The atmosphere makes life on the Earth’s surface possible to regulate our climate, and protects us from destructive cosmic rays. Although telescopes have counted an increasing number of rocky planets, scientists thought that most of their atmospheres had long since disappeared. 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 an atmosphere filled with water vapor but also hold them for long periods.
The study, published March 15 in The Astrophysical Journal, expands our understanding of the planetary formation and helps guide the search for a livable world in other star systems.
“Our model is that these hot rocky exoplanets should have a water-based atmosphere at some stage, which could be quite a long time for some planets, ” said Edwin Kite, an associate professor of experts who specializes in how the planet’s atmosphere evolves.
As telescopes record more and more exoplanets, scientists are trying to figure out what they might look like. In general, telescopes can tell people the physical size of an exoplanet, how close it is to a star, or possibly how big it is. To go further, scientists must infer from what we know about Earth and other planets in the solar system. But the richest planets don’t seem to be like the planets around us.
“What we’ve learned from the Kepler mission is that planets that are a little smaller than Neplope are really rich, and that’s a surprise because we don’t have one in our solar system,” Kite said. “We don’t know what they are made of, but there is strong evidence that they are magma balls in the hydrogen atmosphere.”
There are fewer rocky planets that are similar but do not have a hydrogen atmosphere. So scientists speculate that many planets may start like the big planets, whose atmospheres are made up of hydrogen, but when nearby stars ignite and blow hydrogen away, they lose their atmosphere.
But there are still many details to be filled in these models. Stanford University’s Kite and co-author Laura Schaefer began exploring the potential consequences of some planets being covered by lava oceans. “Liquid magma is very fluid, ” Kite says, so it flips as violently as the oceans on Earth. These magma oceans are likely to draw hydrogen from the atmosphere and react to form water. Some water escapes into the atmosphere, but more water is sucked into the magma.
When a nearby star strips the hydrogen atmosphere, water is pulled out of the atmosphere in the form of water vapor. In the end, the planets are left with a water-based atmosphere. Kite says this phase could last billions of years on some planets.
There are several ways to test this hypothesis. The James Webb Space Telescope, a powerful successor to Hubble, is scheduled to launch later this year; it will be able to measure the composition of the atmosphere of exoplanets. If it could detect planets in the atmosphere that contain water, it would be a signal.
Another way to detect is to look for indirect signs of the atmosphere. Most of these planets are tidally-locked; unlike Earth, they do not rotate as they move around the sun, so they are always hot on one side and cold on the other.
A pair of University of Chicago alumni have come up with a way to use the phenomenon to examine the atmosphere. Scientists Dr. Laura Kreidberg and Dr. Daniel Koll — who now work at the Max Planck Institute for Astronomy and the Massachusetts Institute of Technology, respectively — point out that the atmosphere eases the temperature of the planets, so there is no sharp difference between one side of the day and the other side of the night. If the telescope can measure the intensity of light on the daytime side, it should be able to determine whether there is an atmosphere that redistributes heat.