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Could the Universe Be Swimming in Watery Planets?

A new hypothesis of planet formation means that watery worlds may be common rather than rare

As the Mars Rover Perseverance motors around looking for evidence of past life on a now- mostly dry planet, some researchers are asking, can we be sure that most planets in our galaxy are dry?

A common assumption among exoplanet experts is that most planets got their water via a chance hit early on from an icy asteroid. But researchers from the GLOBE Institute at the University of Copenhagen offer an alternative scenario, based on the millimetre-sized particles of ice and carbon that orbit all the young stars in our Milky Way galaxy.

If masses of these particles are incorporated into a planet from its beginning, it isn’t a matter of chance whether the planet has water. It is a matter of chance if it loses water, which seems to have happened, for example, to Mars. They explain,

“All our data suggest that water was part of Earth’s building blocks, right from the beginning. And because the water molecule is frequently occurring, there is a reasonable probability that it applies to all planets in the Milky Way. The decisive point for whether liquid water is present is the distance of the planet from its star,” says Professor Anders Johansen from the Centre for Star and Planet Formation who has led the study that is published in the journal Science Advances.

Using a computer model, Anders Johansen and his team have calculated how quickly planets are formed, and from which building blocks. The study indicates that it was millimetre-sized dust particles of ice and carbon — which are known to orbit around all young stars in the Milky Way — that 4.5 billion years ago accreted in the formation of what would later become Earth.

“Up to the point where Earth had grown to one percent of its current mass, our planet grew by capturing masses of pebbles filled with ice and carbon. Earth then grew faster and faster until, after five million years, it became as large as we know it today. Along the way, the temperature on the surface rose sharply, causing the ice in the pebbles to evaporate on the way down to the surface so that, today, only 0.1 percent of the planet is made up of water, even though 70 percent of Earth’s surface is covered by water,” says Anders Johansen, who together with his research team in Lund ten years ago put forward the theory that the new study now confirms.

University of Copenhagen – The Faculty of Health and Medical Sciences, “The Milky Way may be swarming with planets with oceans and continents like here on Earth” at ScienceDaily (February 22, 2021) The paper is open access.

Because water is found all over our galaxy, the researchers suggest, it’s quite possible that most planets in our galaxy formed the way Venus, Earth, and Mars did roughly 4.5 billion years ago. In that case, many might be watery like Earth. Venus’s water was stripped by solar winds. Mars’s water may be trapped in minerals or may have escaped into space after Mars lost its magnetic field. Earth retained the magnetic field that keeps the water in. But many other planets in the galaxy may have retained it too.

In any event, a planet may have a great deal of water under the surface that wasn’t just stripped away). One hypothesis about extraterrestrial life — and even intelligent extraterrestrials — is that we might find them living in the oceans under the surfaces of planets and moons. Jupiter’s moons Europa and Ganymede are both assumed to have a huge amount of subsurface water. The same might well be true of many exoplanets and exomoons. Even Mars may have subsurface water:

Some researchers believe that intelligent life is unlikely in these water worlds because aquatic life forms might never, for example, seek to use technology. Think, for example, of dolphins and octopuses. However, others dispute that point. Only further research into exoplanets could tell us.

Which brings us back to Perseverance again:

Here are some sound recordings from Mars.

Note: The Earth-like exoplanet illustration above is a stock image with elements furnished by NASA, © sdecoret / stock.adobe.com


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We won’t find ET on ocean planets, researchers say. We will see few extraterrestrials if a great many promising exoplanets are Waterworlds. Prominent astrobiologists doubt that intelligent life could develop in an entirely aquatic environment. But are they right?


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Could the Universe Be Swimming in Watery Planets?