Exoplanets

17 Known Exoplanets Could Have Oceans of Liquid Water

The search for life is tied to the search for liquid water. That’s why astronomers are so keen on detecting rocky, Earth-like exoplanets in their stars’ habitable zones. In a habitable zone, a planet receives enough energy from its star to maintain liquid water on its surface, given the right atmospheric conditions.

But in our Solar System, we’ve found worlds with liquid water that are way beyond the habitable zone. Can we do the same in other solar systems?

One way to find a subsurface ocean in an icy world is to detect plumes. We’ve found icy plumes erupting from Enceladus and Europa, two of our Solar System’s icy ocean moons. We have no possibility of seeing the same plumes on distant exoplanets to determine if they have oceans under ice.

We’ve found cryovolcanic plumes on two of our Solar System’s frozen ocean moons. The image on the left shows cryovolcanic eruptions at the south pole of Saturn’s moon Enceladus. The Hubble image on the right shows cryovolcanic plumes on Jupiter’s moon Europa. Image Credit Left: NASA/JPL-Caltech/SSI. Image Credit Right: NASA/L. Roth

But some NASA scientists think they may have found another way to determine which exoplanets might have interior oceans.

Even though we know of more than 5,000 exoplanets, we’ve never really seen any of them. There are some directly imaged exoplanets, but they’re little more than dots. They’re interesting, but we can’t learn much from them. We certainly can’t tell if they harbour oceans.

This image shows the exoplanet HIP 65426 b in different bands of infrared light, as seen from the James Webb Space Telescope. Though direct images of exoplanets like this are an important step, they don’t reveal much about the nature of the planet. Image Credit: NASA/ESA/CSA, A Carter (UCSC), the ERS 1386 team, and A. Pagan (STScI).

While we can’t actually see exoplanets in the normal sense of the word, some research scientists have figured out a way to at least determine which exoplanets may have interior oceans under ice. There’s a vast number of exoplanets out there, and this type of work helps astronomers know what exoplanets to focus on.

These results are in a paper published in The Astrophysical Journal titled “Prospects for Cryovolcanic Activity on Cold Ocean Planets.” The lead author is Lynnae Quick from NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

On an ocean world with an icy shell, tidal heating and the radioactive decay of elements in the world’s core provides the heat that keeps the buried ocean from freezing. Water should sometimes erupt through cracks in the icy shells on these worlds, creating cryovolcanic plumes like the ones on Enceladus and Europa. The trick is to figure out which exoplanets might have oceans under icy shells.

The research team determined that 17 known exoplanets could be icy ocean worlds. They used albedo estimates based on Europa and Enceladus to identify the 17 exoplanets. The researchers also calculated the tidal heating based on their orbits and estimated the radiogenic heating from the decay of radioactive elements, partly based on the elemental makeup of asteroids in our Solar System.

They also calculated the amount of geyser activity on each one.

“Our analyses predict that these 17 worlds may have ice-covered surfaces but receive enough internal heating from the decay of radioactive elements and tidal forces from their host stars to maintain internal oceans,” said lead author Quick. “Thanks to the amount of internal heating they experience, all planets in our study could also exhibit cryovolcanic eruptions in the form of geyser-like plumes.”

Two often-studied exoplanets are at the top of the list for cryovolcanism: Proxima Centauri b and LHS 1140 b. This pair could have hundreds to thousands of times more cryovolcanic activity than Europa. Three of the TRAPPIST-1 planets are also among the 17. Readers who follow exoplanet news might recognize some others: Kepler-62f, Kepler-1652b, and GJ-514b.

All of the planets on the list are roughly Earth-sized, with densities that suggest they’re not entirely rocky. Each of them could have considerable amounts of ice and water. Since they’re all much colder than Earth, it’s likely that their surfaces are frozen over, much like Europa and Enceladus.

There are two ways that these planets can experience cryovolcanism, and it depends on the thickness of the ice shells.

These ocean worlds with frozen crusts are not static. Planets with icy shells likely get almost all of their heat from internal sources, as most starlight would be reflected away. Tidal heating varies as a planet orbits its star unless its orbit is perfectly circular.

This figure from the research shows the total internal heating for the 17 exoplanets, with Europa, Earth, and Io included for comparison. “Every planet that we have considered receives more heating from tidal and radiogenic sources than Europa, which is cryovolcanically and tectonically active, and maintains an ocean beneath its icy shell,” the authors write. Image Credit: Quick et al. 2023.

On a planet with a thicker ice crust, liquid water can work its way into the ice shell as tidal heating conditions vary over time, forming pockets of liquid water that contain volatiles. Sometimes, these pockets of water can erupt through a weakness in the icy shell. On planets with thinner shells, the water can erupt directly from the ocean itself.

This schematic from the research helps show how liquid water can erupt through an icy shell. The top panel shows water erupting through thin (<10 km thick) ice shells by the exsolution of volatiles from water-filled fractures connected to a subsurface ocean. The bottom panel shows water erupting from isolated pockets of water in the ice on planets with thicker ice shells. Image Credit: Quick et al. 2023.

The ice thickness helps determine the amount of cryogenic activity, and the researchers determined the likely thickness of shells for the 17 exoplanets. A planet’s temperature determines its thickness, and the researchers determined that surface temperatures for the planets in the sample were lower than thought, up to 33 degrees Celsius (60 F) lower in some cases.

Proxima B’s icy shell is only about 58 meters (190 ft) thick, and LHS-1140b’s shell is only 1.6 km (1 mile) thick. On the other end of the scale is MOA 2007 BLG 192Lb with a shell about 38.6 km (24 miles) thick. For comparison, Europa’s icy shell is about 29 km (18 miles) thick. What does this tell us?

“Since our models predict that oceans could be found relatively close to the surfaces of Proxima Centauri b and LHS 1140 b, and their rate of geyser activity could exceed Europa’s by hundreds to thousands of times, telescopes are most likely to detect geological activity on these planets,” lead author Quick explained.

We don’t have any images of Proxima Centauri b, only artists’ illustrations of what the surface of the planet could look like. Image Credit: ESO/M. Kornmesser

When it comes to Proxima Centauri b, there’s been a lot of uncertainty. The only thing we know for sure is that it’s firmly inside what astronomers think of as the classical habitable zone. Beyond that, we aren’t certain if it’s tidally locked to its star, though that may be likely. If it is, that changes everything about its temperature, atmosphere, and the state of any water it might have held onto. It could have planet-wide oceans or only smaller isolated bodies of water. It could be partially or totally frozen, with a liquid ocean underneath the ice. Or it could be all dry land.

But if this new research is correct, then the search for habitable planets has a more viable target much nearer to Earth than we thought. Earth hosts communities of life at hydrothermal vents in its deep oceans, isolated from sunlight. It’s plausible that the same thing can happen on icy ocean worlds.

“All of the planets considered in this study are large enough and experience enough internal heating for them to contain subsurface oceans at varying depths beneath external ice shells and for them to host active geological processes, notably cryovolcanism,” the authors write in their conclusion. “If experience with the ocean worlds in our solar system is any indicator, the thin ice shells we have estimated for Proxima Cen b, Trappist-1f, and all of the Kepler planets in our study suggest that any eruption products vented into space during episodes of explosive cryovolcanism on these worlds would issue directly from their subsurface oceans.”

That puts these worlds into the same category as Europa. And we’ve detected rocky particles, salts, and organic chemicals in Europa’s plumes. If we can detect plumes on any of these planets, they’ll contain direct evidence of the makeup of their oceans.

New telescopes coming online soon will be able to see Proxima Centauri b. The Giant Magellan Telescope will come online in the next few years and will have the resolving power to easily see PCb. “The angular separation of the closest known exoplanet, Proxima b, from Proxima Centauri, is easily within reach
of the Giant Magellan with adaptive optics,” the Giant Magellan Telescope website says. It’s possible that the powerful telescope will detect any cryovolcanic plumes that escape from the ocean.

This illustration shows what the Giant Magellan Telescope will look like when it comes online. This powerful telescope will be able to image some exoplanets, including Proxima Centauri b. Image: Giant Magellan Telescope – GMTO Corporation

Astronomy will change when the Magellan and our other newer, more powerful telescopes come online over the next few years. And if the Giant Magellan can measure geyser activity on Proxima Centauri b and if it finds a lot of it, the search for life will change, too.

Rather than focusing on the classical habitable zone around stars, we can consider planets further out, where life-supporting oceans might exist and persist under icy caps. If astronomers can figure out a way to detect more plumes and to measure what’s in them, then the search for life will take a giant step forward.

Evan Gough

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