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Up to 95% of an exoplanet’s water could forever be trapped deep within its iron core, changing what we know about water life and making it more habitable than we realized .
“Planet They are much more abundant in water than previously thought,” said Caroline Dorn, professor at exoplanet at ETH Zurich in Switzerland, in a statement.
When planets born Through accretion of debris and sustained collisions with other protoplanets, they become so hot that their entire surface is covered by an ocean of molten rock. This magma eventually cools to form a silicate-rich mantle and solid crust above a deep core of molten iron that accumulates over time as heavier materials move to the center of the planet.
Among the materials from which planets are built is water, and in the early days of the planet, water is present and dissolved in the magma ocean. Previous research has found that young planets of the same type size and mass for Earth — and therefore with moderate internal pressures and temperatures — they are able to draw the water dissolved in the magma down towards their core. In fact, one study found that World in which at least many times more water in its interior than it does on the surface in the form of our familiar ocean.
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“A large proportion of the iron is initially in the hot magma soup in the form of droplets,” said Dorn. The water in the magma can combine with these iron droplets as they move towards the core. “The behavior of the iron droplets as if a raft were being carried down by the water.”
This is fine for Earth-sized planets, but many of the rocky exoplanets that astronomers are discovering are much larger than Earth. The so-called super-world a mass of up to 10 times that of our planet is possible, but it is not clear whether such life, with its harsher internal conditions, could draw down water from the magma ocean as it did on Earth.
Using computer modeling to understand how water interacts with the molten magma surface of a young, hot, rocky planet, Dorn, along with researchers Haiyang Luo and Jie Deng of Princeton University, have pursued that question to answer now, discovering that even on a super-Earth, it is possible to have a lot of planetary water inside it.
“The bigger the planet and the greater its mass, the more water goes with the iron droplets and is integrated into the core,” said Dorn. “In certain circumstances, iron can absorb up to 70 times more water than silicates. However, due to the enormous pressure at the core, the water is no longer in the form of H20 molecules, but it is present in hydrogen and oxygen.”
This water is so deep that it is trapped in the core forever, with no way to get out, so it has no use for life on or near the surface of a planet. However, it may help with habituation in other ways.
By measuring the mass and radius of an exoplanet — using Doppler measurements of radial velocity and transitrespectively — we can calculate the density of these worlds (by dividing the planet’s mass by its volume, which is calculated from its radius). Some exoplanets have densities that suggest a substantial fraction, perhaps up to a quarter of their mass, is made of water.
The assumption was that this water was present on the surface in the form of an ocean ten kilometers deep, but if this new research is correct, then in fact most of the water would be inside the planet, and water-world with. deep global oceans may be rare. Although water is essential for life, a planet with only water on the surface (and no land) may not be habitable. For example, nutrients to feed life wash into the sea from land, and the same runoff process is a critical part of the carbon cycle that sustains planetary climates over long periods of time.
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Supposedly “Hycean” The worlds — named after a portmanteau of hydrogen and oceans — deserve further investigation, according to Dorn, to test the theory that water is brought inside planets. Planets have hydrogen-rich atmospheres, but are also thought to be in there host deep oceans in habitable temperatures.
When a planet’s mantle is cooling, and before the oceans form, some of the water dissolved in its rock can degas and rise to the surface, where it can be released into the atmosphere.
“So if we find water in a planet’s atmosphere, there’s probably a lot more inside,” said Dorn.
In particular, the exoplanet TOI-270dits orbits red ox a star 73 light years from Earth and with a mass 4.78 times that of our planet, is of interest to Dorn, who was part of a team that studied its atmosphere with the James Webb Space Telescope and found methane, carbon dioxide and water vapor.
“There is accumulating evidence that such interactions exist [of water] between the magma ocean in its interior and the atmosphere,” said Dorn.
Because of the ability of water to drain into a planet rather than pooling to great depth on its surface, the potential for habitable planets with shallower oceans out there in the galaxy is even greater.
The research was published today (20 August) in the journal Natural Astronomy.
