An atmosphere is what makes life on Earth's surface possible, regulating our
climate and sheltering us from damaging cosmic rays. But although telescopes
have counted a growing number of rocky planets, scientists had thought most
of their atmospheres long lost.
However, a new study by University of Chicago and Stanford University
researchers suggests a mechanism whereby these planets could not only
develop atmospheres full of water vapor, but keep them for long stretches.
Published March 15 in the Astrophysical Journal Letters, the research
expands our picture of planetary formation and could help direct the search
for habitable worlds in other star systems.
"Our model is saying that these hot, rocky exoplanets should have a
water-dominated atmosphere at some stage, and for some planets, it may be
quite a long time," said Asst. Prof. Edwin Kite, an expert in how planetary
atmospheres evolve over time.
As telescopes document more and more exoplanets, scientists are trying to
figure out what they might look like. Generally, telescopes can tell you
about an exoplanet's physical size, its proximity to its star and if you're
lucky, how much mass it has. To go much further, scientists have to
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 from 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'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. Kite and
co-author Laura Schaefer of Stanford University began to explore some of the
potential consequences of having a planet covered in oceans of melted rock.
"Liquid magma is actually quite runny," Kite said, so it also turns over
vigorously, just like oceans on Earth do. 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 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, Kite said.
There are several 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 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 sun, so one side is always hot and the other cold.
A pair of UChicago alumni have suggested a way to use this phenomenon to
check for an atmosphere. Scientists Laura Kreidberg, PhD'16, and Daniel
Koll, PhD'16--now at the Max Planck Institute for Astronomy and MIT,
respectively--pointed out that an atmosphere would moderate the temperature
for the planet, so there wouldn't be a sharp difference between the day
sides and night sides. If a telescope can measure how strongly the day side
glows, it should be able to tell whether there's an atmosphere
redistributing heat.
Reference:
Edwin S. Kite, Laura Schaefer. Water on Hot Rocky Exoplanets. The
Astrophysical Journal Letters, 2021; 909 (2): L22 DOI:
10.3847/2041-8213/abe7dc
Tags:
Space & Astrophysics