Mankind realized quite early that there are stars in the sky, and there are a lot of them. Then this thought was supplemented by the argument that the stars are similar to our Sun, or at one time were similar. Then it became clear that the Earth and other planets revolve around the Sun, and a reasonable question arises: "Why can't planets revolve around other stars?" The theory did not see any problem in the possible existence of planets outside the solar system, but science always needs facts. And over time, the facts were found.
Exoplanet
What is an exoplanet? Everything is simply outrageous - this is a planet outside the solar system that revolves around a star. The term was formed from the abbreviation extra solar planet, that is, extrasolar planet. But do not be confused: not everything outside the solar system is an exoplanet, there are also celestial bodies - orphans, the so-called planemos, who travel through space outside the orbit of the parent star.
What are the exoplanets? They are very different. The Kepler space telescope observed only two constellations - Cygnus and Lyru - for 8 years, but found about a thousand candidates for exoplanets. And we have 88 constellations, and these two still have something to discover.
Thus, there are many exoplanets, and they are different. The detection methods, which we will talk about later, do not allow us to accurately determine the composition, atmosphere and nature of the discovered planets. What can we say, we cannot even directly see the exoplanet. But even through indirect signs and data, a classification can be made.
The two main classes of exoplanets are small rock planets and giant planets. If we apply this classification to our solar system, then Venus, Mercury, Earth and Mars will go to the first, and Jupiter, Saturn, Uranus and Neptune will go to the second.
Each of the classes can be divided into a number of subclasses. Let's analyze the most basic ones.
Promotional video:
Chthonic planet
The chthonic planet is a gas giant that is rapidly falling on the parent star. In the center of the gas giant there is a small dense nucleolus that holds huge masses of gaseous matter around it. Gradually approaching the parent star, the gas giant begins to evaporate its shell until one core remains.
Artistic depiction of the transit of the chthonic planet HD 209458b in front of its star. European Space Agency, Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS, France) and NASA / wikimedia.org (CC BY 4.0)
Super-earth
The main and only criterion by which a planet can be ranked as a super-earth is its mass. Such planets are usually several times heavier than the Earth, but at the same time they are much smaller than the gas giant. In contrast to the chthonic planets, quite a lot of such celestial bodies were discovered, and in 2007 astronomers found the super-earth Gliese 581-c in the habitable zone.
Gliese 581c Tyrogthekreeper / wikimedia.org (CC BY-SA 3.0)
Hot jupiter
The name of the well-known planet is written with a small letter not by mistake, hot Jupiter is not a specific planet, but a whole planetary class. Unlike our gas giant, hot Jupiters are located almost close to the mother star, which heats their atmosphere to 1500 K. Due to a number of features, in particular, their large size, a lot of hot Jupiters have been discovered.
Cold jupiter
It is to this class that the original Jupiter and Saturn belong - cold Jupiter is located at such a distance from the star that it receives most of its heat from internal processes, and not from radiation.
Ice giant
We also have such planets in our system: Uranus and Neptune are typical representatives of ice giants - planets with a large size and distance from their native star. Due to the fact that the rays weakly heat such planets, almost their entire surface is bound by ice, not only water ice, but also methane and hydrogen sulfide ice.
Voyager 2 image of Neptune in August 1989. NASA / wikimedia.org (CC0 1.0)
The list of exoplanet species can be continued for a very long time. There are oceans planets, carbon planets, hot and cold neptune, and much, much more. But we'll talk about how they are discovered.
Methods for detecting exoplanets
Let's do a simple experiment. Somehow, on a warm summer night, preferably in the south, near the Equator, raise your eyes to the night sky. What will you see? That's right, myriads of stars. Different stars - bright and not very bright, solitary and in constellations. But practically everyone, except for Mercury, Jupiter, the Moon and maybe Mars, will be stars.
The same is the case with the giant telescopes in observatories. The stars, due to their size and radiation, almost completely clog the entire observable space of space, and the planets, which glow with very weak, reflected light, are simply not visible against their background. So if there is somewhere a civilization of our level of development, it most likely guesses about the presence of Jupiter and Saturn near the Sun, but no more.
But exoplanets are found, and very reliably. We have several ways to do this.
The most prolific is the transit, or transit photometry method. The fact is that every star has such an indicator as luminosity. Roughly speaking, luminosity is all the light emitted by a star per unit of time. But if some celestial body passes between the telescope of the observer and the star, then at the time of passage the luminosity falls. And if this process is repeated periodically, it means that the planet revolves around the star. There are pros and cons to this method. The main plus is the ability to determine the size of an exoplanet. Minus - to accurately determine the presence of a planet with a long orbital period, for example, like Jupiter (12 years), you will have to observe the star for a very long time.
Doppler method. Named after the Austrian mathematician Christian Doppler, the method measures the spectral displacement of a star under the influence of a planet. The laws of gravity work in both directions, including for us, therefore not only the Earth attracts us, but we also the Earth. Likewise, in a pair of planet - star. The rotation of the massive exoplanet shifts the radial radial velocity of the parent star, and the instruments show how the planet sways in the red region of the spectrum, then in the violet. The Doppler method allows, together with the transit one, to determine the density of the planet, but again - only if it is large enough.
Gravitational microlensing. This method is tied to the presence of another star between the astronomer's telescope and the observed star, which acts as a gravitational lens. But if the lens star has its own planet, then the light of the observed star will be characteristically distorted.
And finally, the exoplanet can be easily seen. The planets themselves are very weak light sources, so it is very difficult to detect terrestrial celestial bodies using this method. The most likely objects to detect are giants larger than Jupiter, which are far enough away from the star to emit infrared rays by themselves.
Until 2014, the Doppler method, or the radial velocity method, and the transit method shared the leadership in the number of discovered exoplanets. In 2014, thanks to the flagship of the search for exoplanets - the Kepler telescope, the transit method went far ahead.
An interesting fact: the information obtained by Kepler is so extensive that it is freely available for everyone to study. Thus, the Planet Hunters project has already helped to discover three exoplanets.
The possibility of life and the prospects of colonization
Naturally, ordinary people are less interested in hot neptunes and methods for detecting exoplanets. The main interest of the public is the possibility of life and colonization of distant celestial bodies.
Forplayday / bigstock.com
In total, 3,614 exoplanets were discovered in June 2017. Of these, they resemble the Earth - 216. There are plenty to choose from. But the supposed colonization and the possibility of the existence of life are limited by a number of parameters.
Habitable zone
Accustomed to measuring everything by themselves, earthly astronomers have derived the concept of a habitable zone. The essence of the concept is that each star must have a certain zone in which the planets can be inhabited.
The main condition of the habitable zone is the existence of liquid water. Therefore, the planet must be close enough to the star so that the water does not freeze, and far enough so that it does not evaporate. To calculate the center of the habitable zone, an equation was even derived that looks like dAU = √Lstar / Lsun, where d is the average radius of the habitable zone, Lstar is the luminosity of the star, and Lsun is the luminosity of the Sun.
There are 52 planets in the list of habitable exoplanets, according to the University of Puerto Rico. One of them is the mini-earth TRAPPIST - 1d, 21 planets comparable to the Earth, and 30 super-earths.
The main criteria are planet composition, surface temperature, size and atmosphere. The planets are evaluated according to the degree of similarity with the Earth, and even a special numerical criterion has been derived, which consists of all of the above. If a planet is gaining from 0.8 to 1 in the Earth's similarity index, then it can be safely entered into the list of potential colonies. So, take your pick, gentlemen colonists!
Kepler-438b
He was the record holder for similarity to the Earth until 2016. Its ESI (Earth Similarity Index) is 0.88. The planet itself is located 470 light years from Earth in the constellation Lyra, and the parent star of Kepler-438b is only half the size of the Sun. The planet itself is located in the habitable zone of the star, in size exceeds the Earth by 12%.
Proxima Centauri b
The home star of this planet is Proxima Centauri, the closest to the Sun. The planet itself, like the luminary, is located 4.22 light years from us. According to the similarity index, Proxima Centauri is gaining 0.85 and is confidently staying in the top.
TRAPPIST-1 d
At the moment, the planet TRAPPIST, discovered by the telescope, is the most similar to our home Earth. It is also the third from its parent star, slightly inferior to Earth in size and very similar in composition. The estimated surface temperature is +15 degrees Celsius.
Unfortunately, the availability of suitable planets for colonization is far from the most important barrier on the way of human settlement of the Universe. Even to Proxima Centauri b, with current technologies, potential colonists have a very, very long flight time. And until we learn to effectively cover distances of at least 10 light years, it's too early to talk about the conquest of exoplanets.
There are still many variations of exoplanets. But the biggest discoveries lie ahead - ambitious international projects are already being prepared on Earth to create giant telescopes and space observatories that will be able to see what we cannot find now. But I have not yet mentioned that exoplanets have satellites.
