The search for inhabited worlds beyond our solar system is one of the most ambitious tasks in modern astronomy. With the help of space telescopes such as Kepler and TESS, scientists have discovered thousands of exoplanets. Among this multitude, several candidates stand out, located in the so-called "habitable zone" — an area around a star where conditions allow water to exist on the planet's surface in liquid form, which is considered a key condition for life as we know it.

Criteria for habitability and super-Earths

In addition to being in the habitable zone, astronomers also consider other factors. An important role is played by the type of planet: the most interesting are rocky planets similar to Earth, not gas giants. Also, the stability of the host star and the presence of an atmosphere are evaluated. A special class of objects that attract increased interest are "super-Earths," planets whose mass exceeds Earth's but is significantly less than that of gas giants. They may have a stronger gravity, holding a dense atmosphere, and an active geology that promotes the circulation of matter.

TRAPPIST-1 system: seven worlds around a red dwarf

One of the most promising systems is TRAPPIST-1, located about 40 light-years from us. Around the cold red dwarf star, seven rocky planets similar in size to Earth orbit. Three of them — TRAPPIST-1e, f, and g — are located in the center of the habitable zone. This is a unique laboratory for comparative planetary science, allowing the study of worlds with potentially different conditions within one system. However, life around the red dwarf faces challenges: such stars often flare up, subjecting planets to powerful bursts of ultraviolet radiation. Moreover, due to tidal capture, these planets are likely constantly facing the star with one side, creating an extreme temperature contrast between the day and night hemispheres.

Proxima Centauri b: the nearest neighbor

The closest known exoplanet to us, Proxima Centauri b, is located just 4.24 light-years away in the system of the nearest star to the Sun. This rocky planet also orbits a red dwarf and is located within the habitable zone. Its discovery was a sensation, however, the conditions on it are likely extremely harsh. The Proxima Centauri star is known for its high flare activity, which could have led to the erosion of the planet's atmosphere and the impact of harsh radiation on the surface. The question of whether the planet has been able to preserve its atmosphere and magnetic field for protection from radiation remains open.

Kepler-186f: the first Earth-like planet in the habitable zone

The planet Kepler-186f, discovered by the Kepler telescope, became the first rocky exoplanet of Earth-size discovered in the habitable zone of its star. It orbits a red dwarf that is significantly colder and smaller than the Sun. Its size, just 10% larger than Earth's, allows us to assume with a high degree of probability its rocky nature. Although it is in the habitable zone, the amount of energy it receives from its star is equivalent to only a third of what Earth receives from the Sun, placing it on the outer, colder edge of this zone. This may mean that the conditions on its surface are more reminiscent of Martian than Earth-like.

Future research: searching for biomarkers

Discovering planets in the habitable zone is just the first step. The main task for the future, which will fall on the shoulders of new-generation telescopes such as James Webb and observatories that are only being planned, is the analysis of their atmospheres. Astronomers will be looking for biosignatures — chemical markers that may indicate the presence of life. These include oxygen, ozone, methane, and their combinations, which are actively produced by biological processes in Earth's conditions. The detection of such a chemical imbalance in the atmosphere of a distant planet will be a strong, although not definitive, argument in favor of the existence of life on it. In this way, modern astronomy is moving from simple planet counting to their deep characterization, gradually approaching an answer to one of humanity's fundamental questions: Are we alone in the universe?

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