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‘Hycean’ Worlds: A New Candidate for Biosignatures?

We’ve just seen the coinage of a new word that denotes an entirely novel category of planets. Out of research at the University of Cambridge comes a paper on a subset of habitable worlds the scientists have dubbed ‘Hycean’ planets. These are hot, ocean-covered planets with habitable surface conditions under atmospheres rich in hydrogen. The authors believe they are more common than Earth-class worlds (although much depends upon their composition), and should offer considerable advantages when it comes to the detection of biosignatures.

Hycean worlds give us another habitable zone, this one taking in a larger region than the liquid water habitable zone we’ve always considered as the home to Earth. In every respect they challenge our categories. Not so long ago a Cambridge team led by Nikku Madhusudhan found that K2-18b, 2.6 times Earth’s radius and 8.6 times its mass, could maintain liquid water at habitable temperatures beneath its hydrogen atmosphere. The team has now generalized this work with a full investigation of the planetary and stellar properties making life possible on such planets.

Planets between the size of Earth and Neptune are thus far the most common type of planet we’ve found, generally being labeled as ‘super-Earths’ or ‘mini-Neptunes.’ There are no analogues to planets like this in the Solar System; they are classed as super-Earths or mini-Neptunes largely on the basis of their density as inferred by their mass and radius. Some may be predominantly rocky, while others are closer to the ice giants in our system. Some may be water worlds. Some of them are in the habitable zones of nearby M-dwarf stars, making them good candidates for atmospheric studies and possible biosignature detections.

For planets with a hydrogen atmosphere surrounding a layer of high-pressure water covering an inner core of rock and iron may become astrobiologically interesting. It’s true that too dense a hydrogen envelope would create temperature and pressure at the surface that would preclude life. But if the atmosphere is not too thick, life-sustaining temperatures can exist.

The Hycean planets thus represent a new category of potentially habitable worlds, and can be up to 2.6 times the size of Earth, with atmospheric temperatures up to 200 degrees Celsius, while still remaining habitable. They are defined not only by size but also by mass, temperature and atmospheric pressure. Conditions in their oceans may allow at least microbial life.

We are looking at a wide habitable zone as well. Its range takes in planets with orbital separations so large that the only energy source would be internal heat. It also extends to planets orbiting so close to the host star that they are tidally locked, but can support life on their dark sides. The span of possible temperatures allowing life to exist is thus substantial. About the tidally locked worlds the authors refer to as ‘Dark Hycean,’ for example, we learn this:

…we nominally consider the planet-wide average surface and atmospheric temperature to be 500 K. The choice of this temperature is motivated by the atmospheric models for nightsides of Dark Hycean planets…. In particular, we find that planets with equilibrium temperatures of ∼510 K with inefficient day-night energy redistribution can lead to dayside temperatures of ∼500-600 K but nightside surface temperatures ≲400 K. Therefore, while a 510 K temperature is not considered to be habitable, it represents a planet wide average and still allows a nonnegligible fraction of the nightside ocean surface to be at habitable surface temperatures, i.e., below 400 K.

Image: Artist’s conception of the surface of a Hycean planet. Credit: Amanda Smith, Nikku Madhusudhan.

The researchers believe these interesting worlds are common. In their paper, they present a sample of potential Hycean targets that could be useful fodder for next-generation telescopes. All of these orbit red dwarf stars close enough to be suitable targets for the James Webb Space Telescope; none are more than 150 light years away. JWST observations of K2-18b are already being considered and could conceivably provide a biosignature detection. For having looked at five potential biomakers in Hycean atmospheres, the authors note:

Hycean atmospheres may offer even better opportunities for detecting these biomarkers than those of rocky super-earths… For a 10 M planet, the Hycean radius range is ∼2-2.6 R compared to the super-Earth radius of 1.75 R considered in Seager et al. (2013b). The increased radii and lower gravities lead to larger, more easily detectable spectral signatures for Hycean planets. Second, considering that prominent sources of the above biomarkers are thought to be aquatic microorganisms, we expect them to be even more abundant on Hycean worlds compared to predominantly rocky worlds.

The fact that Hycean planets open up the discovery space for worlds that could support life makes them noteworthy. We begin to consider planets of higher mass and radius than before, provided they have a rocky core that, according to the paper, is at least 10% of planetary mass and is of Earth-like composition. Their wide habitable zone expands the area for detection, while presenting a range of challenges that is likewise wide. Mass and radius help us spot a Hycean candidate, but they alone are not sufficient. We also need to learn more about temperatures and pressures in the ocean, and basic properties of the atmosphere:

,,,even if a candidate Hycean planet is in the Hycean HZ it may not necessarily have the right conditions for habitability, e.g., the internal structure and atmospheric properties may be such that the ocean surface pressure and/or temperature is too high… [T]he detection of H2O in the atmosphere does not guarantee the presence of an ocean on the planet, as H2O can be naturally occurring in H2-rich atmospheres as the prominent oxygen bearing species. Conversely, the nondetection of H2O does not preclude the presence of an ocean, since at low atmospheric temperatures H2O can rain out and not be detectable in the atmosphere. Nevertheless, in all these aspects Hycean candidates offer better prospects for establishing their habitability compared to habitable rocky exoplanets, which are inherently harder to characterize.

Given the wide range of transiting worlds we’ve discovered between 1 and 2.6 Earth radii, Hycean worlds offer no shortage of targets and if nothing else provide opportunities for atmospheric characterization that, according to the authors, should be less challenging than similar work on rocky exoplanets. Their large radius and thick atmospheres seem made to order for JWST and future instruments like the Extremely Large Telescope. Although not similar to Earth, Hycean planets can be valuable venues for detecting trace biosignatures. That alone contributes to the larger quest of finding life on planets more similar to our own.

The paper is Madhusudhan et al., “Habitability and Biosignatures of Hycean Worlds,” in process at The Astrophysical Journal (2021). Preprint. The paper on K2-18b is “The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b,” Astrophysical Journal Letters Vol. 891, No. 1 (27 February 2020), L7 (abstract). And I’ve just become aware of (but haven’t yet read) Benncke et al., “Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b,” Astrophysical Journal Letters 887:L14 (10 December 2019). Abstract.

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{ 18 comments… add one }
  • Michael Fidler August 27, 2021, 10:16

    The two closet large exoplanets Proxima c and Barnard’s Star b are Hycean worlds. What would large moons the size of Mars orbiting them do to the interior of these planets. Just awhile back they talked of huge amounts of water being absorbed by magma of such worlds in their interior. Could we see large land masses created by these giant worlds and their moons. I do not think this is going to be as simple as the analysis in this study says, but we may be dealing with a huge variety of worlds. As the mass increases the variety changes exponentially… ;-}

  • Brett August 27, 2021, 10:35

    It’s not just the atmospheric pressure you’d have to worry about – it’s the oceanic depth pressure as well. If the planetary ocean on such a world is tens or hundreds of kilometers deep, then there’s not going to be direct interaction between the rocky part of it and the liquid water part of it – there will be layers of exotic, high-pressure ices. That will cut the water off from both sources of energy as well as minerals necessary for life, with the only alternative (maybe) being the occasional asteroid impact that actually makes it close to the ocean before exploding.

    I get the impulse to look for alternative ideas for life-bearing planets, especially with so many of these disappointing sub-Neptune planets looming large in current exoplanet searches and our inability to reliably find and identify Earth-like planets around sun-like stars in habitable orbits. But I just don’t think these will be life-bearing worlds.

    • John walker August 27, 2021, 12:44

      That high pressure ices would seal the ocean floor and prevent any water/rock interface was also my initial reaction to this announcement. A few calculations later it seems that may not be the case.

      At 10% water by mass one is looking at an ocean depth of perhaps 1000km. And assuming a higher gravity of say 2G the pressure at the ocean floor would be on the order of 2 gigapascals. However, the authors worked with much higher water temperatures. A look at a phase diagram shows water still liquid at 2GPa if the temperature is over 350K or ~80°C.

      So, although in this example the pressure is enormous, the temperature required to prevent ice formation needn’t be inconducive to life.

  • Spaceman August 27, 2021, 12:19

    With atmospheric temperatures above the boiling point of water (373 K), what’s to prevent any planet-wide oceans on a Hycean planet from simply boiling off thereby leading to a Venus-like atmosphere?

    • Alex Tolley August 27, 2021, 14:31

      The boiling point of water changes with pressure. The thick H2 atmosphere and higher gravity probably increase the boiling point until it is above the ambient temperature. Spectral analysis may indicate whether this is true or not.

  • gbaikie August 27, 2021, 23:53

    “The Hycean planets thus represent a new category of potentially habitable worlds, and can be up to 2.6 times the size of Earth, with atmospheric temperatures up to 200 degrees Celsius, while still remaining habitable. They are defined not only by size but also by mass, temperature and atmospheric pressure. Conditions in their oceans may allow at least microbial life.”

    Let’s say 2 times Earth gravity. So with Earth we would have two atm of pressure. What’s boiling point of water? About 120 C.
    And if at same temperature, Earth have higher density of air at sea level. Earth would get more sunlight and counting that the higher density of air would make a more uniform air temperature to land temperature but this would not change ocean surface and air above it.
    If add 10 atm of hydrogen atmosphere, it would lower the surface air density. Or just double gravity, it lowers our troposphere, and add 10 of hydrogen and raises our troposphere higher than it is now. And seems we get more warming from the ozone layer. It both cools and warms.
    But boiling point of water jumps to about 185 C. If instead of 10 atm it was 20 atm pressure added to surface, it’s +210 C.
    And seems Earth would not be much warmer, and more Hydrogen added, the cooler and dimmer Earth would become, and wipe out warming from Ozone layer. And have very high atmosphere- Low earth orbit would start about 200 km. And with brighter star, low earth orbit could around 400 km. A nasty place to leave with rockets. Though much easier to land on the planet.
    It seems one could a lot creatures, stuck on a planet.

  • Edwin August 28, 2021, 5:34

    Hi Paul

    “Planets between the size of Earth and Neptune are thus far the most common type of planet we’ve found, generally being labeled as ‘super-Earths’ or ‘mini-Neptunes.’ There are no analogues to planets like this in the Solar System; ”

    If Browns “Planet 9 turns up you might need to re think your comment above

    The orbit of Planet Nine
    https://arxiv.org/abs/2108.09868

    A very interesting article and an interesting class of planets to investigate and the comments are interesting too, my guess is simple life only on the most favorable of these planets with the best conditions.

    Cheers Edwin

  • torque_xtr August 28, 2021, 8:31

    What is not quite clear to me even from preprint is how surface is so much cooler than effective temp. Reflective clouds could reduce energy input, but upwards of 2-4x solar constant water does not condense, leaving dark Raleigh-scattering atmosphere. Unless there is really strong absorption high in the atmosphere, like alkali metal vapors or Ti-V oxides in hot jovians, there will be no inversion.Even on Venus, most of sunlight that passed through clouds is absorbed above the surface, and yet the profile is close to adiabat. Tholin hazes could create some high absorpion, but I still see no way how an ocean beneath 100 bars of any atmosphere and earthlike insolation could fall into extremophile range. And tholins should be considered much more, since atmosphere of Hycean worlds is reducing and hydrocarbons would inevitably be there.

    Of course I’m not arguing that Hycean worlds could not be classically habitable. But IMO, planets with radii below 2.0-2.2 Re and Martian-to-Jupiter-like insolation are much better candidates. If density and surface gravity is low enough, then atmosphere may reach some fraction of Earth radius and still support ocean surface below that is not superheated. And massive water mantle does not absolutely prevent mineral mixing. There is much chemical interaction between ice and rocks at high P-T and there would be at least some solid-state convective mixing due to internal heat flow.

  • Jim Early August 28, 2021, 11:23

    Does the article address the possible levels of oxygen in these worlds? The lack of free oxygen would limit the types of possible lifeforms. Life on Earth developed while methane was still present in the atmosphere, but the production of oxygen by early life eventually elliminated reducing gases such as methane.

  • Adam Crowl August 29, 2021, 18:56

    Hi Paul
    I do wonder what the proper pronounciation of ‘Hycean’ is. When I first saw it I mentally pronounced it as ‘Hy-cee-an’. However since then the author has stated it’s a portmanteau of ‘Hydrogen’ and ‘Ocean’, thus ‘Hy-shen’ seems the logical way to say it. Perhaps the author can confirm?
    The prospects for hydrogen based biospheres have been examined in several evocative papers. Given such an abundance of water, I do wonder if a natural “Hydrox” world can’t form – one in which free O2 is kept from ignition by being at a low percentage. Given 20 bars surface pressure, breathable partial pressures of O2 become possible.

    • Paul Gilster August 29, 2021, 19:18

      I have to say I really prefer ‘Hy-cee-an’ to ‘Hy-shen’!

      • Robin Datta August 31, 2021, 0:37

        One may also consider the pronunciation of a derivative word; each dictionary has a speaker icon for the word, which if tapped will play the pronunciation on the device’s audio.

    • Geoffrey Hillend August 30, 2021, 16:23

      One would be crushed at 20 bars which is like a depth of 660 feet under water. A depth of 33 feet equals the pressure at sea level or 1013.25 millibars or 14.7 pounds per square inch.

      • Adam August 31, 2021, 1:59

        I’ll let the next deep diver know that when they dive to 190 metres.

  • Wojciech David Jacyk August 30, 2021, 9:50

    I am quite sceptical about them as potential habitable worlds.
    I remember a study on moons with subsurface oceans in Solar System and most of them besides Europa and Enceladus would lack the necessary elements for life due to heat and tectonic movements. Hycean worlds seem too stagnant with deep, extreme pressure environments that don’t seem inviting to creation of life, and even if life would exist, would it even break to the surface or past simple one cell organisms?

  • Alex Tolley August 30, 2021, 12:08

    The arguments for Hycean worlds having life seem to be speculation and the ease of characterizing the atmosphere, IOW, “looking for your keys where the light is, not where you dropped them”.

    The very wide potential HZ of these worlds is quite attractive – from very deep in space where internal temperatures maintain the water in a liquid state, to worlds close to their star and tidally locked. This characteristic would make them more attractive than super-Earth water worlds.

    However, stars do increase in luminosity over time, and the argument that water worlds lacking continents and plate tectonics cannot maintain habitable temperatures as the geological carbon cycle is absent may similarly apply.

    As the authors state:

    One such factor is the role of geochemical cycling
    in regulating atmospheric composition and surface temperature,
    e.g. the carbonate-silicate cycle on Earth (e.g.,
    Walker et al. 1981; Kasting et al. 1993; Franck et al.
    2000; Lammer et al. 2010). While this has been widely
    studied in the context of Earth and terrestrial planets,
    such cycles would evidently be very dierent for Hycean
    planets. Future work will be needed to establish how
    such processes work.

    That the Hycean HZ is much wider may be a mitigating feature, although it may mean that life has to keep evolving to optimize its biology for the prevailing water temperature, which seems to be allowed to extend beyond the tolerance of terrestrial thermophiles.

    With a permanent reducing atmosphere, any life would have to be anerobic, which implies that if life is based on terrestrial-type metabolism, Hycean world life will be unicellular, rather than complex, although microbial mats floating on the surface, or covering the abyssal ocean floor may exist.

    What we may be almost certain of is that any spacecraft approaching such a world will not be observing any structures or signals due to intelligence (but c.f. Solaris) unless the world has been colonized by an external ETI.

    If such worlds are common, then their systems may be good places for terrestrial colonization by humans and terrestrial biospheres. The huge volumes of water will ensure that there is an abundance of this liquid to be used in space habitats. The main difficulty being perhaps the deeper gravity wells to overcome when extracting it.

  • Michael Fidler August 30, 2021, 18:43

    It would be interesting to see a list of nearby transiting Hycean worlds. These would be prime targets for JWST and would give a good picture of their atmospheric structure and molecules.

  • Edwin September 1, 2021, 6:44

    This is interesting

    Dartmouth Engineering Receives $1.25M From NASA To Study Space Ice
    http://astrobiology.com/2021/08/dartmouth-engineering-receives-125m-from-nasa-to-study-space-ice.html

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