Astronomers identify new type of planet that could be home to life

Aliens, if they exist, likely inhabit rocky planets like ours, but new research suggests a whole new kind of habitable world that barely resembles Earth.

Earth is the only functional model we have of a habitable world. So it makes sense that astrobiologists hope to find evidence of aliens on rocky Earth-like exoplanets. Indeed, the size of our planet, its chemical composition, its temperature and its position in the solar system have made it friendly to life, at least life as we know it.

But like new research published in The Astrophysical Journal suggests that a whole new class of planets, dubbed the Hycean worlds, could also be habitable. This type of planet does not exist in our solar system, but there are actually quite a few in the galaxy. This is basically what you might expect if both Earth and Neptune had a baby: The Hycean planets are large and very hot, they have atmospheres filled with hydrogen, and they are covered with a huge planetary ocean.

New article theorizes that Hycean planets are potentially habitable and that microbial life, or even more complex life forms, can exist in their oceans. Additionally, the researchers, led by Nikku Madhusudhan of the Cambridge Institute for Astronomy, have come up with a set of biosignatures that astronomers should look for when studying these worlds, along with a list of promising Hycean candidates for proximity (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 had the idea in studying a potentially habitable mini-Neptune known as K2-18b. Intrigued by the conditions observed on this exoplanet, Madhusudhan and his colleagues embarked on a more in-depth investigation to explore the habitable potential of mini-Neptunes in general, or at least a specific subtype of mini-Neptunes.

Thousands of exoplanets have been detected since the 1990s, the vast majority of which are low mass planets between a and four times the size of the 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 the Earth.

“These objects are smaller than ice giants but too big to have predominantly rocky interiors like super-Earths,” according to the study. “Previous explorations of the interiors of mini-Neptune have revealed that in some cases the pressure and temperature below the [hydrogen]-rich [atmosphere] would be too high to allow livability “, but” temperate mini-Neptunes with the right properties can allow livable conditions in their interiors “.

The newly proposed class of habitable exoplanets can be up to 2.6 times the size of Earth, with atmospheric temperatures of 392 degrees Fahrenheit (200 degrees Celsius). When asked what characteristics of the Hycean worlds lend themselves to habitability, Madhusudhan said that the presence of liquid water, “As the surface would be covered with ocean”, the presence of atmospheric pressure and temperatures similar to those of the Earth on the surface of the ocean, and the hydrogen-rich atmosphere, which provides “both the required surface conditions and protection against harmful radiation”.

Hycean worlds can exist around an assortment of star types, Madhusudhan noted. He believes that a Hycean exoplanet locked by the tides To the potential to be habitable, even if one side never faces its host star (researchers call these exoplanets the Dark Hycean worlds, which sounds so sci-fi). In terms of those exoplanets having the chemical properties required to generate and maintain microbial life, Madhusudhan said the Hycean planets should have “lots of water, radiation, and basic molecules in the atmosphere,” but “beyond that,” the possible chemistry is an open question at this point.

The depth at which life might exist in the Hycean oceans depends on particular surface conditions, but researchers suspect that, in some cases, life could exist deeper than The ocean floor of the Earth. This life, if it existed, would likely be microbial in nature, and Madhusudhan said it is an “open question” whether more complex life could evolve in this exotic environment. Potential obstacles to livability would be excessive UV radiation or a “lack of nutrients necessary to form or maintain life,” he added.

A fortuitous thing about Hycean worlds is that they exist in a larger habitable area than we are used to. Also known as the Goldilocks Zone, this is the band within a star system in which liquid water can exist on the surface.

“A Hycean world can be very far from the star with little irradiation and still be habitable,” Madhusudhan explained. “Likewise, the planet can be a little closer to the star than an Earth-like planet and still be habitable. It is because of the hThe hydrogen-rich atmospheric composition of the Hycean worlds and the fact that their surface temperatures can be considerably higher than those of Earth-like planets.

The new article will be of interest to astrobiologists. Hycean worlds are very common in the Milky Way, which means that habitable exoplanets could be much more numerous than previously thought. The researchers also provided a list of traces of biosignatures that scientists should look for when observing these exoplanets, including oxygen, ozone, methane, and nitrous oxide, as well as rare gases, such as methyl chloride and dimethyl sulfide, which are not indicative of life on Earth but could possibly report of life on the Hycean worlds.

The team identified 11 candidate Hycean worlds orbiting nearby stars, all red dwarfs, which are considered promising targets for biosignature research. The most promising of these is the K2-18b, “for which we have already planned observations with the James Webb Space Telescope,” Madhusudhan said.

Indeed, the James Webb Space Telescope (JWST), with its spectroscopic capabilities, will be a perfect tool to study K2-18b and other mini-Neptunes reputed to be Hyceans. The next-generation telescope is expected to launch later this year, after many years of delay.

What’s cool about the hypothesis offered in this article is that it is testable. If spectroscopic analyzes of Hycean worlds do not give any trace of the proposed biosignatures, it is probably a sign that these exoplanets are dead and inhospitable worlds. But if these biosignatures appeared in our observations, that would be a whole different story and a tantalizing result.