Earth-like celestial body
As earth-like (or even terrestrial and telluric ) celestial body , or (if a planet or Exoplanet is concerned) as rock Planet is in the starry and planetary science a celestial body referred to, which has a solid surface by its own gravity approximately to a sphere is formed , and mostly consists mainly of rock (often with a metallic core). This definition is used to classify planets .
A different, stricter interpretation is sometimes used for the term “earth-like”. Possible additional criteria are the presence of water and a noteworthy, possibly life-friendly atmosphere (see also potentially habitable planets ).
Earth-like celestial bodies consist entirely or almost entirely of non-gaseous components and usually have a shell structure : in the center there is an iron core, over it a thick layer, the so-called mantle , made of silicates and oxides , and on top a thin crust, also made of silicates and oxides, but is enriched in elements that cannot be built into the mantle rock and are "sweated out" over time ( e.g. potassium , rare earths , uranium ). In some cases an atmosphere joins above the crust . In the case of earth, the hydrosphere (oceans, lakes ...) lies in between. In the past, similar diameter and mass as the earth were other conditions, but in particular a similar average density . The latter is roughly between 4 and 8 g / cm³. Today, in addition to being non-gaseous, the main condition is that one's own mass forces the celestial body into (largely) spherical form.
Historically, only the four planets of the inner solar system belonged to the group of celestial bodies called "terrestrial planets" at the time: Mercury , Venus , Earth and Mars . Meanwhile the planetologists include not only Pluto but also the similarly structured earth moon , the Jupiter moons Io and Europa as well as the larger ice moons such as Ganymed , Callisto , Titan and Triton . In the last-mentioned ice moons, the iron core may be missing and an ice coat can replace a silicate coat, but the basic description of these ice moons, their formation and development is very similar to those of the classic models. The dwarf planet Ceres and the large, differentiated asteroids Vesta and Pallas can also be included in this group, as their own size also forces them into a largely spherical shape.
Due to the lack of accuracy and performance of the measurement methods, it is difficult to prove that exoplanets have an Earth-like nature.
Examples outside the solar system
Kepler-186f orbits a red dwarf of spectral class M in the constellation Swan in the northern sky, about 490 light years away . Among the six planetary companions discovered so far, Kepler-186f is considered to be Earth-like. The planet has about 1.1 times the mass of the earth and its radius is also 1.1 times that of the earth. It is very likely that it is an earth-like planet (rock planet), which orbits its central star at a distance of 52.4 million kilometers and requires 130 earth days for one orbit. Its distance to the central star is significantly smaller than that from the earth to the sun , but in direct comparison it receives less energy and is therefore probably on the outer edge of the habitable zone .
Trappist 1 system
The Trappist-1 star system, 40 light-years away, contains a total of seven Earth-like planets, whose mass and diameter are similar to those of Earth and on which liquid water may be present. The system is much smaller than the solar system because the central star is a low-mass red dwarf. NASA assumes that the planets Trappist-1 e, f and g are in the habitable zone. Since red dwarfs get much older than sun-like stars, life on planets would also have significantly more time to develop from them. The astronomer Michaël Gillon who was involved in the discovery sees systems comparable to Trappist-1 as the most promising candidates for possible life.
Exoplanet status unclear
After Kepler-452b was classified as the most Earth-like planet after its discovery in 2015, this assessment has been called into question in a study from 2018. Previous assumptions were that Kepler-452 is an Earth-like planet 1400 light years from Earth, has an orbital period of 385 Earth days, and the diameter of the planet is about 60% larger than that of Earth. According to the assumptions questioned, the planet has twice the gravity of the earth. Its star would also look very much like the sun .
- Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1 (English) - Nature , on February 22, 2017
- Fergal Mullally, Susan E. Thompson, Jeffrey L. Coughlin, Christopher J. Burke, Jason F. Rowe: Kepler's Earth-like Planets Should Not Be Confirmed without Independent Detection: The Case of Kepler-452b . In: The Astronomical Journal . tape 155 , no. 5 , 2018, doi : 10.3847 / 1538-3881 / aabae3 , arxiv : 1803.11307 , bibcode : 2018AJ .... 155..210M .
- Michele Johnson: NASA's Kepler Mission Discovers Bigger, Older Cousin to Earth. July 23, 2015, accessed September 29, 2019 .