circular orbits of the planetary hearts are disturbed by the mutual interactions gravitational and tend to become elliptic, which supports the collision of the hearts and their growth by agglomeration. This phase also cleans the system in formation of innumerable planétésimaux residual which, if they pass very close to of too close planets in formation are destroyed by Force of tide or are expelled in interstellar space.
In a disc circumstellaire of approximately thousandths of solar mass, a Planet telluric (or rock) can be formed into 10 to 100 million years and the scenario which precedes gives an account of their formation successfully.
Phase E: formation of the gas envelopesTo explain the formation of the gas planets - some: 100000 years with 1 million years - as Jupiter or Saturn in a minimal disc of mass, such as previously definite is more problematic.
The giant planets undoubtedly consist of a solid heart (metals + Silicate S + planetary ices) which must then capture by gravity a gas envelope, which requires the attack of a critical mass in-on this side which the Pression due to the energy released by the planétésimaux ones which returns in collision with the planetary heart is sufficient to be opposed to the gravitational collapse of surrounding gas, and the gas envelope remains not very important. With the site of the gas giants of our system, the critical mass is about 15 terrestrial masses what corresponds about to the mass of Neptune or of Uranus.
Beyond the critical mass the accretion will stop only after exhaustion of gas available in the fraction of the disc where the planet was formed, thus opening a furrow in the disc protoplanétaire. One thus obtains gas giants of the Jupiter mass (three hundred terrestrial masses) or Saturn (hundred terrestrial masses).
Still it for that is necessary that all the disc did not already fall down on star. However its lifespan is only of one with a few tens of million years.
Simulations show that to form planets of the Jupiter and Saturn mass the disc must have a mass of three to five times higher than the sufficient minimal mass with the formation of telluric planets in the time assigned by the lifespan of a disc.
Principles of naming of planetsThe names of planets of the solar system are allotted by the commissions of the international astronomical Union (UAI). Those adopt the names of the gods of the Roman Mythologie, in a coherent way. Because of his red color, one named the fourth planet Mars in reference to the Roman god of the war (and thus of the blood) and, more recently, the planet Eris, goddess of the discord for the dwarf Planet whose discovery obliged the astronomers to redefine the concept of planet to the detriment of (134340) Pluto, smaller.
- Solar system
- interstellar Sun
- SETI (project Search for extraterrestrial intelligence )
- Terraformation: how to make a planet livable
- UAI: International Astronomical union
- astronomical Symbols
- '' The Extrasolar Planets Encyclopedia '' of the Observatory of Paris, the list of the exoplanètes is available on the site.
- official Definition in English on the site of the UAI.
- Some drawings of exoplanètes on '' Alien Worlds ''
- planets on the site of NASA
- '' The Planets ''
- Interview on the classification of the solar system
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