Water is a key component of exoplanets, and its distribution – on the surface or deep inside – has a fundamental impact on the planet’s properties. A new study suggests that for Earth-sized planets and planets with more than six times Earth’s mass, the majority of water resides deep within the planet’s core.
Most of the water isn’t stored on the surface of exoplanets, but deep within their cores and mantles. Image courtesy of Sci.News.
“Most of the exoplanets known to date are located close to their stars,” said Professor Caroline Dohn of ETH Zurich.
“That means they consist mainly of hot worlds with oceans of molten magma that haven’t yet cooled enough to form a solid mantle of silicate rock like Earth’s.”
“Water is very soluble in these magma oceans, unlike, say, carbon dioxide, which quickly outgasssssssssssss and rises into the atmosphere.”
“The iron core is beneath a molten silicate mantle. So how does water partition between the silicates and the iron?”
“It takes time for the iron core to form. Most of the iron is initially contained in the hot magma soup in the form of droplets.”
“The water trapped in this soup binds to these iron droplets and together they sink to the center. The iron droplets act like a lift force, being carried downward by the water.”
Until now, such phenomena were known to occur only under moderate pressures, which also exist on Earth.
It was not known what would happen on larger planets with higher internal pressures.
“This is one of the key findings of our study,” Professor Dorn said.
“The larger and more massive the planet, the more likely the water is to be integrated into the core, together with the iron droplets.”
“Under certain circumstances, iron can absorb up to 70 times more water than silicates.”
“But because of the enormous pressure at the core, the water no longer exists in the form of water molecules, but in the form of hydrogen and oxygen.”
The research was sparked by an investigation into the Earth’s water content, which four years ago led to a startling result: the Earth’s surface oceans contain only a tiny fraction of the planet’s total water.
More than 80 of Earth’s oceans may be hidden within it.
This is shown by simulations that calculate how water would have behaved under conditions when the Earth was young, so experiments and seismological measurements are compatible.
New discoveries about the distribution of water within planets will have a dramatic impact on the interpretation of astronomical observational data.
Astronomers can use telescopes in space and on Earth to measure the weight and size of exoplanets under certain conditions.
They use these calculations to create mass-radius diagrams that allow them to draw conclusions about the planet’s composition.
“Ignoring water solubility and distribution, as has been done in the past, can lead to a massive underestimation of the water volume, by up to a factor of ten,” Prof Doern said.
“There’s a lot more water on the planet than we previously thought.”
The distribution of water is also important if we want to understand how planets form and develop: any water that sinks to the core will remain trapped there forever.
However, dissolved water in the mantle’s magma ocean can degas and rise to the surface as the mantle cools.
“So if we find water in a planet’s atmosphere, there’s probably even more water in its interior,” Prof Dorn said.
Water is one of the prerequisites for life to develop, and there has long been speculation as to whether water-rich super-Earths could support life.
Calculations have since suggested that too much water could be detrimental to life, arguing that on such a watery world, an alien layer of high-pressure ice would prevent vital exchange of materials at the interface between the ocean and the planet’s mantle.
Current research has come to a different conclusion: Most of the water on super-Earths is locked away in their cores, rather than on their surfaces as previously assumed, so planets with deep aqueous layers are probably rare.
This has led astronomers to speculate that planets with relatively high water content could potentially form habitable environments like Earth.
“Their study sheds new light on the possibility that worlds rich enough in water to support life may exist,” the authors said.
of study Published in the journal Natural Astronomy.
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H. Luo othersThe interior as the main water reservoir of Super-Earths and Sub-Neptunes. Nat AstronPublished online August 20, 2024; doi: 10.1038/s41550-024-02347-z
Source: www.sci.news
