One study suggests that exoplanets close to their stars can, in fact, hold a thick atmosphere full of water. Above an artist’s illustration of the exoplanet WASP-121b, which appears to have water in its atmosphere. Credit: Engine House VFX, At-Bristol Science Center, University of Exeter
University of Chicago study finds a way in which hot, rocky planets in other systems can form and maintain atmospheres.
An atmosphere is what enables life on the Earth’s surface, regulates our climate and protects us from harmful cosmic rays. But while telescopes have counted a growing number of rocky planets, scientists thought most of their atmospheres were long lost.
However, a new study by researchers at the University of Chicago and Stanford University suggests a mechanism by which these planets can not only develop atmospheres full of water vapor, but also retain them for a long time. Published on March 15, 2021 in the Astrophysical Journal Lettersthe research expands our view of planetary formation and could aid in the search for habitable worlds in other galaxies.
“Our model says these hot, rocky exoplanets should one day have a water-dominated atmosphere, and for some planets that could take quite a long time,” Asst said. Prof. Edwin Kite, an expert on how planetary atmospheres evolve over time.
As telescopes document more and more exoplanets, scientists are trying to figure out what they might look like. In general, telescopes can tell you about one exoplanetphysical size, proximity to the star, and if you’re lucky, how much mass it has. To go much further, scientists need to extrapolate based on what we know about Earth and the other planets in our own solar system. But the most common planets are nothing like the planets we see around us.
“What we already knew about the Kepler mission is that planets are slightly smaller than Neptune are really plentiful, which was a surprise because there aren’t any in our solar system, ”said Kite. “We’re not sure what they are made of, but there is strong evidence that they are magma balls shrouded in a hydrogen atmosphere.”
There are also a healthy number of smaller rocky planets that are similar, but without the hydrogen mantles. So scientists suspected that many planets likely start out as those larger planets with atmospheres made of hydrogen, but lose their atmosphere when the nearby star ignites and blows the hydrogen away.
But many details still need to be completed in those models. Kite and Stanford University co-author Laura Schaefer began investigating some of the possible consequences of having a planet covered in oceans of molten rock.
“Liquid magma is actually quite liquid,” Kite said, so it also spins powerfully, just like oceans on Earth. Chances are, these magma oceans are sucking hydrogen from the atmosphere and reacting to form water. Some of that water escapes to the atmosphere, but much more is slurped into the magma.
Then, after the nearby star clears the hydrogen atmosphere, the water is instead drawn into the atmosphere in the form of water vapor. Ultimately, the planet is left with a water-dominated atmosphere.
This phase could last for billions of years on some planets, Kite said.
There are several ways to test this hypothesis. The James Webb space telescope, the powerful successor to the Hubble telescope, will be launched later this year; it will be able to measure the composition of the atmosphere of an extrasolar planet. If it detects planets with water in their atmosphere, that would be a signal.
Another way to test is to look for indirect signs of atmospheres. Most of these planets are neatly locked; unlike the earth, they don’t rotate when they move around their sun, so one side is always warm and the other side cold.
A few UChicago alumni have suggested a way to use this phenomenon to control the mood. Scientists Laura Kreidberg, PhD’16, and Daniel Koll, PhD’16 – now at the Max Planck Institute for Astronomy and WITH, respectively – pointed out that an atmosphere would moderate the temperature for the planet so there would be no sharp difference between the day and night sides. If a telescope can measure how strongly the day side glows, it should be able to see if there is an atmosphere that redistributes heat.
Reference: “Water on hot rocky exoplanets” by Kite and Schaefer, March 15, 2021, Astrophysical Journal Letters
Financing: NASA