British scientists cited the rationale that the manifestations of life outside the solar system can be looked for not only on exoplanets similar to Earth, but also on others that are very different from it. The article was published in The Astrophysical Journal.
Traditionally, in search of signs of life, astronomers study exoplanets with the same size, mass, temperature and atmospheric composition as Earth, located in the habitat of their parent star - the so-called Goldilocks zone. However, researchers from the Cambridge Institute of Astronomy, led by Dr. Nikku Madhusudhan, believe that life is possible on a completely different type of planet - hot, completely covered by the ocean, with a hydrogen-rich atmosphere.
As an example, they cite K2-18b, an exoplanet orbiting the red dwarf K2-18, located about 111 light-years from Earth. This planet orbits a star in 33 days and contains water in its atmosphere.
There are quite a few such objects among exoplanets - much more than terrestrial planets. Previously, scientists attributed them to super-Earths or mini-Neptuns, depending on their density. Most mini-Neptuns are 1, 6 or more times larger than Earth, but smaller than Neptune. Traditionally, they are considered to be too large to have rocky depths, and the temperature under their atmosphere is too high to support life.
The authors propose to single out such exoplanets in a separate class, which they gave the name Hycean. Scientists have conducted a detailed analysis of the range of conditions on these planets, depending on their position and properties of the parent star, and concluded that under certain conditions, these planets can support microbial life, similar to that found in the most extreme aquatic environments on Earth.
Researchers estimate that the Hycean planets can be 2.6 times the size of Earth and have atmospheric temperatures up to 200 degrees Celsius. Life can exist in the depths of their ocean or on the constant night side. These planets can also be located in a much wider habitable zone. The authors believe that with this approach, the possibilities for detecting biosignatures outside the solar system become much wider.
Usually, scientists refer to the presence of oxygen, ozone, methane and nitrous oxide, which are on Earth, as biosignatures indicating the possibility of the existence of life. The authors add to them methyl chloride and dimethyl sulfide, which are not abundant on Earth, but they can be indicators of life on planets with a hydrogen-rich atmosphere.
"The Hycean planets open up entirely new possibilities for finding life elsewhere," says Nikku Madhusudhan in a Cambridge University press release. "Previously, when we looked for various molecular signatures, we focused on planets like Earth, which was smart to start with. But we think the Hycean planets offer a better chance of finding traces of life."
The researchers note that the larger sizes, higher temperatures and hydrogen-rich atmospheres of the Hycean planets make their signatures much more visible to spectroscopic observations than those of terrestrial planets. Scientists have already compiled a list of potential Hycean worlds for detailed study with the James Webb Space Telescope, which is due to launch soon. All of these exoplanets revolve around red dwarfs at a distance of 35-150 light years from us.