Water on exoplanets is mostly hidden deep inside
- Water is common in our galaxy, both in our own solar system and in other planetary systems. Scientists think many planets larger than Earth have abundant water.
- Most of that water may be inside planets instead of on their surfaces, a new study says. The water is hidden in the planets’ mantles and cores.
- Astronomers can analyze a planet’s atmosphere to help determine how much water it has, both inside and on the surface. The amount of water can also affect a planet’s ability to support life.
More water on exoplanets than previously thought
Water is common in our solar system, and there is growing evidence that it is abundant in many other planetary systems as well. For exoplanets – planets orbiting other stars – scientists have wondered just how much water they have. And for planets that do have water, is it on the surface or deeper inside? On August 20, 2024, researchers in the U.S. and Switzerland suggested in a new study that such water worlds can contain even more water than previously thought. A new computer model simulates the distribution of water on exoplanets and finds most of it is hidden deep inside.
Caroline Dorn at ETH Zurich in Switzerland, along with Haiyang Luo and Jie Deng at Princeton University in New Jersey, published their peer-reviewed findings in Nature Astronomy on August 20, 2024.
Planetary formation is complex
The study focused on exoplanets that are rocky like Earth, but larger, known as super-Earths. It also focused on bigger exoplanets – but still smaller than Neptune – called mini-Neptunes or sub-Neptunes. Astronomers have found a growing number of such worlds. Both can contain a lot of water, either on their surfaces or deep down below a thick atmosphere with no solid surface. For super-Earths, scientists have long used Earth’s own formation as a model for studying other rocky planets. It’s a fairly simple model, with an iron core surrounded by a mantle of silicate bedrock and water oceans on the surface.
But will other rocky exoplanets be similar or different? The answer is probably both. Studies have shown this kind of planetary formation is more complicated than first assumed. Dorn said:
It is only in recent years that we have begun to realize that planets are more complex than we had thought.
Currently, many of these planets discovered so far are very close to their stars. This is largely because closer-in planets are easier to detect than ones farther out. Many of those planets are so hot that their surfaces are molten.
Water in a ‘magma soup’
On young, rocky planets, including Earth at the beginning, the inner iron core takes time to form. This is because the planet is still extremely hot and molten and takes a long time to cool down. Most of the iron is still in this “magma soup.” But there is water also, and it plays a role. It combines with the iron droplets, causing them to sink down in the mantle toward the forming core. As Dorn explained:
The iron core takes time to develop. A large share of the iron is initially contained in the hot magma soup in the form of droplets. The iron droplets behave like an elevator that is conveyed downward by the water.
This is generally typical for rocky planets similar in size and mass to Earth. But what about larger planets like super-Earths or mini-Neptunes? As it turns out, more water will sink down with the iron droplets. Dorn said:
This is one of the key results of our study. The larger the planet and the greater its mass, the more the water tends to go with the iron droplets and become integrated in the core. Under certain circumstances, iron can absorb up to 70 times more water than silicates. However, owing to the enormous pressure at the core, the water no longer takes the form of H2O molecules but is present in hydrogen and oxygen.
Water on exoplanets and water on Earth
A crucial clue to how water behaves on exoplanets was found on Earth itself four years ago. We think of the oceans as being the primary resource of water on our planet. But scientists found that about 80% of Earth’s liquid water is actually inside the planet. The vast oceans contain only a small portion of the overall water reserves. So, with this in mind, larger and more massive planets could have even more of their water inside instead of on the surface. The researchers said the volume of water on these planets may be underestimated by as much as 10 times. Dorn said:
Planets are much more water-abundant than previously assumed.
Clues in a planet’s atmosphere
One way astronomers can try to determine how much water a planet has is to analyze its atmosphere (if it has one, of course). NASA’s Webb space telescope has been doing just that for about two years now. The more water is in a planet’s atmosphere, the more likely the planet has abundant water in its interior. That’s because water in a magma ocean in the planet’s mantle can degas (be removed from the magma). It can then rise to the surface when the mantle starts to cool, and make its way into the atmosphere. Dorn said:
So if we find water in a planet’s atmosphere, there is probably a great deal more in its interior. Only the composition of the upper atmosphere of exoplanets can be measured directly. Our group wishes to make the connection from the atmosphere to the inner depths of celestial bodies.
Water that is even deeper down in the planet’s core, however, will remain trapped there.
TOI-270 d is a primary current example of an exoplanet where these processes seem to be happening, as Dorn noted:
Evidence has been collected there of the actual existence of such interactions between the magma ocean in its interior and the atmosphere.
In this case, TOI-270 d may be a hycean world with a global ocean beneath a thick hydrogen atmosphere. TOI-270 d is about twice the diameter of Earth and is 70 light-years away. Astronomers classify it as a sub-Neptune.
Are these planets habitable?
A big question, of course, is whether any of these kinds of planets could support life. Some of the largest and most massive might have deep global oceans over a layer of high-pressure ice instead of rock. In that scenario, chemical nutrients deeper in the mantle wouldn’t be able to interact with the ocean. That could make it difficult for any life to evolve.
The new study from Dorn and her colleagues, however, said this isn’t necessarily the case. That kind of oceans would be the exception rather than the rule. Planets that simply have most of their water in their mantles should be a lot more common. That might provide Earth-like habitable conditions inside the planets and perhaps even on their surfaces.
This brings to mind K2-18b, a mini-Neptune or sub-Neptune exoplanet that may have a deep global ocean beneath a thick hydrogen atmosphere. The Webb space telescope tentatively detected the molecule dimethyl sulfide in the planet’s atmosphere in 2023. It hasn’t been confirmed yet, but observations continue. On Earth, the molecule is produced primarily by marine life such as plankton in the oceans, so it may be a possible biosignature for life on K2-18b. Maybe!
Bottom line: A new study shows there is likely much more water on exoplanets than previously thought. Most of it would be hidden in a planet’s interior.
Source: The interior as the dominant water reservoir in super-Earths and sub-Neptunes
Read more: Hycean planets might be habitable ocean worlds
Read more: Super-Earths may have long-lasting oceans
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