Aquatic Planets

In a study published in the journal Science Advances, a team of European astronomers shows that water can be delivered to a terrestrial planet in the form of ‘pebble snow’ in the early phases of the planet’s growth.

An artist’s impression of a water-world exoplanet. Image credit:

An artist’s impression of a water-world exoplanet. Image credit:

“All our data suggest that water was part of Earth’s building blocks, right from the beginning,” said Professor Anders Johansen, a researcher in the Centre for Star and Planet Formation at the University of Copenhagen and Lund Observatory.

“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.”

Using a computer model, Professor Johansen and colleagues calculated how quickly planets are formed, and from which building blocks.

Their results indicate that 4.5 billion years ago, millimeter-sized dust particles of ice and carbon accreted in the formation of what would later become Earth.

“Up to the point where Earth had grown to 1% of its current mass, our planet grew by capturing masses of pebbles filled with ice and carbon,” Professor Johansen said.

“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% of the planet is made up of water, even though 70% of Earth’s surface is covered by water.”

The team’s pebble accretion theory is that planets are formed by pebbles that are clumping together, and that the planets then grow larger and larger.

“The water molecule is found everywhere in our Galaxy, and that the theory therefore opens up the possibility that other planets may have been formed in the same way as Earth, Mars and Venus,” Professor Johansen said.

“All planets in the Milky Way may be formed by the same building blocks, meaning that planets with the same amount of water and carbon as Earth — and thus potential places where life may be present — occur frequently around other stars in our Galaxy, provided the temperature is right.”

If planets in our Galaxy had the same building blocks and the same temperature conditions as Earth, there will also be good chances that they may have about the same amount of water and continents as our planet.

“With our model, all planets get the same amount of water, and this suggests that other planets may have not just the same amount of water and oceans, but also the same amount of continents as here on Earth. It provides good opportunities for the emergence of life,” said Professor Martin Bizzarro, a researcher in the Centre for Star and Planet Formation at the University of Copenhagen.

“If, on the other hand, it was random how much water was present on planets, the planets might look vastly different. Some planets would be too dry to develop life, while others would be completely covered by water.”


Anders Johansen et al. 2021. A pebble accretion model for the formation of the terrestrial planets in the Solar System. Science Advances 7 (8): eabc0444; doi: 10.1126/sciadv.abc0444

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