Hycean Worlds: Studying 'Mini-Neptunes' Could Advance Search for Extraterrestrial Life

In the search for extraterrestrial life, scientists turned to potentially habitable exoplanets and have found K2-18b, a planet belonging to a class called 'Hycean worlds.'

A miniature exoplanet, K2-18b, has a radius twice that of Earth and is more than eight times as massive as our planet. The new planet of interest belongs to a class of potentially habitable exoplanets, collectively called 'Hycean Planets' -- planets more numerous and observable than Earth-like planets. These 'Mini-Neptunes' are hot ocean worlds with atmospheres rich in hydrogen.

Astronomers from the University of Cambridge present their examination of these "Mini-Neptunes" in the article "Habitability and Biosignatures of Hycean Worlds." The article appears in The Astrophysical Journal, August 26.

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New Class of Potentially Habitable Exoplanets

In their paper, the researchers explained that the Hycean worlds have densities between the rocky 'super-Earths' and the more extended 'mini-Neptunes.' They argue that these new potentially habitable exoplanets could be "optimal candidates" in the decades-long search for extraterrestrial life in addition to being abundant among exoplanets.

Researchers investigated various properties of these exoplanets, including physical characteristics like mass, radius, temperature, its potential to sustain life as we know it, and observable biosignatures.

In a statement to The Guardian, Dr. Nikku Madhusudhan explained that "Hyceans are basically water worlds with hydrogen-rich atmospheres." Madhusudhan is the lead author in the new investigation and a researcher in Cambridge. He also explained that an exoplanet the size of Earth is so tiny relative to its star that its atmospheric signature is weak, making it challenging to identify potential signs of extraterrestrial life.

On the other hand, Hycean worlds can be twice as large, with an order of magnitude more massive, and have significantly higher temperatures. Atmospheres in these exoplanets could reach 392 degrees Fahrenheit (200 degrees Celsius). Furthermore, the variety of molecules also suggests increased chances of finding hints of extraterrestrial life.

Additionally, the study also covers the tidally locked 'Dark Hycean Worlds.' According to a Washington University news article, tidally locked planets lack side-to-side momentum, resulting in the planet having one side always facing its star. In the Dark Hycean worlds, their permanent nightsides have conditions that can support life. Researchers also covered so-called 'Cold Hycean Worlds' that have "negligible irradiation."

About K2-18b

The planet used in the initial computations in the Cambridge study is K2-18b, which is similar to other mini-Neptunes. Also known as EPIC 201912552 b, the exoplanet orbits its star, the red dwarf K2-18, located some 124 light-years from Earth.

According to the NASA Exoplanet Catalog, it was first discovered in 2015 through the Kepler Space Telescope. A 'super-Earth,' K2-18b, has a mass of about 8.92 times compared to Earth.

While it was initially tagged as a "potentially rocky world," two independent studies in 2019 used data from the Kepler Space Telescope, the Hubble Space Telescope, and the now-retired Spitzer Space Telescope.

These studies revealed that this exoplanet is a water world, with vast amounts of water vapor in the atmosphere. One of the studies, published in the September 2019 Nature Astronomy, notes that "K2-18 b offers an unprecedented opportunity to gain insight into the composition and climate of habitable-zone planets."