Waterful Worlds: More planets able to sustain life than thought

By Stephen Beech via SWNS

More planets may be capable of sustaining life than previously thought as they contain water, according to new research.

Scientists have long known that Earth has an iron core surrounded by a mantle of silicate bedrock with water in the form of oceans on the surface.

They have used that simple planet model until now for investigating exoplanets – planets that orbit another star outside our solar system.

Study author Caroline Dorn, Professor for Exoplanets at ETH Zurich in Switzerland, said: “It is only in recent years that we have begun to realize that planets are more complex than we had thought."

She explained that water is one of the preconditions for life to develop, and there has long been speculation about the potential habitability of water-abundant "Super-Earths" – planets with a mass multiple times greater than the Earth and with a surface covered by a deep, global ocean.

But calculations then suggested that too much water could be hostile to life.

Dorn says the argument was that on such water worlds, a layer of exotic high-pressure ice would prevent the exchange of vital substances at the interface between the ocean and the planet’s mantle.

But the new study has instead concluded that planets with deep water layers are likely to be a rare occurrence as most of the water on Super-Earths is not located on the surface, as has been previously assumed but is instead trapped within the core.

That has led scientists to assume that even planets with a relatively high water content could have "potential" to develop Earth-like habitable conditions.

Dorn and her colleagues say their study, published in the journal Nature Astronomy , casts a new light on the potential existence of water-abundant worlds that could support life.

She worked with colleagues from Princeton University with the help of model calculations based on fundamental laws of physics.

Dorn said: “The iron core takes time to develop.

"A large share of the iron is initially contained in the hot magma soup in the form of droplets.”

She says the water sequestered in the soup combines with the iron droplets and sinks with them to the core.

Dorn said: “The iron droplets behave like a lift that is conveyed downwards by the water.”

Until now that behavior had only been known to be the case for moderate pressures of the sort that also prevail in the Earth.

It was not known what happens in the case of bigger planets with higher-pressure interior conditions.

Dorn said: “This is one of the key results of our study.

“The larger the planet and the greater its mass, the more the water tends to go with the iron droplets and become integrated in the core.

"Under certain circumstances, iron can absorb up to 70 times more water than silicates.

"However, owing to the enormous pressure at the core, the water no longer takes the form of H2O molecules but is present in hydrogen and oxygen."

This study was triggered by an analysis of the Earth’s water content which, surprisingly, found four years ago that 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.

Dorn says that was shown by simulations calculating how water behaves under conditions of the kind that prevailed when the Earth was young.

She said the new findings concerning the distribution of water in planets have "dramatic consequences" for the interpretation of astronomical observation data.

Using telescopes in space and on Earth, astronomers can under certain conditions measure the weight and size of an exoplanet.

They use those 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.

Dorn said: “Planets are much more water-abundant than previously assumed.

“So if we find water in a planet’s atmosphere, there is probably a great deal more in its interior.”

The James Webb Space Telescope , which for two years has been sending data from space to Earth, is seeking to find evidence.

Dorn explained that it is capable of tracking down molecules in the atmosphere of exoplanets.

She said: “Only the composition of the upper atmosphere of exoplanets can be measured directly.

“Our group wishes to make the connection from the atmosphere to the inner depths of celestial bodies.”

Dorn says new data regarding the exoplanet called TOI-270d is particularly interesting.

She added: “Evidence has been collected there of the actual existence of such interactions between the magma ocean in its interior and the atmosphere."

Dorn's list of interesting objects that she wishes to examine more closely also includes the planet K2-18b, which previously hit the headlines because of the probability of there being life on it.

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