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Scientists have reason to believe that the so-called aquatic worlds – exoplanets whose surfaces are completely covered by a single gigantic ocean – are common in the galaxy. However, a new computer simulation suggests that not only are aquatic worlds widespread, but that they are also full of water – and at staggering scales. Imagine oceans hundreds, even thousands of miles deep.
New research published today in the Proceedings of the National Academy of Sciences reinforces the growing thesis that aquatic worlds are a common feature of the Milky Way. With the help of computer simulations, astronomer Li Zeng of Harvard University and his colleagues presented new data showing that planets smaller than Neptune, it is- that is, planets whose radius is about two to four times that of the Earth, are probably aquatic worlds and not gaseous dwarfs. surrounded by thick atmospheres, as it was usually thought.
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To be clear, the aquatic worlds, also called oceanic worlds, are still hypothetical. Unless, of course, we include the moon of Jupiter, Europa and Saturn, Enceladus, who are supposed to have global oceans wrapped in an icy crust. Global formation models suggest that aquatic worlds are real and probably very common. Research conducted in 2017, for example, suggested that most terrestrial habitable planets could be aquatic worlds.
For this new study, Li's team sought to further refine our global training models. Observations made by Kepler and other observatories have allowed astronomers to identify thousands of exoplanets, many of which are located near their host stars (closely, we are talking about distances even closer than Mercury to our Sun) . These data indicate two types of dominant exoplanets ranging from one to four times the size of the Earth: dense rocky worlds (called super-Earths) or planets of medium size and relatively low density. This is the last category that interests the new study because scientists do not know if these exoplanets possess a rocky core surrounded by a thick atmosphere rich in hydrogen (ie a dwarf of gas) or if they contain a significant amount of water, either ice or liquid, or a combination of both (ie, aquatic worlds).
Conventional thinking suggests that they are gas dwarves, because aquatic worlds can only form beyond the "snow line" of a planetary system (distance between the host star and the host star). where it is cold enough for volatile compounds to form solid ice grains). The new computer models, however, suggested that these sub-Neptunes should present a very modest atmosphere in terms of size – certainly nothing on the scale of a gaseous dwarf. At the same time, simulations of planetary growth and development have shown that many mid-sized planets are aquatic worlds.
This discovery later reinforces another emerging theory, that of the migration of the planet. Because aquatic worlds can only be formed outside a star system and so many sub-Neptunes exist in close proximity to their host stars, this research provides theoretical evidence that planets, including worlds, aquatic, slowly move inward.
"Unfathomable, bottomless, very deep."
New research has revealed that the sub-Neptunian aquatic worlds are probably extremely wet. At least 25% of the total mass of these planets is composed of ice and water-dominated fluids. Some of these planets could even be composed of 50% water. Therefore, these aquatic worlds are not just planets devoid of terrestrial surface elements – they are actually water-filled orbs with pieces of rock and metal projected for good measure. By way of comparison, the Earth is mainly rocky with a water content of about 0.025% of its total mass and an atmospheric water content representing only one millionth of its total mass.
The sub-Neptunian aquatic worlds are not only "submerged," Li explained in an email to Gizmodo. They have oceanic depths exceeding "hundreds or thousands of kilometers", "instead of a few kilometers like the oceans of the Earth". some apt descriptors: "Unfathomable. Bottomless. Very deep."
Or at least that's what computer simulations have suggested. The models used in the study simulated the formation processes of planets, influenced by the abundance of nebular gases, water-rich ice, various rock materials consisting mainly of iron and nickel, and influenced by complex chemical processes driven by temperature, cooling rates, and evaporation. , condensation, density and distance to the host host, among other factors.
Looking at the simulations, Li was impressed by the amount of water that appears to be in the galaxy and by its importance during the formation phase of the planet.
"Statistically, these aquatic worlds could be more abundant than rocky planets similar to the Earth," Li told Gizmodo. "Maybe each typical Sun-like star has one or more of these aquatic worlds [and maybe] our solar system is less typical. In general, this type of planetary system architecture including very close super-rocky lands and water-rich sub-Neptunes may be more common to the Milky Way than our type of solar system ", did he declare.
Some of these planets, he said, have oceans deep enough to exert pressures equivalent to one million times our atmospheric pressure at the surface. Under these conditions, the fluid water is compressed into ice phases under high pressure, such as Ice Seven or superionic ice cream, he says.
"These high-pressure ice sheets are essentially like silicate rocks in the deep mantle of the Earth, they are hot and hard," he said. "These are totally different worlds from our own Earth."
Our planet has an obvious surface, but the sub-Neptunian aquatic worlds, not so much. With water compositions between 25 and 50% of the total mass of the planet, these objects would be completely waterlogged. They "may or may not have a well-defined surface," said Li, and they "could be fluid all the way through – up to the greatest depth".
Sean Raymond, an astronomer from the University of Bordeaux who did not participate in the new study, said the document was solid.
"His conclusions are statistical, which means that the authors do not designate specific planets and claim that they are aquatic worlds, but rather focus on the population as a whole," explained Raymond in an email to Gizmodo. "Still, it's a cool paper and a provocative result."
Raymond was particularly impressed by the way in which the study reinforced the credibility of the global migration hypothesis.
"An aquatic world close to its star must be formed much further and then be closer as its orbit contracts." The composition of the planet was defined when it was farther away in a colder orbit, "he said. "The process of orbital shrinkage is called" migration "and is motivated by the severity of the gas disk from which the planets were formed. If aquatic worlds are common, they provide a very strong confirmation that migration really takes place and is a key process in the formation of planets, both around other stars and in our own solar system. "
Raymond also found it interesting that the water content of 25% is about the same as that of external solar system objects such as Pluto and comet 67P. This "corresponds to the history of migration, namely that these aquatic worlds may well be born far from their stars before migrating inland," he said.
"I like simulations like this," wrote in an email to Gizmodo Anders Sandberg, principal investigator at the Future of Humanity Institute at Oxford University, who was not involved not to the new study either. "What's really great is that this document is somehow a bet on what we will discover over the next few years: the actual data is accumulating at a tremendous rate, which will allow us to check whether the prediction is true. "
Indeed, an interesting aspect of this article is that the results tell us what we should look for once we have access to the next generation of space telescopes, including the James Webb telescope and the ESA Ariel Space Telescope. Sandberg also said that the new discovery had implications for the search for extraterrestrial life.
"The water worlds are not perfect for life because the heaviest elements can be buried under hundreds of kilometers of high pressure ice, but they are probably much better than the gas dwarves," he said. he declares.
It remains to be seen whether or not a water-soaked planet could become an intelligent species able to move in space. This is a completely different problem, which certainly deserves to be thought about given the apparent abundance of these planets in the galaxy.
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