‘Hycean’ exoplanets may potentially harbor the extraterrestrial life that has eluded us thus far.

In a matter of a few years, astronomers may be able to identify the initial indications of microbial life beyond our solar system.

As astronomers uncover more and more exoplanets, our solar system, with its small rocky planets on the inside and large gas giants on the outside, appears increasingly peculiar.

When astronomers observe stars in distant solar systems, they seldom come across Venus-like or Earth-like planets. Instead, the typical inner planet seems to be somewhere in between a rocky dwarf and a giant gas planet. These common worlds have been labeled “mini-Neptunes,” believed to be downsized versions of the ice giants in our solar system.

Due to the lack of a local example to examine, the characteristics of mini-Neptunes have remained enigmatic. However, the high atmospheric pressures on these planets have led many researchers to speculate that they might just be barren spheres of hydrogen and helium.

A group of exoplanet researchers now proposes that some mini-Neptunes may not resemble Neptune at all. Through extensive modeling, they have discovered that certain planets with these atmospheric pressures could potentially sustain hot, but not scorching, oceans for billions of years. Considering this possibility, the team suggests that these inner planets could potentially harbor alien microbes, and any life forms on these planets could modify their atmospheres in ways that astronomers will soon be able to detect easily.

“This finding significantly increases our chances of discovering life,” says Nikku Madhusudhan, an astronomer at the University of Cambridge and the primary author of the new study.

A mini-Neptune, revealed

Astronomers have the ability to estimate the mass and size of an exoplanet by observing its effects on its host star, which provides a general understanding of whether the planet is composed mainly of rock or gas. However, when it comes to determining the specific characteristics between a planet’s solid core and its atmosphere, researchers face significant limitations.

Nevertheless, in 2019, a team utilized the Hubble Space Telescope to observe the atmosphere of a mini-Neptune called K2-18b. This observation revealed the presence of water vapor and clouds, prompting Madhusudhan and fellow researchers to analyze the potential compositions of the planet’s interior based on this new atmospheric information.

In the previous year, they announced that certain versions of K2-18b could potentially possess planet-wide oceans that could support some forms of Earth life, despite the extreme heat, pressure, and size of the world. Madhusudhan states, “This is the first time we have demonstrated that planets can be significantly larger than Earth and still maintain habitable conditions.”

A whole new world

The primary goal of numerous researchers is to discover an Earth twin that matches in mass, size, and orbit – close enough to its star to maintain liquid water, but not so close that it evaporates. However, upon realizing that K2-18b (which is eight times the mass of Earth in a volume approximately 17 times larger) might be submerged in water, Madhusudhan pondered whether the field should broaden its perspective. While an Earth analog may be suitable for complex organisms that eventually develop telescopes and the internet, it may be excessive for simple bacteria thriving in hot springs and deep-sea vents.

Madhusudhan suggests, “Let’s not presume [conditions that are] comfortable for humans.”

By discarding conventional assumptions, researchers were able to identify a group of mini-Neptunes as potentially habitable planets. They discovered that a sufficiently dense and hydrogen-rich atmosphere could function as a pressure cooker lid, enabling all-encompassing oceans to endure even under extreme heat experienced by planets in close proximity to their host star. This new category of planets was named “Hycean” worlds (pronounced HI-shun), a blend of “hydrogen” and “ocean.”

The researchers outlined a typical Hycean planet along with two variations that all fit their criteria for habitability (with temperatures below approximately 250 degrees Fahrenheit and pressures less than 1,000 times that of Earth’s atmosphere).

A standard Hycean world would consist entirely of ocean without any continents, characterized by perpetual fog and mist emanating from its warm seas. “Dark Hycean” planets would be those where gravitational forces have synchronized their rotation so that one side constantly faces their star, similar to how one side of our moon always faces the Earth. The illuminated side would scorch while the dark side remained relatively cool. Lastly, the hydrogen atmospheres of “Cold Hycean” worlds could retain sufficient heat to prevent their oceans from freezing even at considerable distances from their host star’s warmth.

The trio of Hycean planets broadens the scope of potentially habitable planets, reflecting a current trend in exoplanet research where scientists are exploring more nuanced concepts of habitability.

According to Björn Benneke, an astronomer at the University of Montreal not associated with the study, the notion of discovering a planet identical to Earth is prevalent. However, there are alternative methods to uncovering life forms beyond dinosaurs roaming on the planet’s surface.

From candidates to confirmation

The Hycean hypothesis stands out from many other proposals in the speculative field of astrobiology due to its testability. The abundance of Mini-Neptunes in the galaxy increases the likelihood of finding planets with the right conditions for deep oceans. Even a small percentage of these planets meeting the requirements would result in a significant number of potential Hycean candidates. Astronomers have identified eleven prime targets for further study, focusing on planets with optimal viewing conditions. Hycean worlds possess atmospheres that are ideal for observation, allowing researchers to detect specific chemicals that could indicate the presence of life. The upcoming James Webb Space Telescope (JWST) is expected to play a crucial role in this endeavor.

Just waiting for discovery

The initial investigations aimed at detecting signs of life in the atmosphere of K2-18b could occur as early as next year, provided that the JWST launch proceeds smoothly this autumn. Madhusudhan predicts that the advanced space telescope will have the capability to identify one of the proposed “biosignatures” (if it exists) with just 20 hours of observation. Furthermore, he is among the researchers who have already obtained approval to conduct such a program during JWST’s first operational cycle.

While a single detection would not definitively prove the presence of alien bacteria on K2-18b, it would encourage other astronomers to search for additional biosignatures that could bolster the argument.

“If that occurs, it will pave the way for further discoveries,” Madhusudhan explains. “We may witness the detection of a biosignature within the next two to three years.”

Benneke, on the other hand, has devised his own strategy for hunting for life. He plans to examine the atmosphere of one of the rocky, Earth-like planets in the TRAPPIST-1 solar system for preliminary biosignatures. Additionally, he intends to study a mini-Neptune orbiting a faint star, which could potentially be a Hycean world.

“There are numerous reasons to be enthusiastic about both of these approaches,” he remarks. “It is an incredibly thrilling era to be alive.”

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