K2-18b

K2-18b is an exoplanet larger than Earth orbiting the dwarf red star K2-18, located in the constellation Leo. The planet was discovered using data from the Kepler space telescope's K2 mission, employing the transit method—a technique in which the star's light dims slightly each time the planet passes in front of it.

By size and mass, K2-18b is classified as a sub-Neptune—a planet larger than Earth but smaller than Neptune. This type of planet does not exist in our solar system, so scientists use it to understand how the atmospheres of such planets form.

Location and orbit

The star K2-18 is cooler and dimmer than the Sun. For this reason, the habitable zone—the region where conditions could allow liquid water to exist on a planet's surface—lies very close to the star. K2-18b orbits near the edge of this zone, with an orbital period of less than one Earth month.

Atmospheric studies

In 2019, research teams using data from the Hubble Space Telescope reported detecting signs of water (H₂O) in K2-18b's atmosphere. This finding generated significant interest, as it was the first time water had been reported in the atmosphere of a planet of intermediate size within a habitable zone. Subsequent analyses noted that some of these signals could also be interpreted as other gases, such as methane.

From 2023 onward, the James Webb Space Telescope has made precise measurements of the planet's atmosphere. This data revealed signals of methane (CH₄) and carbon dioxide (CO₂). Some scientists have proposed that these gases are consistent with a model known as Hycean—a planet with a water ocean beneath a hydrogen-rich atmosphere. These interpretations are not universally accepted within the scientific community.

Scientific significance

K2-18b has become an important test case for assessing the power of modern telescopes to analyze the atmospheres of distant exoplanets. The analysis relies on transmission spectroscopy: as starlight passes through the planet's atmosphere, different molecules absorb specific wavelengths of light, allowing them to be identified. The varying results obtained from the same data demonstrate how difficult it is to extract certain conclusions from weak signals.