September 20, 2023

Buried oceans may be common on icy exoplanets

    Jupiter's icy moon Europa, a dirty white object with crisscrossing dark lines on its surface, is seen against the blackness of space

Jupiter’s icy moon Europa, a dirty white object with crisscrossing dark lines on its surface, is seen against the blackness of space

The chances of finding extraterrestrial life may have received a significant boost.

A new analysis of exoplanets suggests there’s a much higher chance than previously thought that these worlds harbor liquid water, an essential ingredient for life on Earth.

The universe could therefore be filled with more habitable planets than scientists had previously assumed, with a higher probability that these worlds possess environments in which alien life could develop, even if they have icy outer shells.

“We know that the presence of liquid water is essential to life. Our work shows that this water can be found in places that we had not given much thought to,” study leader and scientist at Rutgers University Lujendra Ojha said in a statement. “This significantly increases the chances of finding environments where life could theoretically develop.”

Related: The 10 most Earth-like exoplanets

Ojha and colleagues found that even exoplanets with frozen surfaces may have subsurface oceans of liquid water.

“Before considering this subsurface water, it was estimated to be roughly one rocky planet [in] every 100 stars would have liquid water,” explains Ojha. “The new model shows that, if the conditions are right, this could approach one planet per star. So we are 100 times more likely to find liquid water than we thought.”

Since there are about 100 billion stars in the Milky Way galaxy, “that means really good odds for life emerging elsewhere in the universe,” he added.

How icy worlds could hold liquid water

The researchers examined planets around the most common type of stars in our galaxy, red dwarfs, which are smaller and cooler than the sun. Red dwarfs, also called M dwarfs, not only make up about 70% of the stars in the Milky Way, but they are also the stars around which most Earth-rocky worlds have been found.

The team considered two ways rocky planets with an icy shell could be heated from below, allowing them to retain liquid water underground, the first of which is clearly visible here on Earth.

“As Earthlings, we are lucky right now because we have just the right amount of greenhouse gases in our atmosphere to make liquid water stable at the surface. However, if the Earth were to lose its greenhouse gases, the average global surface temperature would be about minus 18 degrees Celsius. [minus 0.4 degrees Fahrenheit], and most liquid surface water would freeze completely,” Ojha explained. “A few billion years ago, this actually happened on our planet, and liquid surface water completely froze. However, this does not mean that the water was completely solid everywhere.”

Liquid water at that time in Earth’s history was preserved by heating in the form of radioactivity from deep within the planet.

“Heat from radioactivity deep in the Earth can warm water enough to keep it liquid,” Ojha said. “Even today we see this happening in places like Antarctica and the Canadian Arctic, where despite the frigid temperature, there are large underground lakes of liquid water supported by the heat generated by radioactivity.”

The researcher said there is evidence that heating via radioactivity may also currently be taking place near the south pole of Mars.

“We modeled the feasibility of generating and sustaining liquid water on exoplanets orbiting M dwarfs by only considering the heat generated by the planet,” Ojha said. “We found that when one considers the possibility of liquid water generated by radioactivity, it is likely that a high percentage of these exoplanets may have enough heat to sustain liquid water — much more than we had thought.”

Artistic illustration of the Europa Clipper spacecraft flying above Jupiter's icy moon Europa.

Artistic illustration of the Europa Clipper spacecraft flying above Jupiter’s icy moon Europa.

Another possible heating mechanism that could help keep liquid water under a frozen planetary shell, suggested by the team, arises due to the gravitational pull of a larger body, causing the interior of an outwardly frozen world to swirl endlessly. This is also something that is visible elsewhere in our solar system.

“Some of the moons you find in the solar system, e.g. Europa or Enceladus, have substantial subsurface liquid water, even though their surfaces are completely frozen,” Ojha noted, referring to the icy moons of Jupiter and Saturn, respectively.

“This is because their interiors are constantly being churned by the gravitatio
nal effects of the large planets they orbit, such as Saturn and Jupiter,” he added. “This is similar to our moon’s effect on tides, but much stronger.”

Not only has this effect made Europa and Enceladus prime candidates for finding life elsewhere in the solar system, but it also has implications for life-sustaining environments on worlds orbiting other stars.

NASA will soon explore at least one ice world, albeit within the confines of the solar system: The Europa Clipper probe is scheduled to launch toward the Jovian system in 2024, arriving six years later.

Related: Europa Clipper: A Guide to NASA’s New Astrobiology Mission


— Exoplanets: worlds beyond our solar system

— The search for extraterrestrial life (reference)

– Europa: A guide to Jupiter’s icy ocean moon

Abel Méndez, director of the Planetary Habitability Laboratory at the University of Puerto Rico, was not involved in the new research, but commented on the implications of the findings.

“The prospect of oceans hidden under ice caps increases our galaxy’s potential for more habitable worlds,” Méndez said. “The biggest challenge is figuring out ways to detect these habitats through future telescopes.”

The team’s research was recently published in the journal Nature and will be presented by Ojha at the Goldschmidt geochemistry conference in Lyon, France, to be held from Sunday (July 9) to (July 14).

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