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Neptune-Like Hycean Worlds Focus of Search for Extraterrestrial Life

Hycean worlds
The Sun with Jupiter, Saturn, Neptune and the Earth. Scientists are now looking for life on Hycean worlds, or planets that resemble the planet Neptune. Credit: HalloweenNight/PlanetUser/Public Domain

Hycean Worlds — which are planets like Neptune — may be able to host life, according to researchers at Cambridge University. Such planets, which do not resemble Earth much at all, may actually be where we want to look if we are to find life in other universes, they now believe.

As reported in New Atlas on Thursday, “exoplanets” such as these could hold the key to finding that Holy Grail of some astronomers, the discovery of life outside our solar system.

Although more than 4,400 exoplanets have been discovered by astronomers so far, most of their  attention has been focused, intuitively, perhaps, on those that are Earth-like, believing that it may take similar amounts of water, or at least the forms of water, as we have on this planet to support life elsewhere.


However, astronomers have now classified a new type of exoplanet called “Hycean” worlds, which hold out hope for finding life in the heavens. The new term is a combination of the words “hydrogen” and “ocean.”

As scientists know, though, even here on Earth, life constantly shows us just how tough and resourceful it is, by thriving in environments  previously thought to be were too harsh, such as extraordinarily arid deserts, boiling-hot fumaroles in the bottoms of our oceans or or in the freezing darkness under polar icecaps.

Likewise, in other areas in our solar system, some of the most likely places to find aliens don’t initially appear to be very hospitable to us either, such as the icy oceans of Europa or especially, perhaps, the liquid methane of Titan.

Cambridge University astronomers are now taking a closer look at a type of exoplanet that might have been overlooked before – something they call affectionately the mini-Neptune.

Hot “Mini-Neptunes” have enormous oceans of water

As this implies, these are smaller versions of our own solar system’s planet Neptune, with thick atmospheres, rocky or icy layers and in some cases, oceans made of water. They are usually between 1.6 times and 3.9 times larger than Earth.

A sub-class of these mini-Neptunes has now been narrowed down; these “Hycean worlds” are hot mini-Neptunes with hydrogen-rich atmospheres and enormous oceans on their surfaces. The heat and the pressure there would certainly preclude humans living there, but just like with our extremes on the earth, other forms of life might find it very hospitable.

The holotype Hycean planet is “K2-18b,” located approximately 124 light years away, measuring 2.6 times wider and 8.6 times more massive than the Earth.

Like our Earth, it also orbits its host star within what is considered the habitable zone. In 2019 the Cambridge researchers reported they had detected water vapor in its atmosphere, and last year they came up with models showing its probable composition.

According to some scenarios, K2-18b could possibly host certain forms of life.

The astronomers expanded their search to more broadly define Hycean exoplanets, saying now that these worlds can be up to 2.6 times the width of Earth, have up to ten times the mass of Earth, and experience atmospheric temperatures up to a mildly uncomfortable 200°C (392 °F).

Dark Hycean worlds may also harbor life forms

These steamy temperatures might indeed be too extreme for any life; however, their oceans may well be a great deal cooler than that. These types of planets, they state, are likely to be more common as well, somewhere between the the smaller, rockier “Super-Earths” and the gaseous mini-Neptunes, which are larger.

The researchers are looking into whether or not “Dark Hycean” worlds can also support life; these are tidally locked to their star, which entails that one side is always facing that star’s light while their opposite side is cloaked in darkness. The dark side of these worlds may be able to support life, according to the Cambridge astronomers.

There are also “Cold Hycean” worlds, they add, but these don’t receive much radiation at all from their host star.

Hycean planets are actually quite common, they say, and they might very well provide the next frontier in the eternal search for life beyond our planet.

Certain biomarkers of life that scientists look for, including methyl chloride and dimethyl sulphide, could also be fairly easily detected in their atmospheres with telescopes that will be in use soon, such as the new James Webb instrument.

Dr. Nikku Madhusudhan, the lead author of the study, notes that “Essentially, when we’ve been looking for these various molecular signatures, we have been focusing on planets similar to Earth, which is a reasonable place to start. But we think Hycean planets offer a better chance of finding several trace biosignatures.”

The new James Webb telescope has now completed its testing and is undergoing the final preparations so that it can be moved to its launchpad, to be blasted into space to help find life somewhere out there.

In preparation for the time when it will begin looking at the stars, the team from Cambridge has already identified a list of Hycean planets which could be investigated by not only James Webb but other other telescopes.

These Hycean worlds all orbit red dwarf stars no more than 150 light years away from the earth.

“A biosignature detection would transform our understanding of life in the universe,” says Madhusudhan. “We need to be open about where we expect to find life and what form that life could take, as nature continues to surprise us in often unimaginable ways.”

The Cambridge University team’s scientific paper was published in the Astrophysical Journal.

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