“Existing for Billions of Years” –Exoplanets Engulfed by Magma Oceans May Evolve Water-Rich Atmospheres

Exoplanet Wasp 121b



“Liquid magma is actually quite runny,” said University of Chicago planetary scientist, Edwin Kite, about a new study by University of Chicago and Stanford University researchers suggesting a mechanism where planets could develop atmospheres full of water vapor similar to what makes life on Earth’s surface possible, regulating our climate and sheltering us from damaging cosmic rays.

Magma Oceans Suck Hydrogen  Out of the Atmosphere

“So it also turns over vigorously, just like oceans on Earth do,” Kite added. “There’s a good chance these magma oceans are sucking hydrogen out of the atmosphere and reacting to form water. Some of that water escapes to the atmosphere, but much more gets slurped up into the magma. Then, after the nearby star strips away the hydrogen atmosphere, the water gets pulled out into the atmosphere instead in the form of water vapor. Eventually, the planet is left with a water-dominated atmosphere. This stage could persist on some planets for billions of years.”

“Should Work as Well or Better in the Habitable Zone”

“Our study only looked at worlds too hot for life. However, the proposed mechanism should work as well or better in the habitable zone, Kite wrote in an email to the Daily Galaxy. “Therefore,’ he added, “habitable-zone oceans that have H2O with nebula-sourced hydrogen should be common.”

“Kepler’s Sweet Spot” –Earth at Center of Milky Way’s ‘Habitable Zone’

Published March 15 in the Astrophysical Journal Letters, the research expands our picture of planetary formation and could help direct the search for habitable worlds in other star systems.. Although telescopes have counted a growing number of rocky planets, scientists had thought most of their atmospheres long lost.

Could Help Direct the Search for Habitable Worlds

“Our model is saying that these hot, rocky exoplanets should have a water-dominated atmosphere at some stage, and for some planets, it may be quite a long time,” said Kite, an expert in how planetary atmospheres evolve over time.

“Superhabitable” –Exoplanets With Lifespans Up to 70 Billion Years

Generally, telescopes can tell you about an exoplanet’s physical size, its proximity to its star and if you’re lucky, how much mass it has, reports the University of Chicago. “To go much further, scientists have to extrapolate based on what we know about Earth and the other planets in our own solar system. But the most abundant planets don’t seem to be similar to the ones we see around us.”

A Surprise –Planets Smaller than Neptune 

“What we already knew from the Kepler mission is that planets a little smaller than Neptune are really abundant, which was a surprise because there are none in our solar system,” Kite said. “We don’t know for sure what they are made of, but there’s strong evidence they are magma balls cloaked in a hydrogen atmosphere.”

There’s also a healthy number of smaller rocky planets that are similar, but without the hydrogen cloaks. So scientists surmised that many planets probably start out like those larger planets that have atmospheres made out of hydrogen, but lose their atmospheres when the nearby star ignites and blows away the hydrogen.

“Then, after the nearby star strips away the hydrogen atmosphere, the water gets pulled out into the atmosphere instead in the form of water vapor. Eventually, the planet is left with a water-dominated atmosphere that could persist on some planets for billions of years,” Kite said.

JWST –100 Times More Powerful than the Hubble

The James Webb Space Telescope, with its unprecedented infrared sensitivity a 100 times more powerful than the Hubble, is scheduled to launch later this year; it will be able to conduct measurements of the composition of an exoplanet’s atmosphere. If it detects planets with water in their atmospheres, that would be one signal.

Another way to test is to look for indirect signs of atmospheres. Most of these planets are tidally locked; unlike Earth, they don’t spin as they move around their sun, so one side is always hot and the other cold. A pair of U of Chicago alumni astronomers have suggested a way to use this phenomenon to check for an atmosphere.

Scientists Laura Kreidberg, who focuses on atmosphere characterization, with the goal of turning these distant planets into real places – revealing what they’re like and how they came to be, and Daniel Koll, who studies the atmospheres of exoplanets and ‘exotic’ past climates on Earth -now at the Max Planck Institute for Astronomy and MIT, respectively–pointed out that an atmosphere would moderate the temperature for the planet, so there wouldn’t be a sharp difference between the day sides and night sides. If a telescope can measure how strongly the day side glows, it should be able to tell whether there’s an atmosphere redistributing heat.

Source: “Water on hot rocky exoplanets.” Kite and Schaefer, Astrophysical Journal Letters, March 11, 2021.

Maxwell Moe, NASA Einstein Fellow, University of Arizona, via University of Chicago

Image at top of the page: An artist’s illustration shows the exoplanet WASP-121b, which appears to have water in its atmosphere. Scientists led by the University of Exeter and including researchers from PlanetS* found unmistakable evidence for a stratosphere on the enormous exoplanet with an atmosphere hot enough to boil iron. They made the new discovery by observing glowing water molecules in the atmosphere  with NASA’s Hubble Space Telescope.


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