:: OSEL.CZ :: - Hycean worlds are completely different from Earth. Yet they can host life

A ghostly Hycean world. Credit: Amanda Smith, University of Cambridge.

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Exoplanet hunting, in which we enthusiastically discover new worlds in ever-increasing numbers, has told us a fundamental thing about ourselves. And the fact that when it comes to the planets, we have a really lousy imagination. We were fooled by the planets of the solar system, which seemed very different to each other. We felt that they represented a decent portion of the diversity of planets in space. Now we know we were completely out.

Nikku Madhusudhan. Kredit: Kavli Institute for Cosmology, Cambridge.

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In the case of the search for life in space, our blindness is probably even greater. It’s because we know only one planet with life – our Earth. That’s why we usually look for another Earth in the surrounding universe, and we hope that this is where we could be lucky with alien organisms. However, fortunately it turns out that we are right next door again.

Nikku Madhusudhan from the British University of Cambridge and his collaborators have traced a new category of planets that can be habitable by life similar to ours. These are the Hycean worlds (according to the English “hydrogen” plus “ocean”), ie planets practically completely covered by the ocean, which have a foggy atmosphere rich in hydrogen. In other words, they are absolutely fascinating and go far beyond the earthly imagination.

It may not look like it at first glance, but for astrobiologists it’s a gem. Hycanic worlds appear to be more common among exoplanets than Earth-like worlds. And that’s not the end of it. For example, compared to Earth-like planets, they have a much wider habitable zone. Many potential Hycean worlds are larger and hotter than Earth, yet there may be conditions in which at least some terrestrial organisms can withstand.

Probabilistic Hycean World K2-18b. Credit: or ESA / Hubble, M. Kornmesser.

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Ocean planets can be up to 2.6 times larger than Earth and their atmospheres can reach temperatures of up to 200 ° C. Nevertheless, the conditions in their oceans are such that, in theory, terrestrial-like organisms could exist. Madhusudhan and his colleagues distinguish the so-called dark Hycanian worlds, which are bound in rotation with their star, on which the areas where eternal night reigns would be habitable. They also say there are cold Hycean worlds that receive little radiation from the star.

Researchers are convinced that it will be possible to discover biomarkers of the presence of life on the Hyceanian worlds. Detailed spectroscopic observations should handle this in the near future. A certain flaw in beauty is that the issue of biomarkers is not closed. It is still not entirely clear how to reliably confirm the presence of life on a planet. On the contrary, the advantage is that we detect such biomarkers on Hycanian planets much more easily than on Earth-like planets. One of the most promising candidates for such observations is the planet K2-18b, orbiting in a habitable zone of a red dwarf 124 light-years away.

Video: IUCAA Colloquium, March 25, 2021