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 extraterrestrial life.
However, new research suggests that these “hycean” planets would suffer from a catastrophic runaway greenhouse effectthereby limiting their potential for habitability.
Hycean worlds take 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 ours solar system – are covered by thick, dense layers of hydrogen atmosphere and could support vast oceans of liquid water.
Related: The search for extraterrestrial life
Although the existence of Hycean worlds has not been confirmed, the massive exoplanet NASA survey Kepler mission identified several candidate worlds that could be Hycean planets based on estimates of their size and density.
Astronomers are very interested in Hycean worlds. Where there is liquid water there is a potential home for life as we know it. And because of their dense atmospheres, these planets can potentially exist in a much wider range of orbits around their parent planet Stars without sacrificing their habitability, so there’s even a possibility that life may be more common on Hycean worlds than on our own.
However, current research on the habitability of Hycean worlds is not very detailed, relying on relatively simple models of atmospheric dynamics to understand how these planets can function. To remedy this, a team of researchers developed a more sophisticated approach to study how a more detailed treatment of the hyceic atmospheres and oceans would change our understanding of their behavior around different types of stars.
“Supercritical” oceans
The researchers found that the presence of a thick, hydrogen-dominated atmosphere radically altered the behavior of these planets compared to a world of a similar kind Earth. Our planet also has a dense atmosphere, but this atmosphere is made up of heavier elements like nitrogen and oxygen. The ability of these elements to block or let in certain wavelengths of light affects how warm the surface is for a given amount of incident solar radiation.
But hydrogen behaves differently: it blocks and transmits light of different wavelengths, which in turn alters the surface’s response to sunlight. For example, the researchers found that if a planet with an atmospheric pressure 10 to 20 times that of Earth (which is typical of Hycean worlds) would be placed in the same orbit as Earth , its oceans would become “supercritical”. This means that the planet’s temperatures would rise above boiling point, leading to evaporation and the complete disappearance of the oceans.
The researchers also found that the mixture of water vapor and hydrogen in the atmosphere of Hycean planets changes their habitability. Hycean worlds cannot receive nearly as much sunlight as we previously thought before their oceans become too hot to sustain 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, exactly around one astronomical unit (AU), the distance that the earth orbits the sun. But the new calculations shift the inner margin to 1.6 AU for worlds with air pressure similar to Earth. For Hycean worlds at 10x atmospheric pressure, the inner edge of the habitable zone is now estimated at 3.85 AU.
This means Hycean worlds cannot live near their parent stars, limiting their habitability. In fact, the researchers concluded that all known potential Hycean worlds exist within these new habitable boundaries and therefore are unlikely to host liquid water oceans – and no chance for life. The researchers have submitted their work for publication in the Astrophysical Journal, and a preprint is available at arXiv (opens in new tab).
But there is still hope for the habitability of these exoplanets. Hycean worlds can exist well beyond the outer limits of habitable zones for Earth-like planets and sustain oceans of liquid water, so we may still find new promising candidates. Researchers hope to continue their work with even more detailed simulations to capture the complex and intriguing dynamics of these mysterious hypothetical worlds.
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