'Hycean' exoplanets may not be able to support life after all

Artist's illustration of the view from the seas of a potentially habitable "Hycean" exoplanet.
Artist's illustration of the view from the seas of a "Hycean" exoplanet. (Image credit: Amanda Smith, Nikku Madhusudhan )

In 2021, astronomers proposed a new class of exoplanets that contain hydrogen-rich atmospheres and support vast liquid-water oceans, making these hypothetical worlds potential candidates in the search for alien life. 

However, new research suggests that these "Hycean" planets would suffer from a catastrophic runaway greenhouse effect, thus limiting their potential for habitability.

Hycean worlds get their name from a combination of "hydrogen" and "ocean," because these worlds — which would be larger than Earth but smaller than any of the giant planets in our solar system — are covered in thick, dense layers of a hydrogen atmosphere and could support vast liquid-water oceans.

Related: The search for alien life

Although no Hycean worlds have been confirmed to exist, the massive exoplanet survey by NASA's Kepler mission identified several candidate worlds that, based on estimates of their size and density, might be Hycean planets.

Astronomers are very interested in Hycean worlds. Where there's liquid water, there's a potential home for life as we know it. And because of their thick atmospheres, these planets can potentially exist in a much broader range of orbits around their parent stars without sacrificing their habitability, so there's even the chance that life is more common on Hycean worlds than our own.

But current research on the habitability of Hycean worlds isn't very detailed, and it relies on relatively simplistic models of atmospheric dynamics to understand how these planets can work. To remedy this, a team of researchers developed a more sophisticated approach for studying how a more accurate treatment of Hycean atmospheres and oceans would change our understanding of their behavior around different kinds of stars. 

'Supercritical' oceans

The researchers found that the presence of a thick, hydrogen-dominated atmosphere radically changes how these planets behave, compared with a world like Earth. Our planet has a thick atmosphere too, but that atmosphere is composed of heavier elements, like nitrogen and oxygen. The ability of those elements to block or allow in specific wavelengths of light affects how warm the surface is for a given amount of solar radiation coming in.

But hydrogen acts differently: It blocks and admits different wavelengths of light, which, in turn, changes how the surface responds to sunlight. For example, the researchers found that if a planet with an atmospheric pressure 10 to 20 times Earth's (which is typical for Hycean worlds) were to be placed in the same orbit as Earth, its oceans would become "supercritical." That means the planet's temperatures would rise beyond the boiling point, which would lead to the oceans' evaporation and complete disappearance. 

The researchers also found that the mixture of water vapor and hydrogen in Hycean planets' atmospheres changes their habitability. Hycean worlds cannot receive nearly as much sunlight as we previously thought before their oceans become too hot to sustain themselves as a liquid. 

Previous models had placed the inner edge of the habitable zone, the region where surface temperatures on a world are just right to sustain liquid water, right around one astronomical unit (AU), the distance at which Earth orbits the sun. But the new calculations push the inner edge to 1.6 AU for worlds with similar air pressures as Earth. For Hycean worlds with 10 times the air pressure, the inner edge of the habitable zone is now thought to be 3.85 AU.

This means that Hycean worlds cannot live close to their parent stars, thereby limiting their range of habitability. Indeed, the researchers concluded that all the known potential Hycean worlds exist inside these new habitable limits and, therefore, are unlikely to host liquid-water oceans — and any chance for life. The researchers have submitted their work for publication in The Astrophysical Journal, and a preprint is available via arXiv.

But there's still hope for these exoplanets' habitability. Hycean worlds can exist and sustain liquid-water oceans far beyond the outer edge of the habitable zones for Earth-like planets, so we may still find new promising candidates. The researchers hope to continue their work with even more detailed simulations to capture the complex and fascinating dynamics of these mysterious hypothetical worlds.

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Paul Sutter
Space.com Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

  • murgatroyd
    Yet another nail in the coffin of the hypothesis that life must be abundant in the universe (and that to think otherwise is "parochial" or "arrogant"). No, dear Trekkies, it is not. A series of extreeeemely improbable accidents had to occur for this planet to be hosting complex, multicellular life for billions of years un-interrupted. The odds against it happening were literally astronomical, the odds against a Starship being built by a pink or green-skinned Elon Musk anywhere are super-exponentially more astronomical. The trillion-year-old kid setting up this experiment for their Junior Science Fair designed it that way, or that is just the way it fell out as a consequence from unfathomable (to us) objectives. Whether or not a spacefaring civilization exists in some distant galaxy is immaterial. We will never meet or interact with them.*

    *But if we do, I say nuke them from orbit. It's the only way to be sure.
  • rod
    "Although no Hycean worlds have been confirmed to exist, the massive exoplanet survey by NASA's Kepler mission identified several candidate worlds that, based on estimates of their size and density, might be Hycean planets."

    Good to have models, better when observations and measurement using nature show the models are true. Exoplanet studies are moving fast and the very definition of a planet's habitable zone is changing too.

    WHERE TO LOOK FOR LIFE: HOMING IN ON THE HABITABLE ZONE, https://skyandtelescope.org/astronomy-news/where-to-look-for-life-homing-in-on-the-habitable-zone/
    "The so-called habitable zone is the region around a star where, if a planet had an atmosphere, its surface temperature would be just right for liquid water to exist. Yet even though the habitable zone is where many astronomers focus their efforts, its very definition is still being refined. In a study to appear in Astrophysical Journal Letters (preprint available here), Cassandra Hall (University of Georgia) and colleagues identify in what part of the habitable zone (HZ) photosynthesis can occur. This region is the most likely to host planets capable of producing the key signs of life known as biosignatures, Hall and colleagues write, which will aid in efforts to home in on habitability. A HABITABLE ZONE FOR PHOTOSYNTHESIS All life as we know it depends on photosynthesis, directly or indirectly..."

    ref - A New Definition of Exoplanet Habitability: Introducing the Photosynthetic Habitable Zone, https://arxiv.org/abs/2301.13836, 12-April-2023.

    From the conclusion of the paper. "5. CONCLUSION We have demonstrated the existence of a photosynthetic habitable zone (PHZ). It is the distance from the host star where the habitable zone overlaps with where photosynthesis is possible...The PHZ becomes smaller with increasing atmospheric attenuation (i.e., more dense atmospheres), and so may make life less likely on super-Earths, since their larger gravitational field can hold onto more atmosphere. The PHZ also becomes smaller as the conditions for life become less favourable, which we describe as respiration rate relative to maximum possible photosynthetic rate, increasing. We therefore conclude that the parameter space for signs of life is far narrower than the standard HZ...We identify five planets, Kepler-452 b, Kepler-1638 b, Kepler-1544 b and Kepler-62 e and Kepler-62 f, that are consistently in the PHZ in a variety of environments. For Kepler-452 b, we calculate
    that it should have a rotation period of 11 hours. The other four planets are estimated to have rotation periods between 9 and 11 hours. We suggest the search for signs of life elsewhere in the Universe should begin in earnest on the candidate planets we have identified."
  • rod
    My note, I calculated 1.65 km/s rotation rate for Kepler-452 b using the properties shown here, http://exoplanet.eu/catalog/kepler-452_b/, and the conclusion of the PDF report; calculated temperature 265 K, 273 K is freezing of water. Mass is not shown and atmosphere composition is not known too. The radius for Kepler-452 b is 1.63 earth radii. Using 3.8 g cm^-3 mean density indicates the exoplanet could be at least 3 earth masses and surface gravity stronger than Earth too, 1.1289E+00 with possible escape velocity 15.1679878 km/s. When we encounter various unknown properties for exoplanets that may look good for biological life to exist there, I always double check a few parameters :)
  • Joe Brooks
    Thanks for that great article, Dr. Sutter. I enjoyed that and learned something at the same time. The research that you reported on goes beyond the usual simplistic understanding of habitable zones and helps clarify our ideas on the habitability of Super Earths, which would now seem to be quite a bit rarer than previously thought.

    We seem to be getting ever more evidence that seems to confirm the hypothesis that stars like our Sun and planets like our Earth are truly the best places for complex life to thrive.

    While it's true that many icy moons and planets could have complex life swimming in oceans buried under icy crusts that are many miles thick, how would such life ever be able to gaze up at the stars and learn of the vast universe beyond their home world, let alone have an environment like an oxygen atmosphere that allows fire, metallurgy, and the building of telescopes and spaceships? They will likely never venture among the stars, and never send signals to or receive signals from other civilizations. SETI programs like ours will likely never find them. Surface-dwelling complex life is probably quite rare.