For centuries, we have been fascinated by the idea of other worlds and other forms of life. Exoplanets orbiting other star systems have become objects of particular interest in recent decades.

In 1584, Italian thinker Giordano Bruno hypothesized that the universe is infinite and that the stars in it are distant suns with “countless worlds.” In 1917, Van Maanen noticed the first signs of the existence of exoplanets when he discovered the first polluted white dwarf.

Then, in 1992,Alexander Wolszczan and Dale Frylconfirmed the existence of exoplanets.

They discovered a pulsar in the constellation Virgo that rotates more than 160 times per second and emits powerful radio waves. Anomalies in the signal led to the conclusion that two or more planet-like bodies were orbiting the pulsar. These turned out to be the first exoplanets ever discovered. They were named “Poltergeist and Fobetor,” also known as PSR B1257+12 b and PSR B1257+12 c.

In 1995,Michel Mayor and Didier Quelozdiscovered another exoplanet, 51 Pegasi b, orbiting the star 51 Pegasi.

Similar to a “hot Jupiter,” it is at least 150 times more massive than Earth and is closer to its star than Mercury is to the Sun. 51 Peg b is a gas giant with an atmospheric temperature of 540–980 °C, which orbits its star every four days. In 2019, its discoverers received the Nobel Prize in Physics.

Since then, a real hunt for new worlds has begun. Search methods have improved, and the number of discovered planets has grown. To date, the existence of more than 7,500 exoplanets has been confirmed. Most of them are located close to the Solar System, so they were easier to detect. You can check theupdated list of discovered exoplanets in the NASA catalog.

Exoplanets located in the so-called habitable zone — the area around a star where conditions are suitable for liquid water to exist — are the main targets in the search for extraterrestrial life. It can be assumed that there are billions of potentially habitable planets in our galaxy.

Detection methods

Since exoplanets are much smaller than stars and do not shine themselves, finding them is fraught with considerable difficulties. Astronomers have to rely solely on indirect detection methods rather than direct images. Today, there are five methods.

Radial velocity method

A star, with its powerful gravitational field, attracts the planets that orbit around it. But the planets also exert a gravitational pull on the star, causing it to “wobble” slightly. The radial velocity method uses changes in the wavelength of starlight to detect these wobbles. When a star moves toward the observer, the light waves become slightly shorter, shifting toward the blue end of the spectrum. When a star moves away from the observer, the waves become longer, shifting toward the red end of the spectrum. By measuring these changes, it is possible to determine the presence, number, and size of planets.

The change in wavelength caused by the rotation of a planet is extremely insignificant. For example, the Sun's oscillations at a speed of 12 m/s, caused by the gravitational influence of Jupiter, shift its spectral lines by only 0.000004%. However, modern astronomical spectroscopes are capable of detecting changes in the motion of stars of less than 1 m/s, and work is underway to achieve the accuracy of 0.1 m/s required to detect Earth-like planets. Therefore, this method is fundamental to exoplanetary astronomy and has accounted for nearly 20% of all discoveries since 2012. Many observatories around the world use it, including to confirm planets found by other means.

Transit method

When a planet passes between us and its star, it blocks some of the star's light. This causes a temporary decrease in the star's brightness, which is reflected in the light curve as a characteristic “dip” or “fall.” A light curve is a graph showing the change in a star's brightness over time. Analysis of the light curve allows astronomers to determine the size and orbital parameters of an exoplanet. The larger the planet, the deeper and longer the dip in the graph. When there are several planets, the light curves become more complex, requiring additional work to extract information about each planet.

The transit method also allows us to partially determine the composition of a planet's atmosphere. When starlight passes through a planet's atmosphere, different substances in it absorb certain wavelengths. Spectral analysis of this absorption gives us a chemical “fingerprint.” Such studies have revealed the presence of water in the atmospheres of exoplanets (Tsiaras et al., 2019) and even allow us to determine the aggregate state of water in the atmosphere—vapor or liquid.

Using the transit method, NASA's Kepler space telescope has helped discover thousands of potential exoplanets and provided important information about their distribution in our galaxy.

Direct observation

The only sure way to prove an exoplanet exists is to take a picture of it, but that takes a telescope with crazy high resolution. For example, to see Jupiter against the Sun from 600 light-years away, you need a telescope with a mirror that's 8 meters across, and to see Earth, you need one that's 39 meters across. Such a device — the Extremely Large Telescope (ELT) of the European Southern Observatory (ESO) — is being built in the Atacama Desert in Chile and will begin operation in 2026. Higher resolution can be achieved by interferometry, combining data from several telescopes.

Another challenge is the huge contrast between the brightness of the exoplanet and its host star, which is ten billion times brighter. To solve this problem, coronagraphs are used to suppress the light from the star. Direct imaging, like the transit method, will allow us to determine the composition of exoplanet atmospheres. The first and so far most detailed photograph taken by the James Webb Space Telescope in 2022 looks like this:

Gravitational microlensing

When a star or planet passes in front of a more distant star, the light is focused and amplified, which is recorded as a change in brightness. Gravitational microlensing is effective for detecting exoplanets that are difficult to detect by other methods. It does not require direct observation and can be used to study planets that are far away from their stars.

Astrometry

This method is based on measuring small changes in the apparent position of a star caused by the gravitational influence of planets orbiting it. Scientists take a series of images of the star and its surroundings, comparing the distances between reference stars and the star under study. If the star moves relative to the others, this may indicate the presence of planets.

For example, Jupiter's gravitational influence on the Sun causes the Sun to oscillate at an average speed of 12 m/s as it moves around a center of mass located close to its surface.

Astrometry requires the most accurate optics possible, and due to distortions caused by our atmosphere, it is particularly difficult to implement from Earth. For example, a star the size of the Sun, located 42 light-years away from us, will oscillate by only one-fifth of a millionth of a degree under the influence of a planet similar to Jupiter. This is equivalent to observing the International Space Station moving 1.5 mm in its orbit from Earth. And the effect of a planet the size of Earth would be about 1,600 times weaker.

Characteristics of exoplanets

About 100 years ago, Einar Hertzsprung and Henry Norris Russell compiled a diagram showing the colors and luminosities of several dozen stars. They noticed that stars cluster in certain areas of the diagram. This became an important test for models of stellar structure and evolution. Can the same be done for planets?

Since its launch in 2009, NASA's Kepler space telescope has played a key role in the study of exoplanets.Petigura, Howard, and Marcy (2013) estimated the prevalence of Earth-sized planets around Sun-like stars. Here's what the study found:


The size and mass of planets are important in their classification. There is a gap in planet sizes known as the “radius valley” or Fulton gap. Data from the Kepler telescope has shown that planets 1.5–2 times larger than Earth are quite rare. They attract thick atmospheres of hydrogen and helium and turn into gas giants. Smaller planets remain rocky bodies or become cores without atmospheres.

Gas giants

These are large planets, consisting mainly of helium and hydrogen, without a solid surface, with a gaseous shell around the core. They can be significantly larger and closer to their stars than Jupiter and Saturn in our solar system.

“Hot Jupiters” are large gas planets whose orbits are very close to their stars. They are characterized by extremely high temperatures and narrow orbits that cause stellar oscillations. This helps to detect them using the radial velocity method.

The discovery of the youngest transiting hot Jupiter orbiting HIP 67522 (T_eff ∼ 5650 K; M * ∼ 1.2 M_⊙) in the 10-20 million-year-old Sco-Cen OB association has helped scientists understand the processes of planet formation. This planet orbits a star about 17 million years old, completing one revolution in seven days, and is located 490 light-years from Earth. Its size, close to that of Jupiter, indicates its gaseous nature.


Another example is WASP-76 b, a hot Jupiter discovered in 2016 using the transit method, which is unusual for its proximity to its star — 0.033 AU, with a rotation period of 1.8 days. It is 1.83 times wider and 0.92 times heavier than Jupiter. The planet is tidally locked, with one side constantly facing the star and heated to 2400 °C, and a relatively cool night side (1500 °C). On the day side, iron evaporates and then falls as rain on the night side. Such are the extreme conditions on exoplanets!

Neptunian planets

Neptunian exoplanets are similar in size to Neptune or Uranus. They have an atmosphere of hydrogen and helium. They have complex internal structures, but always contain dense metallic cores and rocky shells. There are also mini-Neptunes — planets outside our system that are smaller than Neptune but larger than Earth.

Neptunian exoplanets often have thick clouds that block light and make it difficult to analyze molecules. Although the atmospheres of Uranus and Neptune consist mainly of hydrogen and helium, they also contain water, ammonia, and methane. Because of their frozen outer layers, they are called “ice giants,” although their interiors are warm. In 2014, anice giant exoplanet with an orbit similar to that of Uranus was discovered 25,000 light-years away.

Super-Earths

This is a unique class of planets that does not exist in our solar system. They are larger than Earth but lighter than ice giants such as Neptune and Uranus, and may consist of gas, rock, or a combination of both. They are about twice the size of Earth and can have a mass up to 10 times that of Earth. At the upper end of the super-Earth size range, they are often referred to as sub-Neptunes or mini-Neptunes.

The term “super-Earth” refers solely to size — larger than Earth and smaller than Neptune — but does not indicate any similarity to our planet. The true nature of these planets remains a mystery. Nevertheless, they are common among the exoplanets that have been discovered.

In 2017, thesuper-Earth K2-131b, also known as EPIC 228732031b, was discovered. It is twice the size of Earth and orbits a star similar to the Sun, with an extremely high temperature capable of evaporating metal. It completes a full revolution around its star in just nine hours, while Mercury does so in 88 days.

Earth-like planets

These are rocky planets the size of Earth and smaller, consisting of silicates, water, and carbon. It is not yet known whether they have atmospheres, oceans, or other signs of habitability; further research is needed to determine this.

According to NASA, the TRAPPIST-1 system, located about 40 light-years from Earth, includes seven planets orbiting a cold red dwarf star. The first two planets were discovered in 2016, and the remaining fivein 2017. The uniqueness of this system lies in the fact that all seven planets are similar to Earth in terms of their size and rocky composition. At least three of them are in the habitable zone, making them priority targets for the search for extraterrestrial life.

James Webb measured the temperature of TRAPPIST-1b — for the first time, it was possible to measure the light reflected by an exoplanet. It turned out that this planet probably has no atmosphere.

The potential for detecting life on exoplanets

The multitude of systems discovered to date amazes astronomers. The solar system, with its nearly circular orbits and clear order — small rocky planets on the inside, large gas planets on the outside — turned out to be atypical. The question of how often Earth-like planets suitable for life are found remains open. At present, 70 of the discovered planets are in the habitable zone of their stars, but only 29 of them are likely to be rocky.


But the location of a planet in the habitable zone, even if it is similar in size to Earth, does not guarantee favorable conditions for life. The examples of Venus and Mars confirm this. However, the presence of liquid water is only one of the possible conditions for life. Other factors must also be taken into account, such as atmospheric composition, the presence of a magnetic field, and geological activity.

At the same time, habitable worlds may exist outside the habitable zone, such as the suspected oceans beneath the icy surfaces of Jupiter and Saturn's moons. Space missions are already being planned to explore them further. The JUICE probe was launched in April 2023 to study Jupiter and its icy moons, and the Europa Clipper probe, launched in October 2024, will reach Jupiter in 2030 to study the moon Europa.

Due to the excessive length of the flight, it is not possible to send space probes to the moons of other stars. However, by analyzing the light from these systems, astronomers can search for signs of life. Atoms and molecules in the atmospheres of exoplanets leave characteristic traces in the light spectrum. This allows the detection of substances typical of life, such as oxygen or methane, although many biomarkers can also arise from non-biological processes. For example, the methane detected on Mars is likely to be of geological origin.


New observation tools, such as the James Webb Space Telescope and the Extremely Large Telescope currently under construction, offer great promise. In the future, scientists plan to use them to detect biomarkers in the atmospheres of distant worlds, the combination of which can only be explained by biological processes. What lies behind these discoveries — single-celled life, rich ecosystems with complex life forms, or intelligent beings exploring space — remains to be seen.