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They do not look like stone bodies covered by a deep ocean, but rather stone cores "drowned" in the water.
The name of Jules Verne's famous novel, Twenty Thousand Miles Under the Sea, evokes the idea of a bottomless depth. In fact, it is a road topped in the ocean and not of extreme depth.
However, the "thousands of kilometers" of oceans actually exist. And in the cosmos, they are not at all rare.
A study by researchers at Harvard University suggests that a common type of exoplanet – a world two to four times larger than Earth – is sometimes even water.
Research paper published by the scientific journal Proceedings of the National Academy of Sciences.
The most common exoplanets
Astrophysicists have discovered nearly 4,000 exoplanets. In the data of cosmic observations, the largest are the giants of Jupiter and Saturn.
However, the most common size is the one we do not observe in the solar system: most exoplanets reach larger dimensions than Earth but smaller than Neptune (4 times the size of our planet).
A closer look at exoplanets with this "transitional" size indicates that they fall into two categories of dimensions. And it can be two types of very different worlds.
Ocean World or gas dwarf?
On the one hand, the planets are about a third larger than the Earth. It is generally assumed that it's mainly about stone worlds. This was called the super-Earth.
On the other hand, the planets are about two and a half times larger than our planet. Until now, their true nature has not been clearly defined.
Some researchers have considered "the largest stone of the Earth". Others have advocated the idea of oceanic worlds or worlds with extended envelopes of hydrogen and helium in the form of giant giants like Jupiter and Saturn.
Most astronomers and astrophysicists have preferred the latest model of gaseous dwarfs.
The importance of water
These hypotheses were verified by a research team led by Li Zeng of Harvard University. Scientists have explored the possible composition of exoplants 2-4 times larger than the Earth in modeling global development. They used the latest astrophysical observations.
The modeling was based on the representation of raw materials for the creation of planets around different types of stars. These raw materials are mainly interstellar gas, water ice and rocks (including metals).
Astrophysicists have simulated the geological evolution of bodies under the influence of distance from the parent star, various temperature and pressure conditions as well as chemical processes.
Water played a surprisingly important role in the development of planets. And it seems that it constitutes a significant proportion of the worlds reaching two to four times the size of the Earth.
Water and still water
Li Zeng points out that these aquatic worlds do not look like Earth at all.
Although water covers two-thirds of the surface of our planet, it only accounts for about 0.02% of its weight. In the case of space oceanic worlds, it represents at least 25% of the weight. And sometimes more than half.
This means that the oceans of these bodies reach hundreds of thousands of kilometers deep. "They are unimaginable. Dangerous, "said Li Zeng about these oceans.
According to him, the oceanic worlds can not be imagined as stone bodies covered with a deep ocean, but rather as stone cores drowned in water.
Glowing ice
In the depths of the oceanic worlds, Zeng says that there are such extreme pressures that the water is squeezed to look like the Earth's mantle. This produces a high temperature ice reaching the magma.
According to the scientist, such aquatic worlds can be much more numerous than the analogies of the Earth and slightly larger super-lands, mainly formed of rocks.
"Maybe such a world or more could circulate around each Sun-like star," says Zeng.
In the solar system, only relatively small bodies have a composition similar to that of anticipated oceanic worlds. For example, 25% of the water is the Pluto dwarf planet or comet 67P.
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