During Space Week, one of the major topics in astronomy is once again at the center of attention: the possibility of finding life beyond Earth.

Over the past few decades, scientists have confirmed thousands of planets orbiting stars other than the Sun. Among them, some are located in regions where conditions could allow liquid water to exist, an element considered fundamental to life as we know it.

One of the cases that has generated the greatest interest is K2-18 b, an exoplanet located about 120 light-years from Earth. Observations carried out with space telescopes have detected water vapor and other molecules in its atmosphere, making it one of the most closely studied worlds in the search for potentially habitable conditions.

What is an exoplanet?

An exoplanet is a planet located outside our Solar System that orbits a star other than the Sun.

NASA has now confirmed more than 6,000 exoplanets, a number that continues to increase thanks to new observations. These worlds display enormous diversity: some are small and rocky, others are gas giants, and there are also intermediate-sized planets that have no direct equivalent in our Solar System.

Studying these planets allows scientists to compare other planetary systems with our own and analyze which ones might have conditions compatible with life.

The importance of the habitable zone

One of the first criteria used to study the habitability of an exoplanet is its location relative to the star it orbits.

The so-called habitable zone is the region around a star where temperatures could allow liquid water to exist on the surface of a planet. It is also known as the “Goldilocks zone” because it is neither too hot nor too cold.

However, being located within this zone does not automatically mean that a planet is habitable.

Temperature, the planet’s size, the composition and stability of its star, its atmosphere and other factors can also determine whether conditions are suitable. NASA points out that the habitable zone is mainly a first filter for identifying worlds that deserve more detailed study.

K2-18 b, one of the most closely studied worlds

K2-18 b is an exoplanet approximately 8.6 times more massive than Earth. It orbits a red dwarf star and is located within its habitable zone.

In 2019, researchers used data from the Hubble Space Telescope to detect the molecular signature of water vapor in its atmosphere. It was the first detection of water vapor in the atmosphere of an exoplanet located in the habitable zone.

Later, the James Webb Space Telescope made it possible to study its atmosphere in greater detail. In 2023, observations revealed the presence of methane and carbon dioxide. The combination of these elements is consistent with the hypothesis that K2-18 b could have an ocean beneath a hydrogen-rich atmosphere, although this interpretation remains under investigation.

NASA currently describes K2-18 b as a possible ocean world and notes that further observations are still needed to better understand its characteristics.

Finding water does not mean finding life

The presence of water is one of the reasons K2-18 b is so interesting, but scientists maintain a fundamental distinction: water does not equal life.

On Earth, liquid water is essential for the life we know. That is why finding evidence of water on another world can make it a priority for further research. However, determining whether life actually exists would require much more evidence.

The European Space Agency explains that possible signs of life, known as biosignatures, must be analyzed as part of a body of evidence. A single molecule is not in itself proof of life, since certain compounds can also be produced through non-biological processes.

NASA uses the term biosignature to describe a feature, molecule, substance or structure that could provide evidence of past or present life. For such a signal to be convincing, it must be distinguishable from natural processes that do not involve living organisms.

The role of the James Webb Space Telescope

The James Webb Space Telescope has opened up new possibilities for studying the atmospheres of planets located enormous distances away.

Rather than directly observing the surface of most of these worlds, scientists can analyze how light from their star passes through the planet’s atmosphere. Different molecules absorb specific wavelengths of light, leaving behind a kind of chemical fingerprint that can be studied through spectroscopy.

This method has made it possible to identify molecules such as water, methane and carbon dioxide in the atmospheres of some exoplanets. Webb was not specifically designed to prove the existence of life, but its capabilities are helping lay the groundwork for future research into habitability and possible biosignatures.

Water can also exist far from the habitable zone

The search for water is not limited to planets located within the habitable zone.

In 2024, research cited by CR Hoy raised the possibility that some exoplanets located outside that region could retain liquid water beneath their surfaces.

The study was based on models concerning the formation and evolution of planets with magma oceans. According to this hypothesis, during the early stages of formation of certain planets, some of the water could become trapped inside and later be released toward upper layers as the planet cools.

This possibility expands the number of places scientists can consider when searching for potentially habitable worlds.

The search for life is becoming increasingly precise

Exoplanet research has changed the way scientists search for life beyond Earth.

It is no longer simply a matter of finding a planet similar to our own. Researchers can study its size, its star, its atmosphere, the molecules present and the possible existence of water to determine which worlds deserve more detailed observations.

The European Space Agency notes that missions such as Cheops, Hubble and James Webb are making it possible to study the characteristics of exoplanets and determine which ones could offer conditions compatible with life. It also highlights that future missions, such as Plato and Ariel, will contribute to deeper studies of these worlds.

For now, life has not been confirmed on any exoplanet. What has changed is our ability to identify the places where it might be most interesting to look.

And each new detection of water, atmospheric molecules or potentially habitable conditions adds another piece to one of humanity’s oldest scientific questions: whether life exists somewhere else in the universe.