Planetary scientists discover that there can be many planets with water-rich atmospheres

A study suggests that exoplanets close to their stars may actually retain a thick atmosphere filled with water. Above is 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 The study shows how hot, rocky planets from other systems could form and retain atmospheres.

An atmosphere is what makes life on the Earth’s surface possible, regulating our climate and protecting us from damaging cosmic rays. But although telescopes have counted an increasing number of rocky planets, scientists believed 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 could not only develop atmospheres full of water vapor, but retain them for long periods of time. Posted on March 15, 2021 in the Letters from the Astrophysical Journal, the research broadens our picture of planetary formation and could help guide the search for habitable worlds in other star systems.

“Our model says that these hot, rocky exoplanets should have a water-dominated atmosphere at some point, and for some planets that can be quite long,” Asst said. Teacher. Edwin Kite, an expert on how planetary atmospheres change over time.

As telescopes document more and more exoplanets, scientists are trying to figure out what they might look like. Typically, telescopes can tell you about a exoplanetits physical size, its proximity to its star, and if you’re lucky, its mass. To go much further, scientists must 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.

“What we already knew about 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 is strong evidence that they are magma balls enveloped in an atmosphere of hydrogen.”

There are also quite a few small rocky planets that are similar, but without the hydrogen cloaks. Scientists therefore speculated that many planets probably started out as those larger planets that have atmospheres made of hydrogen, but lose their atmosphere when the neighboring star ignites and blows out hydrogen.

But many details remain to be completed in these models. Kite and co-author Laura Schaefer of Stanford University have begun to explore some of the potential consequences of having a planet covered in molten rock oceans.

“Liquid magma is actually quite liquid,” Kite said, so it turns around just as vigorously, just like the oceans on Earth. There is a good chance that these magmatic oceans will suck hydrogen out of the atmosphere and react to form water. Some of this water escapes into the atmosphere, but much more is injected into the magma.

Then, after the nearby star clears the atmosphere of hydrogen, water is pulled into the atmosphere instead as water vapor. Eventually, the planet ends up with an atmosphere dominated by water.

This stage could persist on some planets for billions of years, Kite said.

There are several ways to test this hypothesis. The James Webb Space Telescope, the powerful successor to the Hubble Telescope, is expected to launch later this year; it will be able to take measurements of the composition of the atmosphere of an exoplanet. 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 tidal locked; unlike the Earth, they do not rotate as they move around their sun, so one side is always hot and the other is cold.

A pair of UChicago alumni suggested a way to use this phenomenon to verify an atmosphere. 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 of the planet, so that there would be no clear difference between the day and night sides. If a telescope can measure the intensity of the glare from the daylight side, it should be able to tell if there is an atmosphere redistributing heat.

Reference: “Water on hot rocky exoplanets” by Kite and Schaefer, March 15, 2021, Letters from the Astrophysical Journal.

Funding: NASA.