Exoplanet

An extrasolar planet, short exoplanet, is a planet outside (Greek ἔξω ) of the dominant gravitational influence of the sun. Extrasolar planets do not belong to the solar system, but another planet orbiting another star system.

In addition, also fall planet -like objects that orbit no stars and among the newly-minted preamble Planemo (of English planetary mass object) exist.

Discovery of the first exoplanets

Already in the 1980s the first exoplanets were discovered, but at that time as either a brown dwarf classified (HD 114762 b ) or due to the still insufficient accuracy temporarily rejected (Gamma Cephei b).

The first planet ever to have been confirmed outside the solar system, orbiting the pulsar PSR 1257 12. The pulsar was discovered in 1990 by the Polish astronomer Aleksander Wolszczan. By accurate measurements of the return period of the beam that reaches the Earth from the pulsar, 1992, three planets with masses of 0.02, 4.3 and 3.9 Earth masses and orbital periods of 25.262, 66.5419 and 98.2114 days be detected. Of 1994 another planet was discovered around the pulsar PSR B1620 -26. On this planet is life as we know it from the earth, practically impossible.

The first definitive detection of an exoplanet in orbit around a star similar to the Sun was made in 1995 by Michel Mayor from the Department of Astronomy at the University of Geneva and his collaborator Didier Queloz using the radial velocity method. The planet 51 Pegasi b orbits in the 4.2- day cycle to the approximately 40 light years from Earth star 51 Pegasi ( constellation Pegasus ) and has 0.46 Jupiter masses.

Exoplanets in orbit around sun-like stars could not be directly observed with telescopes long as they are very faint. You are outshone by the lighter by a multiple star around which they orbit. The resolution of ground-based telescopes now ranges usually not one of them made ​​, presented separately to two such closely spaced objects with such a large difference in brightness as a planet and its star. Thus was the first exoplanet that could be directly optically imaged ( 2M1207 b ), an exoplanet around a brown dwarf.

Indirect detection methods

Direct Observation

On 10 September 2004, the ESO announced that possibly a direct recording of a planet at 225 light years distant brown dwarf 2M1207 is the first time succeeded. Follow-up measurements with the Hubble Space Telescope in 2006 were able to confirm this.

On March 31, 2005, a working group of the astrophysical institute of University Observatory Jena known, a planet of only one to two times the mass of the planet Jupiter in which the sun-like, but with an age of about 2 million years ago, much younger star GQ to have observed Lupi, who is currently in the T Tauri phase. Also, this observation was made with the Very Large Telescope of ESO in the infrared spectral range.

British astronomers have in the vicinity of 520 light-years away from Earth and with an age of about 100,000 years, very young star HL Tau using the Very Large Array discovered an exoplanet in the development phase.

The clearest direct evidence was published on 14 November 2008: On two shots from the Hubble Space Telescope from the years 2004 and 2006 in the range of visible light, a moving spot of light can be seen that describes a Keplerian orbit. It is the planet Fomalhaut b, which has a mass of about three times Jupiter's mass and orbits the star Fomalhaut at a distance of 113 AU ( twelve times the distance between the Sun and Saturn). The planet orbiting Fomalhaut at the inner edge of the dust belt surrounding Fomalhaut. According to the discoverer of the planet is so far the coolest and smallest object that could be imaged outside of the solar system. Fomalhaut is 25 light years from Earth and has twice the mass of the sun.

Almost simultaneously gave astronomers announced that they had succeeded at Gemini North Observatory and the Keck Observatory to map an entire planetary system around the 130 light years distant star HR 8799 in the constellation Pegasus. Observations using adaptive optics in infrared light show three planets whose masses are indicated with seven to ten Jupiter masses. The exoplanet orbiting its host star at a distance of 25, 40 and 70 astronomical units. With an estimated age of 60 million years, they are still young enough to give even heat radiation.

Designation

Exoplanets are designated by the name or the catalog name of the star and an appended lowercase. The numbering is done in the order of discovery, starting with 'b'. Where necessary, is often marked with an attached uppercase 'A' to uniquely distinguish them from him orbiting planets of the star.

Number of known exoplanets

The at March 20, 2014 1779 extrasolar planets in 1102 systems known, including 460 systems with two to seven planets and more than 2,000 planet candidates. Planetary systems are now in the immediate vicinity of the sun as safe proven, general phenomenon. Investigations and measurements of the Institute Astrophysique de Paris showed that, on average, every star in the Milky Way has 1-2 planets.

• Radial velocity method
• Pulsarfrequenz
• Transit method
• Direct Observation
• Astrometric
• Timing
• Microlensing

Discovery of exoplanets

Exoplanets are searched and discovered by teams around the world. Some teams try to survey the sky with ground-based telescopes, while directly with telescopes other missions are sent (eg Kepler space telescope ) into space.

Types of exoplanets

There are no internationally binding system for classification of extrasolar planets. So they tried a classification for the planets. This was then transferred to the extrasolar planets.

In general, the classification was (classic) made ​​accordingly tentatively into the following types:

• Rocky planets ( Earth-like rocky planets, " terrestrial ", in the case of several Earth masses referred to as " super-Earths " )
• Gas giants ( Jupiter- like, in close proximity to the fixed stars also known as " hot Jupiters " ) and gaseous planets ( Neptune similar, in close proximity to the fixed stars also known as "hot Neptunes ").

However, these preliminary classification was soon found to be too inaccurate, especially in many new discoveries was unclear whether these planets have a solid surface or not, whether they are dense as Mercury or as icy distant moons of the solar system.

After David Sudarsky et al, therefore, the external appearance of the planets and Planemos is completely hidden: It is only on the chemical composition. This is - as far as can be determined - like planets also derived from three sizes:

• The albedo of the celestial body,
• The radiation temperature in the EM spectrum and
• Of any mass or density of the planet.

Accordingly, the following types arise in the density range between metallic ( 7 g / cm ³), silicate ( about 3 g / cm ³) and wassereishaltigen planet:

• Metallosilikatische planet (similar to Mercury and Earth)
• Silicate planets ( eg similar. Europe, Earth's moon Io and )
• Hydrosilikatische planet (similar to Ganymede, Callisto, Titan and Pluto),
• Ice planet (similar to Enceladus, with very low silica content) and
• Gas giants (similar to Jupiter and Neptune).

The classification by Sudarsky valid strictly for gas giant planets and distinguishes between the following types ( in some cases also with respect to the cosmochemistry and possible chemical evolution ):

• Gas giant with methane clouds ( corresponding albedo, temperature below 80 Kelvin),
• Gas giant with ammonia clouds ( below 150 Kelvin),
• Gas giants, with steam clouds ( -120 to 60 ° C, albedo to 81 %)
• Gas giant with albedo by 12%; , ( 350-900 K actual "hot jupiter " )
• Gas giant with alkali absorption (900-1500 K, albedo by 3% ),
• Gas giant with Siliziumdioxidwolken ( scorching heat, about 1500 K ).

Some data on the atmosphere of the planet is obtained from the spectrum as a star passage.

However, most of the so far discovered systems are not comparable with the solar system, it most are gas giants that orbits its host star in a very close orbit. Such are called exoplanets astronomers of hot Jupiters. According to one theory, they are, like Jupiter, originated in a relatively large distance from their central star in the accretion disk, but then migrated inward. However, according to another theory, they are condensed from a gas cloud like stars.

Small exoplanets

In recent years increasingly smaller exoplanets were discovered. 2004 was the lower limit of detectability with the radial velocity method at a radial velocity of about 1 m / s A planet that orbits its star in 1 AU distance, therefore had a mass of about 11 Earth masses have to be ever discovered can. In the meantime, however, lower mass and smaller exoplanets have been discovered using the radial velocity and by the microlensing and transit method.

One of the smallest exoplanet yet is in April 2007 by astronomers of the European Southern Observatory ( ESO) discovered second companion of the star Gliese 581: Gliese 581 c at a distance of 20.45 light years. Its orbital period or annual length is only 13 Earth days. The surface temperature is estimated to be 0 to 40 ° C. The planet has an estimated 1.5 times the size of Earth and is about five times heavier than the Earth. The detection of the planet succeeded by a spectrograph, which is operated at La Silla, Chile. It red and blue shifts were investigated, which are dependent on the rotation of the planet ( radial velocity method ).

Another, in 2009 discovered companion is Gliese 581 e With him is one of the lowest mass exoplanets with only 1.9 Earth masses and an orbital period of just over 3 days.

Others of the previously detected little exoplanets are Gliese 876 b, OGLE -2005- BLG - 390Lb, μ Arae d, MOA -2007- BLG -192 - L b and CoRoT -7 b. For some there are so-called super-Earths.

Gliese 876 b has about 7.5 times the mass of Earth. As it orbits at a very small distance in just 47 hours once around its star, its surface temperature is about 200 ° C to 400 ° C.

μ Arae d is about 10 times as heavy as the earth and thus has about two-thirds of the mass of Uranus. In just 9.5 days of exoplanet orbiting the solar system from about 50 light years distant star μ Arae in the constellation altar.

OGLE -2005- BLG - 390Lb was discovered in January 2006 by an international group of researchers using microlensing. This exoplanet is removed from the earth about 25000-28000 light years and has about five times the mass of Earth. It orbits the star OGLE -2005- BLG - 390L ( a red dwarf ) at a distance of 2.6 astronomical units once in ten Earth years. Due to the small size and relatively low radiation of the "Mother Star" and the great distance, the surface temperature of the planet is only about -220 ° C. The development of life forms is thus highly unlikely.

MOA -2007- BLG -192 - L b was discovered in June 2008 and is one of the smallest known exoplanets. It has the 3.2 times Earth's mass and is located at a distance of about 3000 light years. However, recent evidence suggests that the mass of its parent star is much higher and it is this is not a bay horse, but a red dwarf. This results for the exoplanet a newly determined mass of only 1.4 Earth masses.

CoRoT -7 b was determined using the transit method discovered by the CoRoT space telescope in 2009 and is the first unequivocally proven extrasolar rocky planet. The only 1.75 Earth radii large planet has about 5 times the mass of Earth. Because of its mass and its short distance from the parent star, it is impossible that there is a gas or ice giants. This strongly suggests that there must be a terrestrial planet. This assumption was later confirmed by measurements at the La Silla Observatory ESO by findings on the density of the planet. With an orbital period of only about 20 hours, he holds another record among the known exoplanets.

Kepler 37 - b is currently the smallest known exoplanet (February 2013) and with a diameter of about 3900 km, only slightly larger than the Earth's moon.

Exemplary exoplanets

2M1207 b

The gas giant 2M1207 b was discovered in 2004 in orbit around the brown dwarf 2M1207 and was the first exoplanet that could be perceived directly by optical means, thereby offering the possibility of a direct spectroscopic investigation.

Ssc2005 - 10c

The object ssc2005 - 10c at the star HD 69830 meets a "German Shepherd " function to a discovered with the Spitzer Space Telescope NASA asteroid belt, similar to Jupiter for the asteroid belt of the solar system. This belt has about 25 times the mass of the star and is as close as the Venus of the sun.

Gliese 581 b, c, d, e and g

Gliese 581 is a 20.5 light years distant star in the constellation Libra. There is a red dwarf, which is about 500 times weaker than the sun radiates ( spectral class M3, 5, Visual Brightness: 10,56 ). He has a planetary system with four confirmed companions on that as Gliese 581 b, 581 c, 581 d, 581 e are called. The alleged discovery of two more, 581 f and 581 g, has not yet been confirmed.

Gliese 581 b was discovered in 2005 and is similar in mass to Neptune ago. Due to the small distance between this planet, however, has a surface temperature of 150 ° C. estimated The planet has about 17 Earth masses and orbits its star every 5.366 days at a distance of 6 million kilometers.

In April 2007, a team of astronomers at Xavier Delfosse of the European Southern Observatory ( ESO) have discovered a second planet: Gliese 581 c. The orbital period or year length is 13 days. The surface temperature is estimated to be due to the small distance to the weakly radiating central star at 0 to 40 ° C. If it was there to give water, it could be liquid. Thus Gliese 581 c is an interesting research target in the search for extraterrestrial life. The planet has an estimated 1.5 times the size of Earth and is about five times heavier than the Earth. He was with his discovery of the smallest known exoplanet, to the even smaller Kepler - 10b was discovered in early 2011.

At a conference of ESO Michel Mayor announced in April 2009 that the in 2007 discovered Gliese 581 d in its system of planets within the habitable zone is located and possibly houses oceans with liquid water. Gliese 581 d orbits its star in 66.8 days and was found using the HARPS spectrograph at the European Southern Observatory in La Silla, Chile.

The fourth planet is Gliese 581 e, which orbits its star every 3.15 days and is probably a rocky planet.

HD 149026 b

To the yellow star HD 149026 orbits every 2.87 days in 0.046 AU distance of saturn large gas planet HD 149026 b, which seen from Earth performs a transit. He has 36 % of the mass of Jupiter (type: "Hot Jupiter", 72% of its diameter ). About 50 % of its mass ( about 70 Earth masses ) is accounted for by a solid core, more than any other gas giants in which this proportion is known. Located in the constellation Hercules located 250 light-years distant central object (star spectral class: G0; spectroscopic magnitude: 8.15 m) has 1.3 solar masses.

CoRoT -4 b

CoRoT- 4b, only CoRoT- Exo - 4b called, was on 24 July 2008 by scientists at the University of Exeter with the help of the space telescope CoRoT. discovered. This exoplanet is, on the basis of about 0.7 times the mass of Jupiter and about 1.2 times the diameter of Jupiter, a gas giant.

The special thing about this planet is the co-rotation with its central star, CoRoT- fourth The planet rotates at exactly the same period around its own axis as its star and also moves with the same speed corresponding to this. On the always facing the star side of CoRoT -4 b is thus constantly day and on the opposite side there is a star overnight. Since its mass to its star is too low in relation to influence their own revolution, this circulation synchronization must have causes that are the origin of CoRoT- 4 system.

Lalande 21185 B and OGLE -2005- BLG - 390Lb

The Red Dwarf Lalande 21185 is located in the Great Bear ( age 3-10 billion years, 7.5. Magnitude, spectral type M2n ). The 8,312 light-years distant dwarf star with only 0.00568 times the solar luminosity (with only 0.393 times the solar diameter and 0.403 solar masses ) has two known exoplanets, both gas giants, which are far outside. In the vicinity of the central star is room for rocky planets that could have life-friendly temperatures.

OGLE -2005- BLG - 390L b is similar to Lalande 21185 B in the sense that he is also far from the central star and thus is likely to be just as cold.

Epsilon Eridani b

The orange sun-like star Epsilon Eridani has a gas planet, Epsilon Eridani b, as well as a dust disk. This 10.50 light years away, just 800 million years old exoplanet has 1.5 Jupiter masses and orbits the star every 6.85 years in 1.9 to 3.7 AU distance. Maybe Epsilon Eridani has an entire planetary system.

Gliese 876 d

Gliese 876 d is a planet of 11 billion old red dwarf Gliese 876 in Aquarius ( luminosity: 0.0016 × sun, mass: 0.32 × sun, diameter: 0.36 × solar ). At the time of its discovery in June 2005, he was the least massive exoplanet known. The exoplanet of about 8 times the mass of Earth orbiting the star of spectral type M3, 5 ( size class 10.15 ) all 1.94 days at a distance of 0.021 AU. For food he is - whether gas or rocky planet - probably due to its proximity to the star with 200-400 ° C too hot (similar: μ Arae ). Two more, also quite hot exoplanets ( Gliese 876 b and Gliese 876 c) with the 1,6 - and 0,5 times the mass of Jupiter orbiting the star in only 0.21 and 0.13 AU distance.

55 Cancri e

On 55 Cancri e, the exoplanets around the star 55 Cancri double in the constellation Cancer, there must be - similar to Gliese 876 d - due to close proximity to the central star will be extremely hot. The chance to form a gas giant deprived could this object after evaporation of all volatiles from planetary mass probably only make a few kilometers thick crust, decorate on the sulfur fumes and a few clouds of rock dust of the last volcanic eruptions the horizon of the thin residual atmosphere. For chemical processes and material distributions in space and on planets see below cosmochemistry and chemical evolution.

Ypsilon Andromedae b, c, and d

The binary star system Upsilon Andromedae consists of the luminous stronger star Upsilon Andromedae A and the red dwarf star Upsilon Andromedae B. The larger of the two stars, Upsilon Andromedae A, has at least three planets:

The system is located in the constellation Andromeda, is 2.9 to 4.1 billion years old, 43.93 light-years away, and the orbital period of Upsilon Andromedae A and B is 20,000 years.

HD 10180

On August 24, 2010 reported that a planetary system with up to seven planets was found with HD 10180. One of these seven - HD 10180 b - is still unconfirmed.

Gliese 1214 b

GJ 1214 b ( Gliese 1214 b) is a discovered in 2009 extrasolar super-Earth about 40 light -years away in 38 hours the red dwarf GJ 1214 orbits in the constellation Ophiuchus from the earth, the radiation of which is 200 times weaker than that of the sun. The exoplanet GJ 1214 b has an atmosphere that is mainly composed of water vapor.