An exoplanet is what? How do they discover and study exoplanets?

Exoplanet - a planet that is outside the limits of our solar system. Over the past two decades, thousands of such objects have been discovered, mainly with the help of NASA's Kepler Space Telescope.

Exoplanet - what is it?

These space objects are significantly different in size and orbit. Some of them are giant planets, circling near their stars. Some are covered in ice, others with 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 inhabited zone.

The inhabited zone is a range of distances from the star at which the planet's temperature 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 inhabited zone fuzzy.

Theory of the origin of life

Although the exoplanet is the discovery of the 1990s, for many years astronomers have been convinced of their existence. They did not just believe, 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 star and planets. After that, preserving the angular momentum meant that the future of the luminary should rotate faster and faster. However, although it has 99.8% of the mass of the solar system, the planets have 96% of its angular momentum. Astronomers wondered why our star rotates so slowly.

The young luminary had a very strong magnetic field, its lines of force permeated the disk of the swirling gas from which the planets were formed. These lines were associated with charged gas particles, and acted as anchors, slowing the rotation of the emerging Sun and unwinding the gas, which eventually turned into planets. Most of the stars rotate slowly, so astronomers have concluded that the same "magnetic braking" happened to them, which means that the formation of planets was to occur. Hence the logical conclusion: the planets should be searched around the Sun-like stars.

Early discoveries

For this and other reasons, scientists first limited the search for exoplanets to stars similar to the Sun, but the first two discoveries in 1992 were related to the pulsar (the rapidly rotating remains of a star that died as a supernova), called PSR 1257 + 12. The first confirmed exoplanet, circling around the star (photo posted in the article), meeting this requirement, was opened in 1995. It became 51 Pegasi b, the mass of which is commensurate with the mass of Jupiter, and which is 20 times closer to its Sun than Earth. This was 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 Gamma Cepheus. But since its orbit was much smaller than the orbit of Jupiter, the scientists did not state a final detection. Astronomers did not dare to assume that such planets exist. It was so different from our solar system that scientists were extremely cautious.

From large to small

Almost every exoplanet discovered at first is a huge Jupiter-like (or even more) gas giant rotating at a small 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 "rocking" of the star when the planets revolve around it. The large closely spaced cosmic bodies exerted such a significant influence that it could be easily detected.

Prior to the discovery of the exoplanets, the instruments could measure only the motions of the stars to within a kilometer per second, which was insufficient to detect their vibrations under the influence of the planets. Modern instruments are capable of measuring the speed to centimeter per second, in part because of the increased accuracy of the equipment, but also because of the greater experience of astronomers in isolating weak signals from the data.

Information explosion of "Kepler"

To date, there are more than 1000 confirmed exoplanets detected by one satellite. The Kepler Space Telescope was launched into orbit in 2009 and hunted for inhabited planets for four years. It used a method called "transit" - it measured the dimming of the star while passing a cosmic object in front of it.

Kepler revealed an abundance of different types of planets. In addition to gas giants and bodies of the terrestrial group, the telescope helped establish the existence of a new class of "super-earth", the dimensions of which are within the dimensions of the Earth and Neptune. Some of them are located in the inhabited zones of their stars, but astrobiologists are still checking the calculations to find out how life can develop in such worlds.

In 2014, the astronomers of Kepler presented a method of "multiplicity check", which was supposed to increase the speed of translation of the candidate planets into confirmed status. The technique is based on orbital stability - many stars were dimmed at short intervals, which could be caused only 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), who hunted 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 satellites MOST and NASA TESS continue to work, and the Swiss CHEOPS and the ESA PLATO satellite will begin to search for transit from outer 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 oscillations of stars, but many other telescopes are involved in the hunt.

One example is NASA's 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 3000 known planets, it is difficult to choose several of them. Small solid exoplanets in the inhabited zone seem to be the best candidates, but astronomers distinguish others that have broadened our notion of the formation and development of other worlds.

First Swallows

51 Pegasi b. As mentioned above, this was the first proven exoplanet that revolves around a solar-type star. Having half the mass of Jupiter, it is removed from the center of the system by a distance of Mercury. The planet is so close to its luminary that, most likely, one side of it is in the tidal capture - it is constantly facing the star.

HD 209458 b. This was the first exoplanet discovered in 1999 (the photo was placed in the article), which passed by 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" that orbits the star, bright enough to be seen with the naked eye. Thus, astronomers can study the system in more detail than any other. Her "year" is only 17 hours and 41 minutes (this was found when MOST watched the system for two weeks in 2011). Theorists suggest that the 55 Cancri can be rich in carbon and has a diamond core.

HD 80606 b. This exoplanet is the record holder (at the time of its discovery in 2001) due to 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 opened in 2011 and has a kind of sunscreen layer - stratosphere - which absorbs some of the visible and ultraviolet light of the parent star. The planet not only moves in orbit in the opposite direction, but also causes oscillations of the luminary, which the MOST satellite registers.

Twins of the Earth

Kepler-442b. This exoplanet is, as they say, a "twin of the Earth". With its size, mass and temperature regime, it is the most like our planet. Open January 6, 2015, it is in the constellation of Lear at a distance of 1.120 light years. The temperature on the surface of this rocky exoplanet is -40 ° C. Its mass is 2.34 times the mass of the Earth, and gravity is 30% larger. The planet is outside the zone where the tidal capture is active. In a publication published in 2015, she, along with Kepler-186f and 62f, was named the best candidate for potentially populated planets (see photo).

Exoplanet Kepler-78b. It revolves around the Kepler-78 star. At the time of opening in 2013, the planet most resembled the Earth in terms of mass, radius and average density. It was found not only its transit against the background of the luminary, but also the eclipse and reflected light corresponding to the orbital phases. The "year" of the exoplanet lasts only 8.5 hours, since it is 40 times closer to the star than the distance from Mercury to the Sun.