In 2021, astronomers proposed a new class of exoplanets that have hydrogen-rich atmospheres and support vast oceans of liquid water, making these hypothetical worlds potential candidates in the search for alien life.
However, new research indicates that these planets would suffer a catastrophic escape Global Warmingwhich limits their ability to live.
The Hycean worlds got 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 neighborhood – Solar System It is covered in dense, dense layers of hydrogen atmosphere and could support vast liquid water oceans.
Related: The search for alien life
Although the existence of Hycean worlds is not confirmed, these are huge worlds exoplanet A survey conducted by NASA Kepler mission It identified several candidate worlds that, based on estimates of their size and density, might be Hycean planets.
Astronomers are very interested in the Hycean worlds. Wherever there is liquid water, there is a potential home for life as we know it. Because of their thicker atmospheres, these planets are likely to exist in a much wider range of orbits than their parent stars without sacrificing habitability, so there is a chance that life is more common on Hycean worlds than ours.
But current research on the habitability of Hycean worlds is not very detailed, and it relies on relatively simplistic models of atmospheric dynamics to understand how these planets function. To remedy this, a team of researchers developed a more sophisticated approach to study how a more precise treatment of the Hycean atmosphere and oceans would change our understanding of their behavior around different types of stars.
oceans ‘supercritical’
The researchers found that the presence of a dense, hydrogen-dominated atmosphere radically changes how these planets behave, compared to a world like Land. Our planet also has a thick atmosphere, but this atmosphere is made up of heavier elements, such as nitrogen and oxygen. The ability of these elements to block or allow specific wavelengths of light to affect how warm the surface is relative to a given amount of incoming solar radiation.
But hydrogen works 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 of 10 to 20 times that of Earth (typical of Hycean worlds) was placed in the same orbit as Earth, its oceans would become “supercritical.” This means that the planet’s temperatures will rise beyond the boiling point, which will lead to the evaporation and complete disappearance of the oceans.
The researchers also found that the mixture of water vapor and hydrogen in the atmospheres of Hycean planets changes their habitability. Hycean worlds could not receive as much sunlight as previously thought before their oceans became too hot to sustain themselves as a liquid.
Previous models had placed the inner edge of the habitable area, the region in which global surface temperatures are well suited to conserving liquid water, is about one astronomical unit (AU), which is the distance that Earth orbits the sun. But the new calculations pushed the inner edge to 1.6 AU for worlds with air pressures similar to Earth’s. 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 near their parent stars, which limits how habitable they are. In fact, the researchers conclude, all known potential Hycean worlds exist within this new habitable boundary, and are therefore unlikely to host liquid water oceans — and any chance of life. The researchers have submitted their work for publication in The Astrophysical Journal, and a preliminary version is available online arXiv (Opens in a new tab).
But there is still hope for the habitability of these exoplanets. Hycean worlds could exist and maintain oceans of liquid water well beyond the outer edge of the habitable zones of Earth-like planets, so we may still find promising new candidates. The researchers hope to continue their work with more detailed simulations to capture the complex and fascinating dynamics of these mysterious virtual worlds.
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