Super earth
Super earth is a slang term for a large extrasolar terrestrial planet . The designation is based only on the mass, but does not make any statements about the surface properties or habitability of the planet. According to the most common standard, it must be at least as heavy as the earth , but lighter than the planet Uranus , and accordingly have 1 to 14 times the mass of the earth . Other common definitions assume 1 to 10 or 5 to 10 times the mass of the earth. If the mass is even larger, it is called a mega-earth . Furthermore, the planet is made general by the star orbited by itsufficiently irradiated, since as a cold planet of this size it would lose little gas in this system arrangement and would become a gas planet .
There have been some discoveries of super-earths since the discovery of Gliese 876d by a team led by Eugenio Rivera. The solar system does not contain any comparable planets. The largest terrestrial planet in the solar system is Earth and the next larger planet Uranus already has 14 Earth masses.
proof
Due to the lack of accuracy of the measurement methods, an Earth-like nature could not be proven beyond doubt for most of the exoplanets that were classified in the super-earth group. In the case of planets with masses of less than about 14 earth masses, it is difficult to decide when a possibly existing dense gas envelope is no longer regarded as a dense atmosphere, but as a gas envelope around a solid core ( gas planet ). In order to decide whether it is a rocky planet or a Hot Neptune , it is necessary to determine the mean density of the object as precisely as possible. The two predominant methods of detecting exoplanets both provide incomplete information on their own. While with the transit method a statement about the radius of a planet but not about the mass is possible, with the radial velocity method no statement about the radius about the mass is possible. However, with the radial velocity method there is also the uncertainty of the often unknown inclination , so that the mass information is usually only around the lower limit of the possible mass of the planet. For a few planets, a better classification is possible by detecting the planets with both methods.
The first unequivocally proven extrasolar rock planet is the planet CoRoT-7 b (formerly CoRoT-Exo-7 b) discovered at the beginning of 2009 . Another proven rock planet was added later with Kepler-10b . Evidence as a rock planet is based on the density, which must move in the area of the earth-like planets.
Discoveries
First super earth
The first super-earths were found by Wolszczan and Frail around the pulsar PSR B1257 + 12 in 1992. The star's pulses arrived with regular delays, indicating to the scientists that it is accompanied by two satellites. This type of measurement is the timing method. The two outer planets of a total of four discovered satellites of the system have about 4 Earth masses, so they are too small for gas planets .
Other major super-earths
The first super-earth around a main sequence star was discovered in 2005 by a team led by Eugenio Rivera. The planet orbits Gliese 876 and was named Gliese 876 d (two Jupiter-sized planets had previously been discovered in the same system). According to estimates, it has at least six times the mass of the earth and a very short orbital period of only two days. Due to the close proximity of Gliese 876 d to its star, its surface has a temperature of 650 Kelvin .
Two more super-earths were discovered in 2006: OGLE-2005-BLG-390Lb with at least 5.4 times the earth mass detected by the microlensing method, and HD 69830 b with at least 10 earth masses. Both exoplanets orbit a main sequence star, a so-called red dwarf .
In June 2008, David P. Bennett announced the discovery of the exoplanet MOA-2007-BLG-192-Lb at a meeting of the American Astronomical Society. This super-earth has 3.2 times the mass of the earth and orbits a brown dwarf in the constellation Sagittarius . The planet, which is about 300 light years away, probably consists mostly of ice and rock and was detected by the microlensing effect.
In the same month, the discovery of three super-earths around the sun-like star HD 40307 in the constellation Maler was announced by ESO . For the smallest planet of the star 42 light years away, the minimum mass is only around 4 Earth masses.
At the beginning of February 2009 the Thuringian State Observatory announced the discovery of an extrasolar rock planet called CoRoT-7b . The planet, which is only 1.75 Earth radii, was discovered in January with the help of the CoRoT space telescope using the transit method. The discovery is the first proven terrestrial exoplanet and thus the first unequivocal super-earth. With an orbital time of only about 20 hours, the planet holds another record. Because of its estimated mass and its proximity to the parent star, it was next to impossible that it was a gas or ice giant . So it was assumed that it must be an Earth-like planet. After longer observations and measurements with the HARPS instrument on ESO's 3.6-meter telescope, the assumption was confirmed in September of the same year. CoRoT-7 b has about 5 times the mass of the earth and a comparable mean density as the earth.
In April 2009, Michel Mayor of the Geneva Observatory announced the discovery of the planet Gliese 581 e . With a minimum mass of 1.9 Earth masses, it is one of the exoplanets with the lowest mass to date.
The first two super-earths around sun-like stars, 61 Vir b and HD 1461 b , were discovered in late 2009.
Discoveries through the Kepler mission
The Kepler space telescope discovered a large number of suspected super-earths using the transit method. Statistical evaluations suggest that planets from the size of a super-earth to the size of Neptune represent the most common group of planets in the Milky Way .
First super-earths in the habitable zone
In April 2007, a team led by Stéphane Udry (based in Switzerland) announced the discovery of two new super-earths around Gliese 581 , both in the habitable zone where liquid water could exist. Gliese 581 c , which has 5 Earth masses and is 0.073 AU or 11 million kilometers from Gliese 581, is located on the “warm” edge of the habitable zone around Gliese 581, with an estimated average temperature (excluding atmospheric effects) of −3 ° C at a Venus- like albedo and 40 ° C for an earth-like one. It is the first Earth-like exoplanet with life-friendly temperatures. However, according to recent computer models by some scientists, Gliese 581c may be too hot to house liquid water. According to this, the exoplanet would resemble Venus more closely and carbon dioxide and methane would heat the atmosphere to over 100 ° C due to a greenhouse effect, so that no water in liquid form would be expected. The second super-earth in the system, Gliese 581 d, could not be confirmed and probably does not exist.
A super-earth with acceptable temperatures and water resources , K2-18b , was discovered in 2015. The water resources were confirmed in 2019. The super-earth K2-18b orbits a dwarf star in the constellation Leo, 124 light-years from Earth. It has eight times the mass and more than twice the diameter of the earth, reports the research team led by Angelos Tsiaras and Ingo Waldmann from University College London in the journal "Nature Astronomy".
properties
Due to their higher mass, super-earths have different physical properties from Earth. In a study of Gliese 876 d by a team led by Diana Valencia , it turned out that it is possible to infer from the radius measured by the transit method whether the discovered planet is a super-earth. For Gliese 876 , the calculations range from 9,200 km (1.4 Earth radii) for a rocky planet with a large iron core to 12,500 km (2.0 Earth radii) for a watery and icy planet. With this range of radii, the super-earth Gliese 876 d could have a surface gravity between 1.9 g and 3.3 g. High surface gravity (generally higher than Neptune and Saturn- sized planets and in certain cases larger than Jupiter- sized planets) is a prominent feature of super-earths.
Further theoretical work by Valencia and others came to the conclusion that super-earths are geologically more active than Earth, with stronger plate tectonics under greater loads. In fact, their models showed that the earth itself is a "borderline" case and is just big enough to develop plate tectonics.
See also
Web links
Individual evidence
- ↑ a b c Valencia et al .: Radius and structure models of the first super-earth planet. In: The Astrophysical Journal . February 2007
- ↑ Peter N. Spotts: Canada's orbiting telescope tracks mystery 'super Earth' . In: Hamilton Spectator , April 28, 2007 (English)
- ^ Fortney et al .: Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits. April 2007, arxiv : astro-ph / 0612671v3
- ↑ Rivera et al .: 7.5 M ⊕ Planet orbiting the nearby star GJ 876 . 2005, doi: 10.1086 / 491669
- ^ Program Planet. ( Memento of April 28, 2014 in the Internet Archive ) Oasis, Online Abstract Submission and Invitation System
- ↑ A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192 . arxiv : 0806.0025
- ↑ A Trio of Super-Earths . ( Memento of October 7, 2008 in the Internet Archive ) ESO Science, Release No. 19/08: June 16, 2008
- ↑ "Transit of Venus" at Corot-Exo-7. http://corot.tls-tautenburg.de/Exo-7b/ ( Memento from September 5, 2013 in the Internet Archive ) TLS-Tautenburg, February 3, 2009
- ↑ CoRoT discovers extrasolar rocky planets German Aerospace Center, February 3, 2009
- ↑ COROT-7b - Clear evidence for rocky planet . Astronews.com, September 16, 2009
- ↑ Rock planet discovered, water world suspected , astronews.com, April 21, 2009
- ^ Udry et al .: The HARPS search for southern extra-solar planets, XI. An habitable super-Earth (5 M ⊕ ) in a 3-planet system . In: Astronomy and Astrophysics . preprint, 2007, S. preprint.
- ↑ Editor: Researchers find water for the first time on K2-18b. In: https://www.spiegel.de/ . Spiegel Online , September 11, 2019, accessed on September 23, 2019 (German).
- ↑ Earth: A Borderline Planet for Life? CfA Press Release No .: 2008-02 January 09, 2008