Artist’s depiction of a water world.
Image: NASA/JPL-Caltech

Scientists have good reason to believe that so-called water worlds—exoplanets with surfaces covered entirely by a single gigantic ocean—are common in the galaxy. But a new computer simulation suggests that not only are water worlds prevalent, they’re also teeming with water—and at mind-boggling scales. Imagine oceans that are hundreds, and even thousands, of kilometers deep.

New research published today in Proceedings of the National Academy of Sciences bolsters the growing case that water worlds are a common feature of the Milky Way. Using computer simulations, Harvard University astronomer Li Zeng and his colleagues presented new data showing that sub-Neptune-sized planets, that is, planets featuring radii about two to four times that of Earth, are likely to be water worlds, and not gas dwarfs surrounded by thick atmospheres as conventionally believed.

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To be clear, water worlds, also known as ocean worlds, are still hypothetical. Unless, of course, we include Jupiter’s moon Europa and Saturn’s moon Enceladus, both of which are presumed to have global oceans wrapped in an icy crust. Planetary formation models suggest water worlds are real, however, and likely very common. Research from 2017, for example, suggested most habitable Earth-like planets may in fact be water worlds.

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For the new study, Li’s Team sought to refine our planetary formation models even further. Observations made by Kepler and other observatories have allowed astronomers to identify thousands of exoplanets, many of which are located in close proximity to their host stars (by close, we’re talking distances even closer than Mercury is to our Sun). This data is pointing to two dominant types of exoplanets ranging between one and four times the size of Earth: dense rocky worlds (so-called super-Earths) or intermediate-sized planets with relatively low densities. It’s this latter category that’s of interest in the new study, as scientists aren’t sure if these exoplanets feature a rocky core surrounded by a thick hydrogen-rich atmosphere (i.e. a gas dwarf) or if they contain a significant amount of water, either ice or liquid, or a combination of the two (i.e. water worlds).

Conventional thinking suggests they’re gas dwarfs, as water worlds can only form beyond a planetary system’s “snow line” (the distance from the host star where it’s cold enough for volatile compounds to form solid ice grains). The new computer models, however, suggested these sub-Neptunes should feature very modest atmospheres in terms of size—certainly nothing on the scale of a gas dwarf. Meanwhile, simulations of planetary growth and development pointed to many intermediate-sized planets as being water worlds.

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This finding is subsequently bolstering another emerging theory, that of planet migration. Because water worlds can only form in the outer reaches of a star system, and because so many sub-Neptunes exist in close proximity to their host stars, this research is providing theoretical evidence that planets—including water worlds—slowly drift inward over time.

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Sub-Neptunian water worlds, as the new research found, are likely to be exceptionally wet. At least 25 percent of the total mass of these planets would be comprised of water-dominated ices and fluids. Some of these planets could even be comprised of 50 percent water. These water worlds, therefore, aren’t just planets without terrestrial surface features—they’re basically water-logged orbs with bits of rock and metal thrown in for good measure. By comparison, Earth is mostly rocky with a water content around 0.025 percent of its total mass, and an atmospheric water content comprising just one-millionth of its total mass.

Sub-Neptunian water worlds are “not only submerged,” explained Li in an email to Gizmodo, they have ocean depths exceeding “hundreds or thousands of kilometers” deep, “instead of a few kilometers deep as Earth’s oceans.” To which he added a few apt descriptors: “Unfathomable. Bottomless. Very Deep.”

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Or at least, that’s what the computer simulations suggested. The models used in the study simulated planet formation processes, as influenced by the abundance of nebular gasses, water-rich ices, various rocky materials consisting primarily of iron and nickel, and as influenced by complex chemical process driven by temperature, cooling rates, evaporation, condensation, density, and distance to host star, among many other factors.

Looking at the simulations, Li was impressed by the amount of water that seems to be in the galaxy and its prominence during the planet-formation stage.

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“Statistically speaking, these water worlds may be more abundant than Earth-like rocky planets,” Li told Gizmodo. “Perhaps every typical Sun-like star has one or more of these water worlds [and maybe] our Solar System is less typical. Generally speaking, this type of planetary system architecture with close-in rocky super-Earths and water-rich sub-Neptunes may be more common in the Milky Way than our type of solar system,” he said.

Some of these planets, he said, have oceans deep enough to exert pressures equivalent to a million times our atmospheric surface pressure. Under those conditions, fluid water gets compressed into high-pressure phases of ice, such as Ice Seven or superionic ices, he said.

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“These high-pressure ices are essentially like silicate-rocks within Earth’s deep mantle—they’re hot and hard,” he said. “These are utterly different worlds compared to our own Earth.”

Our planet has an obvious surface, but sub-Neptunian water worlds, not so much. With water compositions ranging between 25 to 50 percent of the planet’s total mass, these objects would be completely water-logged. They “may or may not have a well-defined surface,” said Li, and they “could be fluid all the way down—all the way down, to great depth.”

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Sean Raymond, an astronomer from the University of Bordeaux who wasn’t involved with the new study, said the paper is sound.

“Its conclusions are statistical, meaning that the authors are not pointing to specific planets and claiming them to be water worlds but rather focusing on the population as a whole,” Raymond explained to Gizmodo in an email. “Still, it’s a cool paper and a provocative result.”

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Raymond was particularly stoked by how the study added further credence to the planetary migration hypothesis.

“A water world close to its star must have formed much farther away and then moved closer as its orbit shrank. The planet’s composition was set when it was farther away on a colder orbit,” he said. “The process of orbital shrinking is called ‘migration’ and it is driven by the gravity of the disk of gas from which the planets formed. If water worlds are common that provides a really strong confirmation that migration really does happen and is a key process in how planets form—both around other stars and in our own Solar System.”

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Raymond also found it interesting that the water content of 25 percent is about the same as for outer Solar System objects like Pluto and comet 67P. That “fits the migration story, that these water worlds may originally have been born far from their stars before migrating inward,” he said.

“I like simulations like this,” Anders Sandberg, a senior research fellow at Oxford University’s Future of Humanity Institute, also not involved with the new study, wrote in an email to Gizmodo. “The really cool thing is that this paper in a sense is a bet on what we will discover in the next few years: actual data are amassing at a wonderful rate that will allow us to check whether the prediction is true.”

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Indeed, a nice aspect of this paper is that the findings are telling us what we should be looking for once we get access to the next generation of space telescopes, including the James Webb Telescope and the ESA’s Ariel space telescope. Sandberg also said the new finding has implications in the search for alien life.

“While water worlds are not entirely perfect for life since the heavier elements may be buried under hundreds of kilometers of high-pressure ice, they are likely far better than gas dwarfs,” he said.

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As to whether or not a water-soaked planet could foster to an intelligent, space-faring species, that’s another issue entirely, and certainly a question worth thinking about given the apparent abundance of these planets in the galaxy.