An exoplanet is a planet that is outside of our solar system. Over the past two decades, thousands of such objects have been discovered, mainly using the NASA Kepler space telescope.
Exoplanet - what is it?
These space objects are significantly different in size and orbits. Some of them are giant planets orbiting near their stars. Some are covered with ice, others are rocks. NASA and other agencies are looking for a special kind of planet: they need an exoplanet, similar to the Earth, which revolves around a star, similar to the Sun, and is located in the habitable zone.
The habitable zone is the range of distances from the star at which the temperature of the planet allows the existence of liquid oceans of water, which is crucial for life. The earliest definition of the zone was based on simple thermal equilibrium, but modern calculations include many other factors, including the greenhouse effect of the planet’s atmosphere. This makes the boundaries of the habitable area blurred.
Theory of the origin of life
Although the exoplanet is a discovery of the 1990s, for many years astronomers have been convinced of their existence. They not only believed, but based their conclusions on the slow rotation of our own Sun and other stars.
Astronomers have a theory of the origin of life in our solar system. In short, a rotating cloud of gas and dust (the so-called protosolar nebula), under the influence of its own gravity, collapsed and formed our luminary and planets. After that, maintaining the angular momentum meant that the future star should rotate faster and faster. However, although it has 99.8% of the mass of the solar system, planets possess 96% of its angular momentum. Astronomers have wondered why our star rotates so slowly.
The young star had a very strong magnetic field, the lines of force of which penetrated the disk of swirling gas, from which the planets were formed. These lines were connected with charged particles of gas, and acted as anchors, slowing down the rotation of the forming Sun and spinning gas, which eventually turned into planets. Most stars rotate slowly, so astronomers concluded that they also had the same “magnetic drag”, which means that planets should have formed. Hence the logical conclusion: planets must be sought around stars like the Sun.
Early discoveries
For this and other reasons, scientists initially limited the search for exoplanets to stars similar to the Sun, but the first two discoveries in 1992 were related to a pulsar (the rapidly rotating remnants of a star that died as a supernova) called PSR 1257 + 12. The first confirmed exoplanet orbiting a star (photo posted in the article) that meets this requirement was discovered in 1995. She became 51 Pegasi b, the mass of which is comparable with the mass of Jupiter, and which is 20 times closer to its Sun than the Earth. This came as a surprise. But another oddity occurred seven years before, thanks to which it became clear that many exoplanets would be discovered.
In 1988, a group of Canadian scientists discovered a planet the size of Jupiter, moving around the Cepheus Gamma. But since its orbit was much smaller than the orbit of Jupiter, scientists have not announced the final discovery. Astronomers did not dare to suggest that such planets exist. It was so different from our solar system that scientists were extremely careful.
From large to small
Almost every exoplanet discovered at first is a huge Jupiter-like (or even larger) gas giant that rotates a short distance from its parent star. This is explained by the fact that astronomers used the technique of measuring radial velocity, which determines the degree of "swaying" of a star when the planets revolve around it. Large nearby cosmic bodies had such a significant effect that it could be easily detected.
Before the era of discoveries of exoplanets, instruments could only measure the motion of stars with an accuracy of a kilometer per second, which was insufficient to detect their oscillations under the influence of planets. Modern instruments are capable of measuring speeds up to a centimeter per second, partly because of increased equipment accuracy, but also because of the greater experience of astronomers in extracting weak signals from data.
Information explosion "Kepler"
To date, there are more than 1000 confirmed exoplanets discovered by one satellite. The Kepler space telescope was launched into orbit in 2009 and hunted inhabited planets for four years. It used a method called “transit” - measured the dimming of the star during the passage of a space object in front of it.
Kepler revealed an abundance of various types of planets. In addition to the gas giants and the bodies of the terrestrial group, the telescope helped establish the existence of a new class of “super-earths,” whose sizes are within the dimensions of the Earth and Neptune. Some of them are located in the habitable zones of their stars, but astrobiologists are still checking the calculations to find out how life can develop in such worlds.
In 2014, Kepler’s astronomers introduced the “multiplicity check” method, which was supposed to increase the speed of transferring candidate planets to confirmed status. The technique is based on orbital stability - many stars were obscured at short intervals, which could only be caused by planets in small orbits, since if they were stars, they would gravitationally push each other out of the system for several million years.
Other missions
Although the satellites (Kepler and the French CoRoT), hunting for exoplanets, completed their initial missions, scientists are still processing the data obtained with their help, making new discoveries. And they will not remain without work. The MOST and NASA TESS satellites continue to operate, while the Swiss CHEOPS and the ESA PLATO satellite will begin searching for transit from space in the near future. On Earth, the HARPS spectrograph of the 3.6-meter telescope of the European Southern Observatory in Chile conducts a Doppler search for star vibrations, but many other telescopes are involved in hunting.
One example is the NASA Spitzer Space Telescope. Since it is sensitive in the infrared region of the spectrum, it is able to measure the temperature profile of the exoplanet and give an idea of its atmosphere.
Of the more than 3,000 known planets, it is difficult to opt for several of them. Small solid exoplanets in the habitable zone seem to be the best candidates, but astronomers distinguish others that have expanded our understanding of the formation and development of other worlds.
First swallows
51 Pegasi b. As mentioned above, this was the first proven exoplanet orbiting a solar-type star. With half the mass of Jupiter, it is removed from the center of the system at a distance of Mercury. The planet is so close to its luminary that, most likely, one of its sides is in a tidal capture - it is constantly facing the star.
HD 209,458 b. This was the first exoplanet discovered in 1999 (photo posted in the article), which passed in transit past its star (although the Doppler method was used), followed by other discoveries. This is the first planet outside the solar system, in which the parameters of its atmosphere were determined, including the temperature profile and the absence of clouds.
Noteworthy Worlds
55 Cancri e. This exoplanet is what is called a “super-earth,” which orbits around a star bright enough to be seen with the naked eye. Thus, astronomers can study the system in more detail than any other. Its “year” is only 17 hours and 41 minutes (this was established when MOST monitored the system for two weeks in 2011). Theorists suggest that 55 Cancri e can be rich in carbon and has a diamond core.
HD 80606 b. This exoplanet is a record holder (at the time of its discovery in 2001) for the eccentricity of the orbit. It is likely that the path of its movement, similar to the orbit of Halley's comet, may be associated with the influence of another star. In addition, such an extreme orbit is the cause of the extreme variability of the planet's environment.
WASP-33b. It was discovered in 2011 and has a kind of sun-protection layer - the stratosphere - which absorbs part of the visible and ultraviolet light of the parent star. The planet not only moves in the orbit in the opposite direction, but also causes luminary oscillations, which are recorded by the MOST satellite.
Earth Twins
Kepler-442b. This exoplanet is what is called the “double of the Earth”. With its size, mass and temperature regime, it is most similar to our planet. Opened on January 6, 2015, it is located in the constellation Lyra at a distance of 1,120 light-years. The surface temperature of this rocky exoplanet is -40 ° C. Its mass is 2.34 times greater than the mass of the Earth, and gravity is 30% more. The planet is located outside the zone where tidal capture operates. In a paper published in 2015, she, along with Kepler-186f and 62f, was named the best candidate for potentially inhabited planets (see photo).
Exoplanet Kepler-78b. She revolves around the star Kepler-78. At the time of discovery in 2013, the planet most resembled the Earth in mass, radius and average density. Not only was its transit against the background of the luminary, but also an eclipse and reflected light corresponding to the orbital phases. The “year” of the exoplanet lasts only 8.5 hours, because it is 40 times closer to the star than the distance from Mercury to the Sun.