A study suggests that exoplanets near their stars may retain a thick atmosphere full of water. Above is the illustration of an artist from the exoplanet WASP-121b, who appears to have water in its atmosphere. Credit: Engine House VFX, At-Bristol Science Center, University of Exeter
University of Chicago study finds way that hot, rocky planets in other systems can shape and hold atmosphere.
An atmosphere is what makes life on earth possible, that regulates our climate and protects us from cosmic rays. But although telescopes count a growing number of rocky planets, scientists thought most of their atmosphere was lost.
However, a new study by researchers from the University of Chicago and Stanford University points to a mechanism by which these planets could not only develop atmospheres full of water vapor, but could also last a long time. Published on March 15, 2021 in the Astrophysical Journal Letters, the research expands our image of planetary formation and may help to direct the search for habitable worlds in other star systems.
“Our model says that these hot, rocky exoplanets must have a water-dominating atmosphere at some point, and for some planets it can 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 a exoplanet‘s physical size, its proximity to its star and if you’m lucky, how much mass does it have? To go much further, scientists need to extrapolate based on what we know about the earth and the other planets in our own solar system. But most planets do not look like the ones we see around us.
‘What we already knew about the Kepler mission is that planets are a little smaller than Neptune is really plentiful, which was a surprise because there is nothing in our solar system, ‘Kite said. “We do not know for sure what it consists of, but there is strong evidence that it is magma balls covered in a hydrogen atmosphere.”
There are also a healthy number of smaller rocky planets that are similar but without hydrogen mantles. Scientists have therefore suspected that many planets are likely to start like the larger planets that have atmospheric hydrogen, but lose their atmosphere when the star ignites in the environment and blows the hydrogen away.
Many details still need to be filled in the models. Kite and co-author Laura Schaefer of Stanford University have investigated some potential effects of a planet covered in oceans of molten rock.
“Liquid magma is actually very runny,” Kite said. It also rotates powerfully, just like oceans on earth. The magma oceans absorb hydrogen from the atmosphere and react to form water. Some of the water escapes to the atmosphere, but much more is swallowed up in the magma.
After the star in the environment has removed the hydrogen atmosphere, the water is drawn into the atmosphere in the form of water vapor. Eventually, the planet is left with a water-dominating atmosphere.
This stage could last billions of years on some planets, Kite said.
There are different ways to test this hypothesis. The James Webb Space Telescope, the powerful successor to the Hubble Telescope, is scheduled to launch later this year; it will be able to measure the composition of the atmosphere of an exoplanet. If it detects planets with water in their atmosphere, it’s one signal.
Another way to test is to look for indirect signs of atmosphere. Most of these planets are tidal; unlike the earth, they do not rotate as they move around their sun, so one side is always hot and the other one cold.
A few alumni of UChicago have suggested a way to use this phenomenon to look at an atmosphere. Scientists Laura Kreidberg, PhD’16, and Daniel Koll, PhD’16 – now at the Max Planck Institute of Astronomy and MIT, respectively – pointed out that an atmosphere would moderate the temperature for the planet, so there would not be a sharp difference between the day and night time side. If a telescope can measure how strongly the day side glows, it must know if there is an atmosphere that redistributes heat.
Reference: “Water on Hot Rocky Exoplanets” by Kite and Schaefer, March 15, 2021, Astrophysical Journal Letters.
Funding: NASA.