Aliens, should they exist, likely inhabit rocky planets like ours, but new research proposes an entirely new type of habitable world — one that scarcely resembles Earth.
Earth is the only functional model we have of a habitable world. It makes sense, therefore, that astrobiologists are hopeful of finding evidence of extraterrestrials on rocky Earth-like exoplanets. Indeed, the size of our planet, its chemical composition, temperature, and its position in the solar system have made it friendly to life, at least life as we know it.
But as new research published in The Astrophysical Journal suggests, an entirely new class of planet, dubbed Hycean worlds, could likewise be habitable. This type of planet does not exist in our solar system, but they’re actually quite plentiful in the galaxy. They’re basically what you might expect if Earth and Neptune had a baby: Hycean planets are big and very warm, they’ve got atmospheres filled with hydrogen, and they’re covered in a massive planet-wide ocean.
The new paper theorizes that Hycean planets are potentially habitable and that microbial life, or even more complex forms of life, can exist within their oceans. Moreover, the researchers, led by Nikku Madhusudhan from Cambridge’s Institute of Astronomy, proposed a set of biosignatures that astronomers should look for when studying these worlds, along with a list of promising Hycean candidates nearby (cosmologically speaking).
Madhusudhan coined the term Hycean, and it’s a portmanteau of the words hydrogen and ocean, as he explained to me in an email. The astronomer came up with the idea while studying a potentially habitable mini-Neptune known as K2-18b. Intrigued by the conditions seen on this exoplanet, Madhusudhan and his colleagues embarked on a fuller investigation to explore the habitable potential of mini-Neptunes in general, or a least a specific subtype of mini-Neptunes.
Thousands of exoplanets have been detected since the 1990s — the vast majority of them being low-mass planets between one and four times the size of Earth. For the new study, the researchers focused on mini-Neptunes, which are (obviously) smaller than Neptune but about 1.6 times the size of Earth.
“These objects are smaller than ice giants but too large to have predominantly rocky interiors like super-Earths,” according to the study. “Past explorations of mini-Neptune interiors have found that in some cases the pressure and temperature beneath the [hydrogen]-rich [atmosphere] would be too high to allow for habitability,” but “temperate mini-Neptunes with the right properties can allow for habitable conditions in their interiors.”
The newly proposed class of habitable exoplanet can be up to 2.6 times the size of Earth, with atmospheric temperatures of 392 degrees Fahrenheit (200 degrees Celsius). When asked which features on Hycean worlds are amenable to habitability, Madhusudhan said the presence of liquid water, “as the surface would be covered with ocean,” the presence of Earth-like air pressure and temperatures at the ocean surface, and the hydrogen-rich atmosphere, which provides “both the required surface conditions and protection from harmful radiation.”
Hycean worlds can exist around an assortment of star types, Madhusudhan said. He believes that a tidally locked Hycean exoplanet has the potential to be habitable, even though one side never faces its host star (the researchers refer to these exoplanets as Dark Hycean worlds, which sounds so sci-fi). In terms of these exoplanets having the requisite chemical properties to spawn and sustain microbial life, Madhusudhan said Hycean planets should have “plenty of water, radiation, and basic molecules in the atmosphere,” but “beyond this, “the possible chemistry is an open question at this point.”
The depth at which life might exist within Hycean oceans depends on the particular surface conditions, but the researchers suspect that, in some cases, life could exist deeper than Earth’s ocean floor. This life, should it exist, would likely be microbial in nature, and Madhusudhan said it’s an “open question” as to whether more complex life could evolve in this exotic environment. Potential barriers to habitability would be excessive UV radiation or a “dearth of nutrients necessary to form or sustain life,” he added.
A fortuitous thing about Hycean worlds is that they exist within a more expansive habitable zone than what we’re accustomed to. Also known as the Goldilocks zone, this is the band inside a star system within which liquid water can exist at the surface.
“A Hycean world can be very far from the star with little irradiation and still be habitable,” explained Madhusudhan. “Similarly, the planet can be a bit closer to the star than an Earth-like planet and still be habitable. This is because of the hydrogen-rich atmospheric composition of Hycean worlds and the fact that their surface temperatures can be significantly higher than those on Earth-like planets.”
The new paper will be of interest to astrobiologists. Hycean worlds are very common in the Milky Way, which means habitable exoplanets could be far more numerous than previously thought. The researchers also provided a list of trace biosignatures that scientists should be searching for when observing these exoplanets, including oxygen, ozone, methane, and nitrous oxide, and also uncommon gases, like methyl chloride and dimethyl sulphide, which aren’t indicative of life on Earth but could possibly signal of life on Hycean worlds.
The team identified 11 candidate Hycean worlds orbiting nearby stars — all red dwarfs — which are considered promising targets for biosignature searches. The most promising of these is K2-18b, “for which we already have observations planned with the James Webb Space Telescope,” said Madhusudhan.
Indeed, the James Webb space telescope (JWST), with its spectroscopic capabilities, will be a perfect tool for investigating K2-18b and other mini-Neptunes deemed to be Hyceans. The next-gen telescope is scheduled to launch later this year, after many years of delay.
The cool thing about the hypothesis proposed in this paper is that it’s testable. Should spectroscopic scans of Hycean worlds yield zero traces of the proposed biosignatures, it’s likely a sign that these exoplanets are dead, inhospitable worlds. But should these biosignatures appear in our observations, that would be a different story entirely, and a tantalising result.