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We know that the Earth has an iron core surrounded by a mantle of silicate bedrock and water (oceans) on its surface. Science has used this simple planet model until today for investigating exoplanets ⁘ planets that orbit another star outside our solar system. ⁘It is only in recent years that we have begun to realise that planets are more complex than we had thought,⁘ says Caroline Dorn, Professor for Exoplanets at ETH Zurich.
Most of the exoplanets known today are located close to their star. This means they primarily comprise hot worlds of oceans of molten magma that have not yet cooled to form a solid mantle of silicate bedrock like the Earth. Water dissolves very well in these magma oceans ⁘ unlike, for instance, carbon dioxide, which quickly outgasses and rises into the atmosphere.
The iron core is located beneath the molten mantle of silicates. So how is the water distributed between the silicates and the iron? This is precisely what Dorn has investigated in collaboration with Haiyang Luo and Jie Deng from Princeton University with the help of model calculations based on fundamental laws of physics. The researchers present their results in the journal Nature Astronomy .
This study was triggered by investigations of the Earth⁘s water content, which yielded a surprising result four years ago: the oceans on the Earth⁘s surface only contain a small fraction of our planet⁘s overall water. The content of more than 80 of the Earth⁘s oceans could be hidden in its interior. This is shown by simulations calculating how water behaves under conditions of the kind that prevailed when the Earth was young. Experiments and seismological measurements are accordingly compatible.
The new findings concerning the distribution of water in planets have dramatic consequences for the interpretation of astronomical observation data. Using their telescopes in space and on the Earth, astronomers can under certain conditions measure the weight and size of an exoplanet. They use these calculations to draw up mass-radius diagrams that permit conclusions to be drawn about the planet⁘s composition. If in doing so ⁘ as has been the case so far ⁘ the solubility and distribution of water are ignored, the volume of water can be dramatically underestimated by up to ten times. ⁘Planets are much more water-abundant than previously assumed,⁘ says Dorn.
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