K2-18b

K2-18b is the most likely planet to host alien life. Located about 124 light-years from Earth, this planet is described as a "super-Earth" or "mini-Neptune". It is 2.6 times larger than Earth and about 8.6 times more massive.

K2-18b is located in the habitable zone of its star, K2-18. This means that liquid water could exist on its surface. In 2019, the Hubble Space Telescope detected the presence of water vapor in its atmosphere. Later observations by the James Webb Space Telescope (JWST) in 2023 and 2025 found the presence of carbon dioxide and methane molecules in its atmosphere. These findings increase the possibility that the planet is a Hycean World.  That is, K2-18b may be a planet with a large ocean above a rocky surface and a hydrogen-rich atmosphere surrounding it.

According to the latest studies, the presence of chemicals dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) has been detected in the atmosphere of K2-18b. On Earth, DMS is mainly produced by marine microorganisms. Therefore, the presence of DMS is considered a "biosignature" for the possibility of extraterrestrial life. In 2025, a group of scientists claimed that there was a 99.7% chance that life existed on this planet. However, for this to be accepted as a scientific discovery, a higher "5-sigma" confidence level is needed. Some scientists also point out that DMS could have formed without biological processes. However, the amount of DMS on K2-18b is estimated to be thousands of times higher than on Earth.  Therefore, studies of this planet are considered a milestone in the efforts to find extraterrestrial life.