Data suggest that 80% of ‘hot’ rocky exoplanets — rocky planets with orbital period less than 100 days — formed as gas-rich sub-Neptunes that subsequently lost most of their atmospheres, but whether they exoplanets still have atmospheres is unknown. In a new paper in the Astrophysical Journal Letters, a duo of U.S. astronomers predicts that rocky planets with orbital periods between 10 and 100 days still retain water-dominated atmospheres.
“What we already knew from NASA’s 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,” said Dr. Edwin Kite, an astronomer in the Department of the Geophysical Sciences at the University of Chicago.
“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. But lots of details remain to be filled out in those models.”
Dr. Kite and his colleague, Dr. Laura Schaefer of Stanford University, explored some of the potential consequences of having a planet covered in oceans of melted rock.
“Liquid magma is actually quite runny, so it also turns over vigorously, just like oceans on Earth do,” Dr. Kite said.
“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 sub-Neptune’s 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,” Dr. Kite said.
The team’s prediction is imminently testable with the upcoming James Webb Space Telescope (JWST).
“JWST, the powerful successor to the Hubble Telescope, is scheduled to launch later this year,” the researchers said.
“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 host star, so one side is always hot and the other cold.”
Edwin S. Kite & Laura Schaefer. 2021. Water on Hot Rocky Exoplanets. ApJL 909, L22; doi: 10.3847/2041-8213/abe7dc