Today, astronomy is experiencing another round of development. New discoveries occur with increasing frequency. The number of planets discovered outside the solar system is already in the thousands. And these are only confirmed planets, not counting possible candidates.
To systematize the discovered planets, scientists are looking for ways to classify them according to common features. Today, there are several generally accepted classification models, but most often exoplanets are divided into gas and terrestrial planets. The latter will be discussed in this article.
Earth-type exoplanets
Such objects are of particular interest to scientists, since one of the most important tasks of modern science is the search for extraterrestrial life, and the probability of finding it on an earth-like planet is much higher than on a gas one. So what are the planets of this type?
1
Mini-earth
As the name implies, this type of object has dimensions not larger than that of the Earth. In the solar system, Earth, Venus, Mars and Mercury can be attributed to this class. The smaller the earth-type planet has, the smaller its gravitational component. Together with a weak magnetic field, this leads to the fact that the atmosphere is not able to linger on the surface and disappears into outer space.
Typically, such objects are close to their parent stars, which leads to strong heating of the surface. Due to its small size, mini-earths are quite difficult to detect. Most often they are found using the transit method, which is effective for finding planets orbiting at a close distance from the star.
The first planets discovered in this class were Kepler-20 e and Kepler-20 f, orbiting a red dwarf 945 light years distant from us.
A few examples of mini-earth
Kepler-20 e
In the photo: Comparative dimensions of the Earth and Kepler20e
Kepler-20 e is the second remotest planet from the parent star, however, it has an orbit diameter 6 times smaller than that of Mercury. Such proximity to the star makes the temperature on the surface of the mini-earth very high - about 740 ° C, which translates it into a category of potentially uninhabited.
Kepler-20 f
In the photo: Comparative dimensions of the Earth and Kepler20f
This mini-earth has dimensions slightly larger than that of the Earth. Its radius is 3.4% larger than the earth, although it has 0.66 earth masses. The planet is the fourth remotest from the star, the diameter of its orbit is more than 3 times smaller than the diameter of the orbit of Mercury. One year on the Kepler-20 f lasts only 19.5 days.
Despite the similarity in size and mass with the Earth, the conditions on the Kepler-20 f are significantly different from the usual for us. Due to its proximity to the star, the average surface temperature here is about 432 ° C, it is too high to maintain water in liquid form and enough to melt many metals. But it is possible that the Kepler-20 f atmosphere contains a large amount of water vapor.
PSR B1257 + 12 b
An amazing mini-earth, which is located at a distance of 2300 light years from us in the constellation Virgo. The planet is unique in that it revolves around a pulsar, a compact space object made up of a neutron star.
Mini-Earth, one of the three planets found in the orbit of the pulsar PSR B1257 + 12. With its dimensions, it is about 2 times larger than the moon, and has a mass of 50 times less than the earth.
Kepler-37 b
This mini-earth revolves around the yellow dwarf Kepler-37, located in the constellation Lyra at a distance of 126 light years from us. At the time of its discovery, it was the smallest exoplanet of all known. Its radius (3900 km.) Only slightly exceeds the radius of the moon (3476 km.). The diameter of the planet’s orbit is approximately 4 times smaller than the diameter of the orbit of Mercury, which makes the surface conditions close to Mercury.
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Super earth
Super-Earth is a class of planets that are similar in mass, which ranges from 1 to 10 Earth masses. Some sources speak of masses from 5 to 10 terrestrial.
Perhaps this is one of the simplest types of classification of space objects, because neither proximity to the star, nor composition in this class is taken into account, only mass is important. Although here there are some borderline cases. For example, Planet Mu Altar c, which is located 50.6 light years from us, has a mass of 10.5 terrestrial (or 3% of the mass of Jupiter).
Most often, super-earths are found in stars belonging to the yellow and red dwarfs, whose mass is equal to from 35% to 85% of the sun. Another hallmark of stars having super-earths is their metal depletion.
Of course, such types of space objects can have completely different composition, temperature and other characteristics, but scientists are inclined to believe that most of them are stone planets that have a geology similar to the earth. And, if such a planet is located in the habitable zone of a star, then it is very possible that it will be very similar to our Earth, even if it is much larger.
Examples of Some SuperEarths
PSR B1257 + 12 c
This super-earth revolves around a neutron star already known to us, one of the planets of which is a mini-earth (thebiggest.ru wrote about it a little higher). It is also surprising that the discreet name “PSR B1257 + 12 c” hides the first discovered exoplanet in history! The discovery happened in 1991, when the Polish astronomer Alexander Volshchan noticed periodic changes in the intensity of the signals of the pulsar PSR 1257 + 12, which he had discovered a year earlier. Later it turned out that at least 3 objects rotate in the orbit of the pulsar, two of which are super-earths, and one is a mini-earth.
Note: “AE” is an Astronomical Unit. This term is called a unit of length equal to the average distance between the Earth and the Sun, and this is about 150 million km.
The diameter of the orbit of the super-earth PSR B1257 + 12 s is 0.3AE. It is very difficult to imagine the conditions on this planet, but it is obvious that they are very different from all the planets known to us. The pulsar has a colossal magnetic field, the planet is subject to powerful ionizing radiation. Many scientists suggest that here, under certain conditions, life is possible. On Earth, there are some life forms that are resistant to various types of radiation, including ionizing. In addition, the temperature of the pulsar can reach a million degrees Kelvin, and the pulsar wind is able to heat the planet in orbit.
Kepler-442 b
The radius of Kepler-442 b is 30% greater than the earth's, and the mass is more than 2.3 times that of the earth. There is an exoplanet at a distance of 1120 light years from us. It is of great interest to astronomers in that it rotates in the so-called "habitable zone" of its star - an orange dwarf with a mass of 0.61 solar. The radius of the orbit of rotation of Kepler-442 b is 0.41AE, but due to the weaker luminosity of the parent star, the conditions on the surface can be very similar to terrestrial.
Gliese 832 c
This exoplanet, orbiting a red dwarf at a distance of 16 light years from us, has one of the highest Earth similarity indices among all the planets known today. Although Gliese 832 c is more than 6 times closer to the parent star than Earth, it receives about the same amount of heat. Its mass is slightly more than 5 times larger than the earth, and in size it is slightly less than one and a half times larger than the Earth. Further studies of the planet should shed light on the composition and density of the atmosphere of Gliese 832 c, as well as on the possibility of living organisms on it.
Proxima Centauri b
The first mention of this super-earth appeared in 2013, however, data on it was double-checked and received final confirmation only in 2016. Interest in the planet is caused by the fact that it revolves around the yellow dwarf Proxima Centauri, and this is the closest star to us. Its dimensions and mass are almost 10 times inferior to our Sun. It is located at a distance of 4.3 light years, or 40 trillion. km from U.S.
Let us return to the characteristics of Proxima Centauri b. The planet makes a complete revolution around the star in 270 hours (about 11 days). This speed is due to the proximity to the star, because the radius of the orbit of rotation of the super-earth is 20 times smaller than the radius of the Earth’s orbit and even 7 times smaller than the orbit of Mercury. Such proximity to a dim star creates the conditions for liquid water on the planet, which makes Proxima Centauri b potentially viable. The average temperature on the planet’s surface is −39 ° С. The radius of Proxima Centauri b is 10-11% greater than the Earth, and the mass is 27% more than the mass of the Earth.
According to recent data, an exoplanet, without its own magnetic field, is subject to cosmic radiation, hundreds of times higher than the radiation received by the Earth. Such radiation could destroy almost all living organisms of the Earth, although we know some types of bacteria that can survive in more extreme conditions. Scientists have found several models in which life is able to protect itself from the powerful radiation of a star. But in March 2017, a strong flash was observed on the parent star, during which the brightness of the star increased tenfold by as much as 10 seconds. At the time of the outbreak, a huge emission of radiation occurred, which could easily make lifeless any known life forms.
3
Chthonic planet
The next type of terrestrial planets are chthonic exoplanets. These include gas giants, which during evolution lost the gas shell, exposing their solid core.
There are few planets of this type found, but the phenomena during which similar objects are formed are quite common in space. “Weathering” of gas occurs due to the proximity of the gas giant to the star. The stellar wind gradually blows off the gas component of the planet, leaving only heavy elements.
A few examples
CoRoT-7 b
CoRoT-7 b was discovered in 2009. In addition to the chthonic ones, it belongs to the type of super-earths, as well as to lava and iron planets. CoRoT-7 b revolves around a yellow dwarf at a distance of 489 light years from us. The radius of the planet is one and a half times greater than the Earth, and its mass exceeds 7.4 times the Earth. This means that the average density of the planet above the earth is about 2 times.
It is not surprising that CoRoT-7 b lost its gas envelope, because the radius of the planet’s orbit is 22 times smaller than the radius of the orbit of Mercury. Despite the fact that the parent star CoRoT-7 is slightly smaller than the Sun, the temperature in such a close orbit is very high. Probably a huge lava ocean is raging on the surface of CoRoT-7 b, the temperature of which is above 2500 ° C, this temperature is enough to melt almost all known metals and minerals. Due to the large tidal forces, the planet is probably always turned one side towards the star. This makes it possible on the colder side of the precipitation in the form of lava and stones.
HD 209,458 b
This planet is not chthonic, but we put it on this list in "advance". In the very distant future, this gas giant may lose most of its matter, becoming a chthonic exoplanet. HD 209458 b is perhaps the most studied exoplanet in the world.
It belongs to the category of hot Jupiters, revolves around a yellow dwarf, located in the constellation Pegasus at a distance of 153 sv. of the year.
HD 209458 b has dimensions that are almost one and a half times larger than Jupiter, with a mass equal to 0.6 Jupiter. The planet is removed from the star by a distance equal to 1/8 of the radius of the orbit of Mercury. The proximity to the star leads to the fact that one side of the exoplanet warms up to huge temperatures, and the other (the reverse side) is much colder. Like all planets that are very close to the star, HD 209458 b is always turned one side towards the star. The temperature difference between the sunny and dark sides leads to the fact that severe storms are raging on the surface, the wind speed of which is 2 km / s. In addition, the upper parts of the atmosphere under the influence of stellar wind are blown into outer space, forming a huge plume behind the planet, similar to a large comet tail.
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