Not so long ago, planets outside the Solar System could only be read about in science fiction stories or in purely theoretical considerations by scientists. Only thirty years ago, the existence of the first such objects was proven by scientific methods. Later, the term “exoplanet” was coined to describe them. Since then, their research has been advancing, perhaps most actively of all other fields of astronomy. Now the number of confirmed exoplanets is approaching five thousand. For many of them, physical characteristics and even atmospheric properties are known. How did we manage to make such progress?
The word “planet” in Greek means “one that wanders the sky”. This name was given by ancient Greek stargazers to some bright celestial bodies that changed their position relative to the “starry background” — constellations. It was only after the invention of the telescope in the early seventeenth century that people were able to see the surfaces of these luminaries.
However, Giordano Bruno, in his work “On Infinity, the Universe and Worlds,” suggested that, in addition to our planet, our world, there are an incalculable number of other worlds where life may also exist.
And he was right in a sense! Today, we know that there are planets outside our solar system that orbit other stars – exoplanets (or extrasolar planets).
The first officially confirmed exoplanet is PSR B1 257+12 s, which was discovered in 1992 by Alexander Volshchan at the Arecibo Radio Observatory. This exoplanet is one of three extrasolar planets orbiting the pulsar PSR B1 257+12. And in 1995, astronomers Michel Major and Didier Kelo discovered the exoplanet 51 Pegasi b, which orbits a main sequence star. For this discovery, they were awarded the 2019 Nobel Prize in Physics (the other half went to James Peebles for his research in cosmology).
According to NASA, as of July 15, 2024, there were 5690 confirmed exoplanets, 7606 candidates for verification, and 4238 planetary systems.
Like the planets of the Solar System, exoplanets have many physical properties, such as mass, radius, volume, density, albedo, surface temperature, orbital period, eccentricity, and orbital inclination. In terms of mass, they range from a few Moon masses to more than twenty-five Jupiter masses. The lightest is the exoplanet Draugr, whose mass is about twice that of the Moon. And the heaviest is HR 2562 b with a mass of about thirty Jupiterian masses.
The smallest known exoplanet, SDSS J1228+1040 b, has a radius of 0.0101 Earth’s, which means it is a hundred times smaller than the Earth! But the largest extrasolar planet is HD 100546 b, whose radius reaches almost four radii of Jupiter.
However, it’s too early to be satisfied with this interesting picture. The fact is that today there is no definite line where planets end and stars begin. Yes, there is a definition of the term “planet” from the International Astronomical Union (IAU), which once excluded the beloved Pluto from the number of large planets in the solar system. However, this definition does not apply to exoplanets. Therefore, it should be noted that many of them may be so-called sub-brown dwarfs — objects smaller than brown dwarfs. That is, these are self-luminous space bodies that occupy an intermediate position between a star and a planet. There is still no consensus among astronomers on whether to classify brown and sub-brown dwarfs as planets or stars, and whether to consider the process of exoplanet formation a classification feature, since the formation of sub-brown dwarfs, for example, is similar to the formation of “ordinary” stars.
The hottest known extrasolar planet is KELT-9b, which is so hot internally that it breaks hydrogen molecules in its atmosphere, and its surface temperature reaches 4300 °C. It is classified as a so-called “hot Jupiter” — an exoplanet with a mass of half to several Jupiter masses that orbit at a distance of less than 0.15 AU from its star.
The rotational period of exoplanets varies from several hours (for those closest to the central luminary) to thousands of years. Some of them are so far away from their star that it is difficult to say whether they are even gravitationally bound to it. The fastest exoplanet known to date, SWIFT J1756.9-2508 b, orbits its star in 48 minutes and 56.5 seconds, which is slightly longer than one lesson at school. And the slowest rotating exoplanet is COCONUTS-2b with a period of 1.1 million years. It’s a long time to wait for your next birthday on this planet…
Methods for detecting exoplanets
It is very difficult to observe exoplanets. First, because they are too far away. Secondly, because they are quite small compared to galaxies and stars. And thirdly, because of their low luminosity. Combining the first two reasons, we can also say that their angular sizes on the sky are very small. To put this into perspective, looking at an exoplanet through a telescope is about the same as looking at 50 pennies lying on the surface of Pluto from Earth.
Yet astronomers manage to do it! But how do they do it?
Exoplanet scientists have come up with interesting methods for detecting extrasolar planets. The main methods include: direct method, transit photometry, gravitational microlensing method, radial velocity method, and astrometric method.
The direct method, or direct imaging method, makes it possible to essentially “photograph” a planet near its parent star. The fact is that such bodies emit little light compared to the stars, so the latter “illuminate” them. But if you use a coronagraph (a device that covers the disk of a star to collect the radiation around it), you can detect a faint object near the star. Why nearby? If a planet is located far enough from the star, it reflects little light, and its surface temperature is low. It’s different if the planet is large enough and close enough to its sun to fall into its warm embrace. In this case, it will emit a lot of infrared waves (because it is very hot), which will stand out against the background of the total radiation of the system. This will look like a small hot spot on the image. All the observatories and instruments that make it possible to obtain images of such planets are located on Earth.
The transit method, or transit photometric method, is one of the indirect ways of detecting extrasolar planets in exoplanetology. It is based on the fact that we can detect the presence of a satellite in a star due to the periodic even drop in its luminosity. Most exoplanets have been discovered using this method. Observations were made both from space (CoRoT, Kepler, and TESS) and on Earth (the MEarth Project, SuperWASP, KELT, and HATNet ground-based projects). But the orbital plane of such satellites must be oriented in a special way — to have the smallest possible angle between the orbital plane and our line of sight.
It is a well-known fact that, according to the General Theory of Relativity, massive bodies curve space and time around them. This makes it possible to detect exoplanets orbiting stars. Light coming from a distant luminary to us, passing near an exoplanet, begins to move along a curved trajectory. That is, the planet acts as a lens, concentrating the light of distant background objects. This method is called gravitational microlensing.
It is most fruitful for planets located between the Earth and the center of the Milky Way, since the galactic center provides a large number of background stars. Observations are usually carried out using networks of robotic telescopes (e.g., OGLE and MOA).
The oldest of the methods is astrometric. It helps to detect the influence of the exoplanet’s gravitational field on the star, which is characterized by a change in its trajectory through the sky. Currently, there are only three officially confirmed exoplanets discovered using this method. One of them is HD 176051 b in the constellation Lyra. But in the near future, space missions such as Gaia and Nano-JASMINE will allow us to add to the ranks of this modest one-man show.
Exoplanets can also be detected using the radial velocity method. It is also called the Doppler method and even Doppler spectroscopy. The existence of exoplanets is proved by analyzing changes in the spectrum of a particular star. If a sufficiently massive object orbits around it, it slightly “sways” the central luminary with its gravity. We observe this “swaying” in the form of a periodic shift of spectral lines. Until 2010, this method was the most successful for “hunting” for extrasolar planets. However, after the launch of the Kepler spacecraft, the transit photometric method became the main tool for detecting them, and Doppler spectroscopy moved to second position.
Among the spacecraft that took part in the discovery of extrasolar planets, it is worth mentioning Kepler, Hubble, Spitzer and TESS. There were there were too many ground-based telescopes used, so we won’t even list them here.
The number of exoplanets discovered using different methods as of July 15, 2024:
radial velocity method (1092);
transit method (4224);
direct imaging method (82);
gravitational microlensing method (223);
astrometric method (3).
Habitable zone
It is obvious that we humans are interested in whether there is life on these cosmic bodies. So far, no signs of it have been detected. However, there is an orbital region around the star called the habitable zone, or “Goldilocks zone”. If an exoplanet is located in this zone, it is possible that water may exist on its surface in a liquid state. This feature is now considered a prerequisite for the emergence of life.
If an exoplanet in the habitable zone is also similar in mass and size to the Earth, it is called an Earth-like exoplanet, or “exo-Earth.” When its mass is greater than the Earth’s, but much smaller than that of giant planets, such an object is called “super-Earth”.
Thus, in the habitable zone, the planet’s surface is neither too cold nor too hot. However, the location of this zone depends on the size and temperature of the central star. For example, if it has a low temperature by stellar standards, then the habitable zone is located near its surface. But if it is a hot blue supergiant, this zone will be very far from the hot luminary.
It should also be noted that life requires a number of other conditions besides the distance from the star. These include the chemical composition of the planets, magnetic fields, radiation levels, etc.
The discovery of seven exoplanets orbiting the ultracold red dwarf TRAPPIST-1 received special publicity. According to scientists, at least three of them are located in the habitable region of space.
All known exoplanets are in our Milky Way galaxy. This is not surprising, because, as mentioned above, it is extremely difficult to detect them due to their tiny angular size. The most distant exoplanet discovered to date is located near the center of the Galaxy, i.e. at a distance of about twenty-five thousand light years.
Most of the known exoplanets were discovered with the Kepler Space Telescope using the transit method. Its cone-shaped field of view is shown in orange. Ground-based telescopes using transit and other methods of “hunting” for planets have discovered many objects of this class in the vicinity of the Sun, shown in an orange circle. The most distant exoplanets were found using microlensing.
But do exoplanets exist in other galaxies? It is logical to assume that they do. The Bruno-Copernican principle states that our planet does not occupy a privileged position in the Universe. Therefore, it is likely that other star systems also have many different exoplanets. This is confirmed by recent observations by astronomers at the Harvard-Smithsonian Center for Astrophysics, who, using data from the Chandra telescope (NASA) and XMM-Newton (ESA), may have registered such a “transiter”. The potential exoplanet M51-ULS-1b is located at a distance of 28 million light-years from us in the spiral galaxy M51, also known as the Whirlpool.
Exoplanetology is a fairly young field of astronomy, but it has already made so many discoveries! And it is hard to imagine how many interesting things it will tell us in the future.
Author: Vasyl Pryts, This article was published in Universe Space Tech magazine #6(187) 2021. You can buy this issue in electronic or paper versions in our shop.
