Exoplanète

A exoplanète or planet extrasolaire indicates a Planet orbiting around a star other than the Sun.

For a long time, the existence of exoplanète could not be proven by the Observation. The distance, but also the lack of luminosity of these so small celestial objects compared to stars around they orbit made detection impossible. It is only in the Années 1990 that the first are detected in an indirect way. Since, several hundreds of planets are observed and their number increases quickly.

A skew in the methods of detections used makes that one mainly detected rather particular planets compared with those present in the Solar system. The discovery of these planets obliged the astronomers to re-examine the models of formations of the planetary systems which they had worked out while being based on the Solar system.

Since the methods improved, number of work of this field aim at highlighting planets resembling the Earth and being able to lodge a Vie comparable with that which exists there.

History

First steps

For a long time the Man wonders about the question: “ do we Somme alone in the Universe? ”. What involves the question of knowing if there exists or not of another planets on which could develop other forms of Vie. Christiaan Huygens is the first Astronome to plan the use of the instruments of observation in order to detect such planets.

During the 20th century, thanks to the technological advancements of the Telescope S, such as the detecting with coupling of load (CCC), the Image processing, as well as the Space telescope Hubble, which allow more precise measurements of the movement of the stars, much of astronomers hoped to detect planets extrasolaires. In the Years 1980 and at the beginning of the Years 1990, some advertisements are made, included in the media, then, after checks (that can take months, years), finally contradicted (it is the force of the scientific Méthode). The astronomical community is in despair, and some conclude from it already that the solar system would be perhaps only one singularity… It is necessary to wait the year 1995 so that the discovery of the first exoplanète is confirmed.

Discovered

The discovery of the first planet extrasolaire was announced the October 6th 1995 by Michel Mayor and Didier Queloz (of the Observatoire of Geneva), according to observations which they carried out with the Observatoire of High-Provence thanks to the method radial speeds. The star host is 51 Pegasi, in the constellation of PEGASE, to approximately 40 light-years of the Earth.

Since then, more than 200 planets were detected, of which much by a team carried out by Geoffrey Marcy the the University of California to Berkeley.

More half were discovered with the UNIGE (Université of Geneva) by international teams.

The first system where several planets were detected was Upsilon Andromedae, in the constellation of Andromède. The second was 55 Cancri. This last is more the known planetary great system to date (except ours) because it contains at least five planets.

The majority of planets detected for the moment are gas giant having an eccentric orbit very , some finally proved to be brown dwarf . The fact of primarily discovering gas giants close to their star is generally interpreted like a skew of the observation: it is much simpler to discover a massive planet turning quickly around its star by the method the radial speed which detects planet by interpolating its presence by the fluctuations of the trajectory of star.

In the first half of 2005, a polemic agitated the astronomical world. Teams of NASA and ESO announced discoveries thanks to VLT and with the Space telescope Spitzer. Finally, it seems that Europe obtained the first direct planet extrasolaires images well. In fact, they orbit around dwarf the brown GPCC-2M1207 and of star GQ Lupi. However, the companion of GQ Lupi is probably dwarf brown.

Inventory

At November 13rd, 2007 one counts:

265 exoplanètes
252 planetary systems including 26 multiples

Exoplanètes remarkable

It is the November 27th 2001 which one detects the first gas giantess, Osiris, containing Oxygène and Carbone in its atmosphere. This planet being very close to its star, she sees her atmosphere being puffed up by the latter. This phenomenon pushed the scientists to imagine a particular class of exoplanètes, the planets chtoniennes, which are rock residues of gas giants to the atmosphere puffed up by their star.

the August 25th 2004, a planet, Driven Arae C or the Venus de Mu Arae , of 14 terrestrial masses was discovered. This mass being in on this side theoretical limit of 15 terrestrial masses in lower part of which a planet can be telluric, the scientists think that it can be a question of a very large rock planet, the first of this type which would be thus discovered. Nevertheless, it can just as easily be a question of a very small gas planet.

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 looking at the disturbances exerted by planets on their stars or by measuring a fall of luminosity during a transit. 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 American infra-red space telescope Spitzer, NASA decided to benefit from the occasion to announce both at the same time discovered.
It is however important to specify that both exoplanètes observed had already been detected before thanks to the technique radial speed.

the July 14th 2005, the astrophysicist Maciej Konacki of the California Institute off Technology (Caltech) announced in the review Nature the discovery of a exoplanète (HD 188753 Ab) in a system of three stars which is at 149 light-years of the Earth. Thanks to the telescope Keck 1 of Hawaii, it could find this planet whose revolution around its star is done in less than four days. The current models (July 2005) of formation of the planets do not explain how such a planet can be formed in a so unstable environment from a gravitational point of view. This planet was called “Tatooine planet” by its discoverer in homage to the planet of the same name in the film the Star Wars . Sights of artists of stellar system HD 188753 are available on the site of NASA.

the January 26th 2006, Probing Lensing Anomalies NETwork (PLANET) directed by French Jean-Philippe Beaulieu discovered the planet OGLE-2005-BLG-390Lb which seems to be the known first exoplanète telluric. This planet is located at 22 000 light-years of the Earth. Its mass is worth approximately five times that of the Earth, its temperature (average of surface) is estimated at -220°C (53K), which lets suppose that it is about a solid planet.

the May 17th 2006, a team of planet researcher (of which Michel Mayor forms part) announces the discovery, thanks to the spectrograph HARPS, of three planets of the " type; neptunien" around star of the solar type HD 69830. The masses are respectively of 10,12 and 18 times the land mass (what is relatively weak, Jupiter made 317 times the mass of the Earth). This system probably has a belt of asteroids with approximately 1 UA of star.

the August 4th 2006, Ray Jayawardhana and Valentine Ivanov located, thanks to the New Technology Telescope of 3,5 m of the Observatoire of Silla of the southern European Observatoire (ESO), Oph 162225-240515, a double system with two planets turning one around the other and floating freely in space.

the September 18th 2006, a team of astronomers of Smithsonian announces the probable discovered one of a new type of planet: with a ray equivalent to 1,38 times that of Jupiter but not having even half of its mass, it is the least dense exoplanète never discovered! That confers a density lower than that to him of the cork, which would enable him to float comfortably if there was an enough bulky water container to contain it. The object is baptized HAT-P-1; its star is the principal star of a system double, located at some 450 light-years of the Earth in the constellation of the Lizard and known under the very poetic name ADS 16402. The two stars are similar to the Sun but younger, approximately 3,6 billion years.

the October 5th 2006, Kailash Sahu, of Space Telescopes Institute Science of Baltimore, and his/her colleagues American, Chilean, Swedish and Italian would have discovered, thanks to the space telescope Hubble, 5 exoplanètes of a new class baptized “planets at period of ultra-short revolution” (USPP: Planet Ultra-shorts-Period) because they make the turn of their star in less than one terrestrial day, 0,4 day (less than 10 a.m.) for fastest! The objects seem to be giant gas planets of weak density similar to Jupiter, turning around stars smaller than the Sun.

the April 25th 2007, the telescope Harps of 3,6 m of the Observatoire of Silla of ESO to the Chile announces the discovery of a planet “of terrestrial type livable”: Gliese 581 C, orbiting around the star Gliese 581 located at only 20,5 light-years of the Earth. Three laboratories associated with CNRS took part in the discovery, with researchers of the Observatoire of Geneva and Center of astronomy of Lisbon.

Methods of detections

To detect a exoplanète in a direct way is not an easy thing, and this for several reasons:

a planet does not produce a light: it does nothing but reflect that which it receives from its star, which is well little.
the distance which separates us from the star is infinitely more important than that which separates the exoplanète and its star: the resolving power of the instruments of detection must thus be very high to be able to distinguish them.

Thus, the only methods of detection which functioned until very recently are called “indirect” methods, because they do not detect the photons directly coming from planet. There exist several methods, present and future to detect a exoplanète. The majority are detected since the observatory S on the ground.

By radial speed

By the transit

Primary education transit (indirect method)

This method of indirect detection is based on the study of the luminosity of star. Indeed, if this one varies that periodically can come owing to the fact that a planet passes in front.

This method was proposed for the first time in 1951 by Otto Struve of the Observatoire Yerkes of the Université of Chicago. She was proposed again twice: in 1971 by Franck Rosenblatt of the University Cornell, then in 1980 by William Borucki of the Research center Hearts of NASA, in California.

Although the variation of luminosity of a star is more easily locatable than the variation its radial speed, this method appears not very effective in term of quantity of planets detected compared to the sum of stars observed. Indeed, one can use it only if we observe the stellar system almost by the section. One can show that for random orientations of the orbit, the geometrical probability of detection by this method is inversely proportional to the distance between star and planet. One estimates at 5% of stars with a exoplanète the detectable quantity with this method.

However, it with the advantage of requiring the use only of reasonable telescopes of dimensions.

In our own solar system, one can also observe planet transits: the transits of Venus and Mercure can however be observed at most only some times per century.

Secondary transit (semi-direct method)

The principle rests on the secondary Transit, i.e. when the planet passes behind the star. In this case one can detect the photons coming from the enlightened hemisphere of planet, which makes this method a method into semi-direct. In short, one studies the light signal coming from a Planet eclipsed by his star and one withdraws then the light signal emitted by the star (which one measured before), one obtains the signature of planet then.

The first detection of the secondary transit was made with the Space telescope Hubble in 2003 on the star HD 209458 (see this bond for more details (in English)).

Recently, of the teams of astronomers succeeded in detecting two exoplanètes in a direct way, by the use of the satellite Spitzer. Those, which were already known, were located thanks to the light Infrarouge which they emitted.

That opens new opportunities in the field of the observation. Indeed, the researchers from now on will be able to try to compare certain essential characteristics of the exoplanètes located until there, such as the color, the reflectivity and the temperature. This will make it possible to better include/understand the way in which those come to be formed.

By astrometry

It rests on the detection of the angular disturbances of the trajectory of a star. More the mass of the planet, and outdistances it which separates star from planet are large, more the system is close to us and thus visible.

This method, although it is known for a long time, had not been used yet because of the negligible variations which it was to locate. But it will be soon possible thing with in particular the installation of the mode doubles field of the Broad Very Telescopes Interferometer (VLTI) called PRIMA.

By the effect of microlentille gravitational

This method is based on the curve of the Lumière emitted by a distant star or a Quasar, when a massive object is aligned “sufficiently” with this source, phenomenon called “gravitational Lentille”. The distortion of the light is due to the gravitational field of the object lens, one of the consequences of the General relativity, like described it Albert Einstein in 1915. It results from this an effect of lens, formation of two deformed images of distant star, even more.

In the case of the search for exoplanètes, the target planet, orbits around the star lens, provides extra information about it, making it possible to determine its mass and its distance from star. One speaks about microlentille because the planet does not emit or very little light.

This technique makes it possible to observe stars of mass even relatively low, since the observations are not based on received radiation.

Direct

The combined use of systems of adaptive correction in real-time called Optical and of the Coronographie recently made it possible to detect a exoplanète directly using the VLT.

Enormous efforts are currently devoted to the improvement of the techniques of adaptive optics, stellar coronography, and image processing, in order to develop an astronomical imagery with very high contrast able to detect exoplanètes in the long term size of the Earth. These methods are detailed in the principal page.

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