The search for planets beyond our Solar System (extrasolar planets) has grown by leaps and bounds in the past decade. A total of 4,514 exoplanets have been confirmed in 3,346 planetary systems, with another 7,721 candidates awaiting confirmation. At present, astrobiologists are largely focused on the “low hanging fruit” approach of looking for exoplanets that are similar in size, mass, and atmospheric composition to Earth (aka. “Earth-like.”)

However, astrobiologists are also interested in finding examples of “exotic life,” the kind that emerged under conditions that are not “Earth-like.” For example, a team of astronomers from the University of Cambridge recently conducted a study that showed how life could emerge on ocean-covered planets with hydrogen-rich atmospheres (aka. “Hycean” planets). These findings could have significant implications for exoplanet studies and the field of astrobiology.

The research was led by Dr. Nikku Madhusudhan, a Reader in Astrophysics and Exoplanetary Science from the University of Cambridge’s Institute of Astronomy (IoA). He was joined by Ph.D. astrophysics student Anjali Piette (Dr. Madhusudhan is her supervisor) and fellow IoA member Dr. Savvas Constantinou. The study that describes their findings, titled “Habitability and Biosignatures of Hycean Worlds,” recently appeared in The Astrophysical Journal.


An artist’s conception of how common exoplanets are throughout the Milky Way Galaxy. Image Credit: Wikipedia

Life on Little Ice Giants?

Of all the exoplanets that have been discovered in the past 30 years, the vast majority have either been predominantly rocky planets several times the mass of Earth (“Super-Earths”) or ice giants with hydrogen-rich atmospheres (“mini-Neptunes”) or somewhere in between. Whereas Super-Earths account for about 30% (1,383) of all exoplanets discovered to date, mini-Neptunes are the most plentiful, accounting for 34% (1,531).

Most mini-Neptunes are over 1.7 and 3.9 times the size of Earth and are believed to have interiors composed of ice, rock, and oceans of volatile elements. Previous studies of such planets have found that the pressure and temperature conditions beneath their hydrogen-rich atmospheres would be too great to support life. However, in a previous study, Nikku Madhusudhan and his team found that these planets could support life under certain conditions.

In particular, they examined the exoplanet K2-18b, a mini-Neptune that was the focal point of a lot of attention in 2019 when two different teams reported detecting water vapor in its hydrogen-rich atmosphere. The results Dr. Madhusudhan and his team obtained from this study led them to investigate the full range of planetary and stellar properties that would allow mini-Neptunes to be potentially habitable.

This led them to identify a new class of planets that they named “Hycean,” which is a portmanteau of the words “hydrogen” and “ocean.” Like so-called “water worlds,” Hycean worlds would be covered in planet-wide oceans but would have atmospheres dominated by hydrogen (consistent with gas giants). The presence of this atmospheric hydrogen would allow for a greenhouse effect, which would help ensure that the surface oceans remain liquid.


Artist’s impression of K2-18b. Credit: Amanda Smith

Characteristics

Planets that fall into this classification would have a diameter about 2.6 times that of Earth and have atmospheric temperatures of up to 200°C (392°F), depending on the nature of their host star and the planet’s proximity to it. This is similar to what scientists think conditions were like on Earth billions of years ago, at a time when the first single-celled microbial organisms began to emerge.

As Dr. Madhusudhan summarized in a recent University of
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