An exoplanet is usually called cosmic body, which “dwells” outside solar system and, accordingly, orbits another star. Such objects are usually quite dim and have relatively small dimensions. That is why they were discovered recently - in 1980, with the help of improved technical instruments and methods. Scientists continue to study exoplanets in distant star systems to this day.
Today, science knows of the existence of 1821 such objects, 1135 of which belong to planetary systems. It should be noted that the number of objects that fit the parameters of an exoplanet is much larger. After the end of the Kepler mission, scientists counted only 2,750 such bodies. But in order to make sure that these objects belong specifically to exoplanets, we need additional research with the use of ground vehicles.
The number of exoplanets located in our galaxy can reach 100 billion, of which 5-20% may be similar to Earth. It is also known that about a third of all Sun-like stars have already formed Earth-like objects.
It should be noted that most of known exoplanets were discovered not through visual observation, but through the use of various detection techniques. So far, the overwhelming majority of discovered planets are gas giants. But scientists are convinced that talking about Milky Way predominantly dominated by Jupiter-like objects, it's too early. And there is a simple explanation for this: the lack of effective research methods. After all, it is much easier to notice a massive short-period object than a smaller body.
History of discoveries
According to generally accepted opinion, the first person to declare the possibility of finding planets in other star systems was Captain Jacob, an astronomer at the Madras Observatory, back in the mid-19th century. Already in those days there was a version that in binary system 70 Ophiuchus is “inhabited” by the planet.
At the end of the same century, the American scientist Thomas D.D. Xi discovered a moving dim body in the same system. Then it was even possible to calculate the period of its revolution - 36 years. But new calculations made by F.R. Multon, refuted Xi’s beliefs. Today scientists also question the findings planetary bodies in the zone of system 70 Ophiuchus.
The first attempts to search for planets in distant star systems used data on the positions of nearby stars. In 1916, Edward Barnard managed to identify a certain “red star” that moved across the sky at a higher speed than other luminaries. This object was named "Barnard's Flying Star".
In fact, it turned out to be the closest luminary to the Sun. Its mass is almost 7 times less than our star. Scientists have suggested that if there are still planets in its system, this would certainly have a noticeable effect on the “red star”. In the mid-20th century, Peter Van de Kamp announced the discovery of an object similar to Jupiter. But just ten years later, J. Gaywood proved that Barnard’s star moves without any slowdowns or fluctuations. This meant that the probability of finding large bodies near it was practically zero.
In the late 80s of the 20th century, scientists around the world began to measure the speed of movement of the stars closest to the Sun, conducting a separate search for exoplanets, using advanced spectrometers.
One of the first serious discoveries of an extrasolar planet belongs to Canadian scientists B. Campbell, S. Young and G. Walker. Then, in 1988, researchers identified a planet under the “protection” of the subgiant Gamma Cepheus A. But the veracity of the find was confirmed only by 2002.
Immediately after this discovery, scientists were able to “see” a supermassive planet near the star HD 114762 A. Just as in the first case, the object acquired the status of a planet much later - only by 1999.
For the first time, exoplanets were discovered near the neutron giant PSR 1257+12. The author of this discovery was Alexander Volshchan. These objects were classified as “secondary” due to the fact that the star system to which they belong was formed as a result of a supernova explosion.
In 1995, French scientists Michel Mayor and Didier Queloz recorded vibrations emanating from the region of 51 Pegasus. Body sway data was taken into account while working with a powerful, ultra-precise spectrometer. It turned out that the cause of these wobbles is a planet reminiscent of Jupiter orbiting in the zone of the star, which is also at a relatively close distance from its “sun”. Among astronomers, such objects are called “hot Jupiters.”
A little later, using the Doppler method, which consists of measuring stellar radial velocity, more than 100 exoplanets were discovered.
In mid-2004 in star systemμ Altar was the first time a planet was spotted - hot Neptune. It turned out that full turn This object completes its orbit around its star in 9.5 days, and is located 0.09 AU from it. average temperature on the surface of the planet is +626 °C. The dimensions of hot Neptune are 14 times larger than the dimensions of the Earth.
The first planet similar to ours was discovered in the region of the sun-like star Gliese 876. The mass of the found object exceeded the mass of the Earth by almost 14 times.
In 2004, scientists for the first time managed to obtain a snapshot of an object claiming to be an exoplant, which lived in the brown dwarf system 2M1207.
In 2008, scientists were able to obtain a photograph of a single planetary system, which depicted 3 objects at once under the “protection” of HR 8799, which belongs to the large constellation Pegasus. This planetary system is the first to be discovered near a hot white star.
In the same year, astronomers were lucky enough to “catch” the planet Fomalhaut b, moving around the luminary Fomalhaut.
In 2011, analyzing images from the Kepler telescope, scientists discovered a super-Earth located in the region of Kepler-22 b.
A few days later, near the star Kepler-20, astronomers for the first time recorded exoplanets with dimensions identical to Earth.
At the beginning of 2012, American astrophysicists discovered another exoplanet - GJ 1214 b with water on the surface and an orbital period of 38 hours around its axis. According to scientists, the temperature of the substance is upper layers“finds” is about 230 °C.
Methods and tools for studying exoplanets
Astronomical satellites
- COROT (ESA is a special machine that conducts observations from Earth orbit. Its work is based on studying the light curves of many luminaries at the moment when other objects - planets - pass in front of them. This machine was launched 8 years ago. Scientists hoped to make intriguing discoveries thanks to it - find super-Earths. As a result, by 2010, during the COROT mission, 7 exoplanets and 1 star related to brown dwarfs were discovered.
- Kepler (NASA) is an extraterrestrial machine with the Schmidt system, capable of simultaneously observing 100 thousand stars. It was launched in 2009. While working with the device, scientists hoped to detect 600 new planets, the sizes of which exceed those of Earth by 2-2.2 times. Kepler's planned operational life was initially limited to 3.5 years. Scientists then decided to extend his stay in space until 2016. But already in 2013, the main systems of the machine became unusable. It is known that by 2012 he managed to discover 132 exoplanets, as well as identify about 2,750 serious planet candidates located near distant stars.
Ground observatories
Leading studies produced by the transit method
- SuperWASP is one of the best ground-based machines that has helped discover about 70 exoplanets using the transit observation method. The SuperWASP system includes 2 observatories.
- The HATNet project is a system consisting of 6 so-called “automata” - telescopes with a fairly wide field of coverage, located in the Arizona and Hawaiian observatories. With the help of these machines, 33 more exoplanets became known.
Advanced Radial Velocity Observations (Doppler)
- HARPS is a spectrograph attached to one of the machines of the Chilean Observatory, whose observations are based on the radial velocity method.
- The Keck Observatory is a large observatory consisting of a pair of powerful reflecting telescopes. The diameter of each of the three mirrors of the device reaches 10 meters.
Other planned missions:
- Gaia - new space observatory. the main objective its launch - the creation of a 3D map of the Milky Way. Also, with the help of Gaia, scientists hope to discover about 10 thousand more exoplanets.
Projects in progress:
- TESS is under development. The project will be completed by 2017.
- EChO - in progress theoretical study all project details. With the “consent” of ESA, the launch is planned for 2022.
- ATLAST - intensive work is underway on the project. The launch is expected only after 2025.
In addition to the imminent implementation space missions, scientists also plan to improve ground-based instruments. For example, the building of the European aircraft under construction is extremely large telescope a device will be “landed” that will make it possible to study the atmosphere of exoplanets.
The most common methods for detecting exoplanets in our Galaxy
1. Doppler method - one of the most popular methods, the essence of which is to calculate the radial speed of the star. Thanks to this method, planets with a size several times larger than our planet can also be detected. Observed giant planets are usually located relatively close to their star. As they revolve around the star, they rock it. It is precisely such changes in the shift of the star’s spectrum that can be detected using the Doppler method. It also allows you to accurately calculate the amplitude of velocity fluctuations directly for the “star-planet” pair, the mass of the observed object, eccentricity and orbital period. WITH active use Using this method in the search and detection of bodies of this type, scientists were able to register more than 600 new planets.
2. Transit method consists in determining the moment when the planet will cross the disk of the star. The first sign that a body is passing through the disk of a star is a weakening of its luminosity. To determine the size and density of planets, the transit research method is usually combined with the Doppler method. It should be noted that this method can only record those objects whose shaved areas belong to the same plane as the observation point. Thanks to the transit method, about 185 planets were discovered.
3. Gravitational microlensing method . It consists in choosing a third object - a star, acting as a lens that will focus with its gravitational field the glow of that star and its system. If planets orbit around the “lens”, then this will be indicated by the appearance of an asymmetrical light curve and, probably, the absence of achromaticity will be noticed. It should be noted that this method has quite limitations in practical application. Only 13 planets were discovered this way.
4 Astrometric method . Its essence is tracking changes in the star’s own motion under the influence of the planet’s gravity. Using this research method, scientists were able to establish more accurate masses of many exoplanes. For example, Epsilon Eridani b.
5. Radio observation of pulsars . If there are planets in their systems, then this can be easily recognized by the emitted signal. It will be oscillating in nature. Flows of radiation of enormous power will form in space conical surfaces. And, if, for example, the Earth is on one of them, then its radiation will be immediately registered. Using this method, 5 more planets were found.
6 Direct observation . This method consists of obtaining direct images of exoplanets, suppressing the brightness of the star. This method is most effective when observing planets that are hot and distant from their stars.
In the near future, the James Webb Telescope will be able to directly “go” to exoplanets and study their atmospheres in detail.
Nomenclature
The name of the exoplanet consists of two parts. The first part is the name of the star to which it belongs. The second part of the name is a Latin small letter. The very first discovered planet, “assigned” to a particular star system, will be called planet “b”, the next - “c”, then “d”. The letter “a” does not appear in the names of the planets, since this symbol implies the luminary itself. It is necessary to emphasize that planets are not named according to their proximity to the star. That is, object “c” may be located at a smaller distance from the center of the system than object “b”.
There are also exceptions to the names of exoplanets. Even before the discovery of the 51 Pegasi star system, the names of exoplanets sounded different. One of the first objects discovered near the pulsar PSR 1257+12 was given names with capital letters. For example, PSR 1257+12 B. Moreover, immediately after finding a new planet located at a smaller distance from the star, it was given the name PSR 1257+12 A instead of D.
Soon all the old names were changed to new ones that correspond more later rules names.
It is no secret that many exoplanets also have their own “nicknames”. For example, the planet 51 Pegasus b has a second name - “Bellerophon”. According to scientists, assigning personal names to exoplanets is considered ineffective.
Properties of exoplanets
About 10% of luminaries have planets. Their number is also growing with the discovery of new, more effective ways their research and improvement of technology.
First discovered planets belonged to the type of giant planets. This is due to the fact that in the past, smaller objects were much more difficult to detect than they are today. In our time modern technology allows you to record bodies similar in mass indicators to Neptune. About 200 exoplanets discovered by the Kepler telescope have about the same mass as Earth, and 680 of them are similar in size to super-Earths. There are currently more than 1000 planets with mass indicators like Neptune, and more than 200 of Jupiter.
Scientists note pronounced dependence the presence of giant planets in the system from the percentage heavy metals as part of a star. Systems that include planets of this group most often refer to systems with stars solar type. Red dwarfs have a much smaller number of such planets. Recent observations using gravitational microlensing indicate that currently known systems are dominated by planets with masses similar to Uranus and Neptune.
Scientists were able to calculate the diameter of most of the discovered planets, which allowed them to calculate their density, and laid the foundation for new theories related to the presence of massive nuclei that consist of heavy metals. Tristan Guillot, in collaboration with a European group of scientists, was able to establish that when comparing the density of objects with the percentage of heavy elements in their stars, there is a certain pattern. Planets born in star systems similar to the Sun predominantly have medium-sized nuclei, which cannot be said about objects formed near stars with the highest concentration of metals.
Scientists have found that exoplanets with an interior consisting of several layers (core, crust and mantle) have the ability to release heat, which could actively participate in the creation and preservation optimal conditions for the existence of living beings on them.
The planet, which is similar to Earth in most respects, was discovered in 2009. The surface temperature of Gliese 581 c ranges from 0 to 40 °C. This fact suggests that there may be water here, or even life.
Some planetary systems
Upsilon Andromeda d
- a planet belonging to the category of gas giants containing water vapor - clouds. One of the most popular topics related to exoplanetorology is the question of the real existence of large satellites among giant gas planets. To date, scientists have not been able to detect a single object resembling the “moon” of a giant exoplanet.
51 Pegasus
- a star similar to the Sun is the first discovered luminary in the system of which scientists found an exoplanet.
υAndromeda - one of the first stars in the region of which several exoplanets were discovered at once
Tau Ceti - the closest star to the Sun, where the rotation of five planets was recorded at once, but this discovery is still awaiting confirmation.
εEridani - one of the closest stars from the Sun, which can also be seen with the naked eye.
55 Cancer
- 5 planets were discovered here. Astronomers identified one of them as a hot super-Earth, which turned out to be 2 times larger than the Earth.
γCepheus
- one of the first binary star structures where an exoplanet was found.
Gliese876 belongs to the type of luminaries called red dwarfs. It became the first star of its type where several planets were found.
HD209458 - a star in the region of which one of the most interesting space “finds” of scientists is located - the “evaporating planet” HD 209458 b.
KOI-961, KOI-961 d and KOI-961 b
- planets living near the star KOI-961, which is a red dwarf. The size of the radii of the “finds” is also close to the size of the radius of the Earth.
OGLE-235/MOA-53
- an exoplanet first discovered during a test of the gravitational lensing method.
μAltar
. It turned out that this system contains one of the “lightest” exoplanets, presumably related to bodies terrestrial group.
PSR1257+12
- a pulsar in which it exists unique system planets, first found outside our star system. The approximate mass of one of its objects is 0.025 of total mass Earth.
HD188753 - another unique space “complex” consisting of three stars. A big surprise for scientists was the discovery of a planet in this area HD188753Ab
HD189733 - star system where the planet lives HD189733b .- the first exoplanet in the history of astronomy for which scientists made a temperature map.
HD85512b, Gliese 581 c, Kepler-22 b, Gliese 581 d - unique exoplanets outside the solar system, which are similar to Earth in many respects.
WASP-17b - a planet rotating in the opposite side rotation of its star.
Gliese 581 g - a planet on which liquid water probably exists.
COROT-7 b - one of the first identified super-Earths recorded using the transit method. Its dimensions exceed those of Earth by about 1.5 times.
OGLE-TR-56 - a star discovered by scientists using the transit method.
HD10180 - a star near which it was possible to record a group with the largest number planets. Today there are 9 of them.
Kepler-10b - the densest planet (8.8 g/cm³).
Kepler-11 - a luminary included in the constellation Cygnus, where 6 planets were also discovered.
WASP-19 b - a planet with a period of revolution of 19 hours around its axis, which corresponds to about 0.788 of our days.
GJ1214b - the only ocean planet known to us.
KOI-961d - one of the smallest distant planets
WASP-33b - the hottest exoplanet. Its temperature is 3200 °C.
GJ1214b And WASP-43b - owners of the most “compressed” orbits. GJ1214b - among the terrestrial planets, and WASP-43b - among the red-hot Jupiters.
KIC6185331b And KIC10905746b - the first planets discovered not by professionals, but by “amateurs”.
Kepler-20f And Kepler-20e - one of the known exoplanets, the sizes of which are very close to the size of the Earth.
KOI-961 , KOI-961d And KOI-961 b - planets orbiting near a star KOI-961 , related to red dwarfs. The size of their radii is also close to the size of the Earth's radius.
HD37605c - the so-called “cold Jupiter”, first discovered in 2012.
47 Ursa Major - a star containing 3 cold Jupiters - and 47 Ursa Major d , 47 Ursa Major c And 47 Ursa Major b
GD66b - first gas planet, consisting mostly of helium.
WASP-12b - a planet in the region of which there is likely to be an exomoon.
HIP11952 c And HIP11952 b - planets found in the system HIP11952 . They are considered the oldest. Their age is approximately 12.8 billion years.
Alpha Centauri Bb - perhaps the closest exoplanet to us.
GU Pisces b - a planet that “keeps” at a record distance from its star (300 billion km)
Consequences of exoplanet discovery
The discovery of new exoplanets has become a real breakthrough in astronomy. These discoveries helped scientists make important conclusions. For example, state the fact that planetary systems are one of the most common systems in space.
Unfortunately, today there is no generally accepted theory about the formation of planets. But, having received certain statistics, new important details should become clear on this topic in the near future.
It turned out that most star systems containing planets are very different from ours. Scientists explain this by the selectivity of the research methods used. After all, it is much easier to detect short-period major planets, than smaller ones like Earth. The identification of planets similar to ours today can be carried out exclusively by the transit method.
"Closing" of exoplanets
A detailed study of the star WASP-9 using the HARPS spectrometer helped confirm traces of another stellar spectrum in it. This means that the existence of the exoplanet WASP-9 b is completely refuted.
The total number of exoplanets in the Milky Way galaxy is more than 100 billion. An exoplanet is a planet that is outside our solar system. Currently, scientists have discovered only a small fraction of them. About the 10 most incredible planets in this post.
The darkest exoplanet is the distant, Jupiter-sized gas giant TrES-2b.
Measurements have shown that planet TrES-2b reflects less than one percent of light, making it blacker than coal and naturally darker than any planet in the solar system. The work on this planet was published in the journal of the Royal Astronomical Society Monthly Notices. Planet TrES-2b reflects less light even than black acrylic paint, so it is truly a dark world.
TrES-4
The most big planet of those found in the Universe is TrES-4. It was discovered in 2006 and is located in the constellation Hercules. The planet, called TrES-4, orbits a star that is about 1,400 light-years away from planet Earth.
Researchers claim that the diameter of the discovered planet is almost 2 times (more precisely 1.7) larger than the diameter of Jupiter (this is the largest planet in the solar system). The temperature of TrES-4 is about 1260 degrees Celsius.
COROT-7b
A year on COROT-7b lasts just over 20 hours. It is not surprising that the weather in this world is, to put it mildly, exotic.
Astronomers have suggested that the planet consists of cast and solid rock, and not of frozen gases, which will certainly boil away under such conditions. The temperature, according to scientists, drops from +2000 C on the illuminated surface to -200 C on the night.
WASP-12b
Astronomers saw a cosmic cataclysm: a star was consuming its own planet, which was in close proximity to it. It's about about exoplanet WASP-12b. It was discovered in 2008.
WASP-12b, like most known exoplanets discovered by astronomers, is a large gaseous world. However, unlike most other exoplanets, WASP-12b orbits its star at very close range- a little more than 1.5 million kilometers (75 times closer than the Earth to the Sun).
The vast world of WASP-12b has already stared into the face of its death, researchers say. The most important problem of the planet is its size. It has grown to such an extent that it cannot hold its matter against the gravitational forces of its native star. WASP-12b is giving up its matter to the star at a tremendous rate: six billion tons every second. In this case, the planet will be completely destroyed by the star in about ten million years. By cosmic standards, this is quite a bit.
Kepler-10b
Using a space telescope, astronomers were able to discover the smallest rocky exoplanet, with a diameter of about 1.4 times the diameter of Earth.
The new planet was designated Kepler-10b. The star it orbits is about 560 light-years from Earth in the constellation Draco and is similar to our Sun. Belonging to the class of “super-Earths,” Kepler-10b is in an orbit quite close to its star, orbiting it in just 0.84 Earth days, while the temperature on it reaches several thousand degrees Celsius. Scientists estimate that with a diameter of 1.4 times the diameter of the Earth, Kepler-10b has a mass of 4.5 times the Earth's.
HD 189733b
HD 189733b is a Jupiter-sized planet orbiting its star 63 light-years away. And although this planet is similar in size to Jupiter, due to its proximity to its star, it is significantly hotter than the dominant gas giant of our solar system. Like other hot Jupiters found, this planet's rotation is synchronized with its orbital motion- the planet is always turned to the star with one side. The orbital period is 2.2 Earth days.
Kepler-16b
Analysis of data on the Kepler-16 system showed that the exoplanet Kepler-16b, discovered in it in June 2011, orbits two stars at once. If an observer could find himself on the surface of the planet, he would see two suns rising and setting, just like on the planet Tatooine from the fantastic Star Wars saga.
In June 2011, scientists announced that the system contained a planet, which they designated Kepler-16b. After further detailed research, they found that Kepler-16b revolves around a binary star system in an orbit of approximately equal to the orbit Venus, and makes one revolution in 229 days.
Thanks to the joint efforts of amateur astronomers participating in the Planet Hunters project and professional astronomers, a planet was discovered in a four-star system. The planet orbits two stars, which in turn orbit two more stars.
PSR 1257 b and PSR 1257 c
2 planets orbit a dying star.
Kepler-36b and Kepler-36c
Exoplanets Kepler-36b and Kepler-36c - these new planets were discovered by the Kepler telescope. These unusual exoplanets are strikingly close to each other.
Astronomers have discovered a pair of neighboring explanets with different densities orbiting very close to each other. Exoplanets are too close to their star and are not in the so-called "habitable zone" of the star system, that is, the zone where liquid water may exist on the surface, but that is not what makes them interesting. Astronomers were surprised by the very close proximity of these two completely different planets: The orbits of the planets are as close as any other orbits of previously discovered planets.
The search for Earth-like exoplanets is a mission of the Kepler observatory, which will enter orbit in early 2009. Over four years, Kepler will survey about 100,000 stars like our Sun in search of planets similar to Earth.
The first exoplanet detected by direct observation in visible light is Fomalhaut b. Photographs taken by Hubble two years apart show the movement of the planet, completing a full revolution in 872 years.
The Kepler observatory is the first NASA mission capable of detecting planets the size of Earth or even smaller. The Kepler instrument is an ultrasensitive photometer equipped with a Schmidt telescope with an aperture of 0.95 m and a field of view of 12°. The measuring part of the photometer consists of 42 CCD matrices with dimensions of 50 x 25 mm and a resolution of 2200 x 1024 p.
Kepler will measure with great accuracy the intensity of light coming from distant stars and will be able to detect its change as a planet passes across the star's disk.
Around the suns, countless and similar // With a hive of fire, there, in the heights, // In the sparkling cold spaces, // Rotating, drinking in the wondrous light, // Swarms of tragic planets.
Emil Verhaerne, cycle “Evenings” (translated by V. Bryusov)
In 1842, the French thinker Auguste Comte, in the second book of the Course of Positive Philosophy, declared that the “chemical and mineralogical” composition of stars would forever remain a mystery to science. Meanwhile, thirty years earlier, the German physicist Joseph Fraunhofer discovered characteristic dark lines in the emission spectra of some stars, which, as we now know, represent the signature of the elements that make up their atmospheres.
At the time when the author of this article visited astronomical circle Moscow Planetarium, popular books claimed that with the help of terrestrial telescopes it was impossible to detect a single extrasolar planet. In the 1990s, this prediction fell apart, although scientific methods, which made it possible to refute it (first radio astronomical observations, and later Doppler analysis spectral lines), were created much earlier.
By the end of 2008, about 310 so-called exoplanets were known to orbit the stars of our Galaxy. There is no doubt that luminaries from other galaxies also have planetary retinues, they just have not yet been discovered due to their enormous distances. Considering that the first satellite of an ordinary star was officially discovered only 13 years ago, we have to admit that from the very beginning the capture of exoplanets took on a very high pace. And since in last years exoplanets are usually found in the process of automatic scanning of the night sky (the technology of which is being improved by leaps and bounds), the number of such discoveries has a chance to increase significantly in the near future.
See or guess?
The easiest way to search for exoplanets is direct observation. This is exactly how they once looked for circumsolar planets lying behind Saturn: just look through a telescope (more precisely, analyze digitized stellar images). In principle (and more recently in practice) this is a completely solvable problem - if only the telescope were more powerful and the matrix more sensitive.
However, the chances of success are slim. Let's say for a solar-type star at a distance of 15 light years from us, around which at a distance of approximately 5 astronomical units a gas giant the size of Jupiter is circling. In the earth's sky, the angular discrepancy between such a star and its satellite will be approximately one arc second, which is quite accessible to modern telescopes. But the problem is that the contrast is too small. In the optical spectrum, the power of stellar radiation exceeds the reflected planetary reflection by a billion times, and in the infrared range by a million. Therefore, such discoveries are still possible only in exceptional cases. In 2004, one of the eight-meter telescopes of the European Southern Observatory detected a planet with a mass of five Jupiters orbiting a brown dwarf 2 M 1207 (70 parsecs from the Sun) at a distance of two Neptune's orbital radii (55 astronomical units). However, French and American astronomers, who published a message about this discovery a year later, were very lucky. Mother star in in this case shines so weakly that the infrared contrast between its radiation and planetary light is only 100:1. The first ever “direct” photograph of a star-planetary pair (however, taken using adaptive optics) deservedly made it onto the pages of newspapers. Subsequently, using infrared photography, it was possible to find several more exoplanet candidates (according to various estimates, from five to seven). And in November 2008, American astronomers reported the first identification of a previously unknown exoplanet in photographs in visible light(this celestial body with a mass of half to three times the mass of Jupiter orbits the favorite star of science fiction writers Fomalhaut from the constellation Southern Pisces). However, one can hope that new images of this kind will be brought in the next decade by the James Webb orbital telescope and ground-based telescopes of a particularly large caliber that have not yet been built.
Unlucky astrometry
You can be convinced of the existence of exoplanets indirect methods. Their presence is evidenced both by the anomalies in the motion of the parent stars and by the specific features of their radiation.
Deals with the movement of luminaries in the earth's firmament oldest branch astronomy - astrometry. This science is capable of finding invisible stellar satellites: a star with a cosmic companion and its satellite orbit around general center mass, and the displacement of the star can be registered with precision goniometric equipment. It is easiest to detect a planet if the star has a noticeable own movement(displaces in the earth's sky relative to other stars). Back in 1844, the German astronomer Friedrich Bessel came to the conclusion that the smallest aberrations of Sirius's own motion indicate the presence of a satellite. True, it turned out to be not a planet, but a star—more precisely, white dwarf(the second in the history of astronomy) - which 18 years later was examined through a telescope by the American Alvan Clark.
Extrasolar planets began to be systematically searched using astrometric methods. The first in this matter was the Dutchman Piet Van de Kamp who moved to the United States. In 1938, he began periodically photographing a few specially selected stars with the 61-centimeter telescope at the Sprowl Observatory in Pennsylvania. Six years later he announced the discovery of a strange celestial body, which, if desired, could be considered a candidate for the role of an exoplanet.
It happened like this. De Camp was especially interested in a dim star in the constellation Ophiuchus, which was made famous throughout the world in 1916 by the American astronomer Edward Emerson Barnard. Based on many years of observations, he showed that this red dwarf has a record proper motion, shifting by 10.3 arcseconds annually. In addition, it is located very close to the Sun, only 5.96 light years(only Alpha Centauri is closer). De Camp quite logically decided to look for the planetary retinue of a star with such unique characteristics and soon came to the conclusion that he was not mistaken. In 1944, he reported at a meeting of the American philosophical society that Barnard's star has a non-luminous companion whose mass is 60 times more mass Jupiter. That's a lot for a planet, but not enough for a star. De Camp was careful to call his hypothetical body simply an intermediate mass object.
De Camp was not the first to come up with such an announcement. In 1943, his Sprow Observatory colleague Kai Aage Strand and McCormack Observatory astronomers Dirk Reil and Eric Holmberg made similar claims. Strand reported the discovery of a companion with a mass of 16 Jupiters in the star 61 Cygni, and Reil and Holmberg discovered a body one and a half times lighter, belonging to the double star system 70 Ophiuchi. However, these applications could not be confirmed, and the authors abandoned them. But de Camp did not give up. In 1963, he reported that he was absolutely sure that Barnard's star had a cold companion, but reduced its mass to 1.6 Jupiterian. A little later, he gave her another planet of a smaller caliber. However, over time, these conclusions were repeatedly refuted and de Camp’s planets joined the list of astronomical misconceptions. A similar fate befell another American astronomer, George Gatewood. We have to admit that astrometry has not yet been useful for the search for exoplanets.
First successes: radio search
The first success in the search for exoplanets came not from optics, but from radio technology. However, this is natural. As you know, there are plenty of sources of strictly periodic radio signals in space - radio pulsars (these are rapidly rotating neutron stars with a strong magnetic field). Generated on their magnetic poles powerful directed beams of radio waves describe conical surfaces in space. If our planet is on such a surface, the beam crosses it at every revolution. The radiation is recorded on Earth in the form of periodic pulses, which is why the sources themselves are called pulsars. If planets orbit around a pulsar, then with their attraction they slightly change the nature of its rotation and cause oscillations in the radio signal received on Earth.
Planetary retinues have been sought in pulsars since the early 1970s. But it was only in 1992 that Pole Alexander Wolszczan and Canadian Dale Frey, working in the United States, provably discovered two planets orbiting the millisecond pulsar PSR 1257+12, 980 light-years distant from the Sun. Later calculations showed that there are not two planets, but three. The lightest of them is twice as heavy as the Moon, the masses of the others are 4.3 and 3.9 times the mass of our planet. Of course, they are not suitable to serve as a refuge for life of any conceivable type.
Apparently, pulsars are not rich in planets. In any case, later radio astronomers managed to discover only one more representative of this family. It turned out to be the pulsar PSR 1620−26, around which a body with a mass of two and a half Jupiters orbits. And it is quite obvious that the equipment with which these discoveries were made works exclusively for pulsars and is not suitable for searching for non-luminous satellites of ordinary stars.
Doppler spectroscopy
Astrometric methods, in principle (but not yet in practice), make it possible to detect exoplanets by displacements of two-dimensional trajectories of stars by celestial sphere. Therefore, they should give maximum effect if the plane of the planetary orbit is perpendicular to the line of sight to the star. If from the Earth this planetary system is visible not from the front, but in profile, the movement of the planet will most strongly influence not the position of the star on the celestial sphere, but its radial speed relative to the Earth. Moving in our direction, the satellite planet will pull the star along with it, and this speed will increase; when the planet moves away, the radial speed of the star will decrease somewhat. As a result, the star, from the point of view of earthly observers, will sway like a pendulum in the direction “toward us - away from us.” It is impossible to detect such a shift visually, but in the first position a Doppler shift of the spectral lines of stellar radiation occurs to the blue side, and in the second position to the red. Since the planet revolves around the star in a closed trajectory with a stable year, such displacements will turn out to be strictly periodic. They can be easily detected using sensitive spectroscopes. This method (also called the radial velocity method) works even if the angle in question is not 90 degrees, but is still non-zero. Of course, the duration of observations should be at least a planetary year, or even better - several years.
Exoplanet hunters realized the potential of this method back in the 1970s. And they not only realized it, but also started working. In 1988, Canadian astronomers Bruce Campbell, Gordon Walker and Stephenson Young reported that they had allegedly discovered the dark moon of Gamma Cephei. However, they admitted that their equipment was not sensitive enough to confidently claim a discovery. Four years later, their findings were questioned, but in 2003 they were fully confirmed. So in this sense this year can be considered an anniversary - the first discovery of an exoplanet took place 20 years ago. Similarly, Harvard astrophysicist David Latham in 1989 announced the possible identification of a planet near the star HD 114762, but confirmation of this discovery had to wait seven whole years (however, it is still unknown whether it is a planet or a brown dwarf).
In the early 1990s, several scientific teams were already seriously engaged in a spectrometric search for both non-luminous and very dim companions of solar-type stars. Using this method, they hoped to discover not only exoplanets, but also brown dwarfs, long-predicted by theorists, infrared stars with a mass less than 8% of the mass of the Sun, in the depths of which thermonuclear combustion of ordinary hydrogen is impossible (though deuterium can burn there, but its reserves do not last long). Both hopes came true 13 years ago, and by an interesting coincidence, at the same time.
Race for exoplanets
Among the many exoplanet hunters, three have taken the lead scientific groups. One was composed by the already mentioned Canadians Campbell and Walker, the second by the Americans Geoffrey Marcy and Paul Butler (a chemist, but with astronomical aspirations), the third by Michel Mayor, a professor of astronomy at the University of Geneva, and his graduate student Didier Queloz. The Canadians may well have been the first to achieve recognized success, since they did more than others to develop instruments that would allow them to notice the “swaying” of stars. However, they were unlucky again. In 1994, they again claimed the possible discovery of an exoplanet, but their conclusions were not confirmed. Luck didn't want to smile on the Americans either. That same year, Marcy reported that they had monitored a third of the list of specially selected stars, but still had no results.
The Swiss, meanwhile, have begun a systematic search for exoplanets using a spectrometer high resolution ELODIE, mounted in 1983 on the 1958 193-cm telescope of the Haute-Provence Observatory in Southern France. On November 23, 1995, they published an article in Nature, from which the world learned about the long-awaited discovery of a planet orbiting an ordinary star. Just a few weeks later, the Americans confirmed this result and reported the registration of another couple of exoplanets. Planetary astronomy has once and for all moved beyond the solar system. And later similar discoveries rained down one after another.
Scientists immediately realized that exoplanets are different from the satellites of the Sun. The first of them was discovered near the star 51 Pegasi. It orbits in a circular path with a radius of 7.5 million kilometers, completing one revolution in just 4.2 days, and has a very significant mass (0.47 the mass of Jupiter). For comparison, tiny Mercury never comes closer than 46 million kilometers to the Sun and makes a full revolution in 88 days. Both planets reported by the Americans also raised eyebrows. These were clearly gas giants - 2.54 and 7.44 Jupiter masses. At the same time, they also turned out to be suspiciously close to their stars - 47 Ursa Major and 70 Virgo: their semimajor axes are equal to 2.1 and 0.48 AU, respectively. (Jupiter is 5.2 AU away from the Sun). The second planet also moves in an extremely elongated orbit with an eccentricity of 0.4, twice that of Mercury.
Stellar eclipses
Exoplanets are also caught using photometry—determining fluctuations in the apparent brightness of starlight. Of course, this is only possible if the planet periodically passes between the Earth and its star. Reduction amplitude luminous flux is proportional to the square of the ratio of the radii of the eclipsing and eclipsed body. So, if the diameter of the planet is equal to one tenth the diameter of the star (this is the ratio geometric parameters Jupiter and the Sun), it will block one hundredth of starlight, and the planet earth-sized will reduce the star's brightness by one part in ten thousand.
The photometric method not only brings information about the presence and composition of the planet's atmosphere, but also expands the capabilities of Doppler spectroscopy. Indeed, if a planet eclipses a star, then Doppler spectroscopy gives not a minimum, but a real estimate of the planetary mass (see sidebar). In the fall of 1999, David Charbonnet and Timothy Brown used a combination of these two methods for the first time - they spectrometrically identified the presence of a companion to the star HD 209458, and then recorded periodic dips in the curve of its brightness fluctuations. The data obtained made it possible to find out that the mass of the planet is 0.69 times the mass of Jupiter, and its diameter is one and a half Jupiterian times. Later, the eclipse effect of this planet was confirmed with much greater accuracy by instruments. orbital telescope Hubble and the Hipparchus astrometric satellite.
Another type of photometric capture of extrasolar planets is based on the phenomenon of gravitational microlensing. It was originally used to search for dim, low-mass stars. Caught between Earth and the distant bright luminary, such a star bends its rays with its gravity and temporarily increases its visible brightness. If the star has a satellite, the light curve changes slightly. The distant planet was first spotted this way in 2003. The method itself is very effective, but, unfortunately, does not allow for repeated observations.
The successful pursuit of exoplanets not only provided a wealth of information to astronomy, but also brought public attention to the science and greatly increased its prestige. And this had a positive effect on the financing of new projects. Therefore, it is not surprising that the development of next-generation instruments designed for such searches is in full swing. But more about them in the next issue.
Most of the stars have a system of planets. The question arises, how many are there in total? There must be billions of extraterrestrial worlds in our Galaxy alone!
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IN clear night, when light interference is not a serious factor, the sky looks breathtaking: the view opens great amount stars But, of course, we can only see a small part of the stars that actually exist in our Galaxy. What's even more amazing is that most of them have their own planetary system. The question arises, how many exoplanets are there? There must be billions of extraterrestrial worlds in our Galaxy alone!
So let's assume that the eight planets that exist within the solar system represent the average. The next step is to multiply this number by the number of stars that exist within the Milky Way. The actual number of stars in our Galaxy is a matter of some debate. Essentially, astronomers are forced to make rough estimates because we cannot view the Milky Way from the outside. And given that it is in the shape of a barred spiral, the galactic disk is the most difficult to study due to the interference of light from its many stars. As a result, the estimate is based on calculations of the mass of our Galaxy, as well as mass fraction stars in it. From this data, scientists estimate that the Milky Way contains between 100 and 400 billion stars.
Thus, the Milky Way galaxy could have between 800 billion and 3.2 trillion planets. However, in order to determine how many of them are habitable, we must consider the number of exoplanets studied so far.
As of October 13, 2016, astronomers have confirmed the presence of 3,397 exoplanets out of 4,696 potential candidates that were discovered between 2009 and 2015. Some of these planets were observed directly through direct imaging. However, the vast majority have been detected indirectly using the radial velocity or transit method.
The histogram shows the dynamics of exoplanet discovery by year. Credit: NASA Ames/W. Stenzel, Princeton/T. Morton
During its initial 4-year mission, the Kepler space telescope observed about 150,000 stars, which were mostly M-class stars, also known as red dwarfs. When Kepler entered a new phase of the K2 mission in November 2013, it shifted its focus to studying K- and G-class stars, which are almost as bright and hot as the Sun.
According to a recent study conducted by NASA Ames Research Center, Kepler found that about 24% of M-class stars may have potentially habitable planets comparable in size to Earth (those that are no more than 1. 6 times the radius of the Earth). Based on the number of M-class stars, there may be about 10 billion potentially habitable, Earth-like worlds in our Galaxy.
Additionally, analysis of the K2 results suggests that about one quarter of large stars may also have Earth-like planets orbiting within habitable zones. Thus, it can be estimated that there are literally tens of billions of planets potentially suitable for the development of life in the Milky Way alone.
In the coming years the mission space telescopes James Webb and TESS will be able to detect smaller planets orbiting dim stars, and may even determine whether any of them harbor life. Once these new missions get underway, we will have more accurate estimates of the size and number of planets that exist in our Galaxy. Until then, their estimated number is encouraging: the chances of extraterrestrial intelligence are very high!
