Planetary Formation

Planets Might Protect their Water Until their Star Settles Down

Creating rocky planets is a messy, dangerous, hot business. Planetesimals accrete together, which creates heat and pressure on the newborn world. The nearby adolescent star bombards them with intense radiation. That likely “bakes off” any surface oceans, lakes, or rivers, which is a disaster if you’re looking for places where life might arise or exist. That’s because life needs water and planets around these stars are among the most likely to harbor life. But, that doesn’t look too hopeful if the radiation steams the water away.

Scientists at the University of Cambridge in the UK created a complex model that describes a world with most of its water locked deep below the surface, not in pools or oceans, but in rocks. Technically, it’s trapped in minerals deep beneath the surface. If conditions are right on worlds around these most common stars in the Galaxy, there could be enough water in them to equal several Earth oceans.

Clare Guimond, a Ph.D. student at Cambridge, along with two other researchers, came up with the model, which describes newborns around M-type worlds orbiting red dwarf stars. “We wanted to investigate whether these planets, after such a tumultuous upbringing, could rehabilitate themselves and go on to host surface water,” she said. Her team’s work shows that these planets could be a very good way to replace liquid surface water chased off in the host star’s early life. “The model gives us an upper limit on how much water a planet could carry at depth, based on these minerals and their ability to take water into their structure.”

Sequestering Water on a Forming World

M-type red dwarfs are the most common stars in the Galaxy. That makes them good subjects to study the variables of planetary formation. They form just as other stars do. Once past infancy, they also tend to be outbursty and temperamental, just like other stars. However, they stay colicky much longer than other stars. That doesn’t bode well for the surfaces of any planets (or protoplanets) nearby. If it isn’t baked away, the water migrates underground. But, would it happen with every rocky planet? What size world does it take to do this?

The team found that a planet’s size and amount of water-bearing minerals determine how much water it can “hide.” Most ends up in the upper mantle. That rocky layer lies directly below the crust. It’s usually rich in so-called “anhydrous minerals.” Volcanoes feed from this layer, and their eruptions can eventually bring steam and vapor back to the surface through eruptions.

Olivine is a mineral found in Earth’s crust and, under pressure, is transformed into the anhydrous minerals wadsleyite and ringwoodite. Such minerals can store water deep beneath the surface of a planet. Image Credit: Tom Trower

The new research showed that larger planets — around two to three times bigger than Earth — typically have drier rocky mantles. That’s because the water-rich upper mantle makes up a smaller proportion of its total mass.

Hidden Water and Planetary Science

This new model helps planetary scientists understand not only the conditions at Earth’s birth but the water-rich objects that accrete to form planets. However, it’s really more aimed at the formation environment of larger rocky planets around M-type red dwarfs. Thanks to their star’s storm adolescence, those worlds likely experienced chaotic climate conditions for long periods. Those could have worked to send liquid water deep underground. Once their stars settled down, the water could emerge in various ways.

The model could also explain how early Venus could have transitioned from being a barren hellscape to an aqua world. The question of Venus’s water is still hotly debated, of course. However, if it had liquid pools and oceans four billion years ago, how did they happen? “If that [happened] Venus must have found a way to cool itself and regain surface water after being born around a fiery Sun,” said Guimond’s research partner Oliver Shorttle. “It’s possible that it tapped into its interior water in order to do this.”

Science teams have identified clay-type minerals on the asteroid Bennu. Water from such objects is contributed to larger worlds during the process of accretion. Courtesy NASA/OSIRIS-REx mission.

Implications for Exoplanet Searches

Finally, the current research may give new guidelines in the search for habitable exoplanets in the rest of the Galaxy. “This could help refine our triaging of which planets to study first,” said Shorttle. “When we’re looking for the planets that can best hold water you probably do not want one significantly more massive or wildly smaller than Earth.”

The factors in Guimond’s model also have implications for the formation and mineralogy of rocky planets. More specifically, it can explain what’s stored inside a planet, particularly between the surface and the mantle. Future research will likely look at the habitability and climates of both rocky and surface water-rich worlds.

For More Information

Scientists have New Tool to Estimate How Much Water Might be Hidden Beneath a Planet’s Surface

Mantle Mineralogy Limits to Rocky Planet Water Inventories

Carolyn Collins Petersen

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