Team performs first laboratory simulation of exoplanet atmospheric chemistry

Lead author Sarah Hörst, right, and assistant research
scientist Chao He examine samples of simulated atmospheres
in a dry nitrogen glove box, where they are stored to avoid
contamination from Earth’s atmosphere. Credit: Will

Scientists have conducted the first lab experiments on haze
formation in simulated exoplanet atmospheres, an important
step for understanding upcoming observations of planets
outside the solar system with the James Webb Space Telescope.

The simulations are necessary to establish models of the
atmospheres of far-distant worlds, models that can be used to
look for signs of life outside the solar system. Results of the
studies appeared this week in Nature Astronomy.

“One of the reasons why we’re starting to do this work is to
understand if having a haze layer on these planets would make
them more or less habitable,” said the paper’s lead author,
Sarah Hörst, assistant professor of Earth and planetary
sciences at the Johns Hopkins University.

With telescopes available today, planetary scientists and
astronomers can learn what gases make up the atmospheres of
exoplanets. “Each gas has a fingerprint that’s unique to it,”
Hörst said. “If you measure a large enough spectral range, you
can look at how all the fingerprints are superimposed on top of
each other.”

Current telescopes, however, do not work as well with every
type of exoplanet. They fall short with exoplanets that have
hazy atmospheres. Haze consists of solid particles suspended in
gas, altering the way light interacts with the gas. This muting
of spectral fingerprints makes measuring the gas composition
more challenging.

Hörst believes this research can help the exoplanet science
community determine which types of atmospheres are likely to be
hazy. With haze clouding up a telescope’s ability to tell
scientists which gases make up an exoplanet’s atmosphere – if
not the amounts of them – our ability to detect life elsewhere
is a murkier prospect.

Hörst uses a flashlight to look inside the experimental
chamber when the experiment is running, to see if haze is
being formed. Credit: Will Kirk/JHU

Planets larger than Earth and smaller than Neptune, called
super-Earths and mini-Neptunes, are the predominant types of
exoplanets, or planets outside our solar system. As this class
of planets is not found in our solar system, our limited
knowledge makes them more difficult to study.

With the coming launch of the James Webb Space Telescope,
scientists hope to be able to examine the atmospheres of these
exoplanets in greater detail. JWST will be capable of looking
back even further in time than Hubble with a light collecting
area around 6.25 times greater. Orbiting around the sun a
million miles from Earth, JWST will help researchers measure
the composition of extrasolar planet atmospheres and even
search for the building blocks of life.

“Part of what we’re trying to help people figure out is
basically where you would want to look,” said Hörst of future
uses of the James Webb Space Telescope.

Given that our solar system has no super-Earths or
mini-Neptunes for comparison, scientists don’t have “ground
truths” for the atmospheres of these exoplanets. Using computer
models, Hörst’s team was able to put together a series of
atmospheric compositions that model super-Earths or
mini-Neptunes. By varying levels of three dominant gases
(carbon dioxide, hydrogen, gaseous water), four other gases
(helium, carbon monoxide, methane, nitrogen) and three sets of
temperatures, they assembled nine different “planets.”

The computer modeling proposed different percentages of gases,
which the scientists mixed in a chamber and heated. Over three
days, the heated mixture flowed through a plasma discharge, a
setup that initiated chemical reactions within the chamber.

“The energy breaks up the gas molecules that we start with.
They react with each other and make new things and sometimes
they’ll make a solid particle [creating haze] and sometimes
they won’t,” Hörst said.

Lead author Sarah Hörst, right, and assistant research
scientist Chao He examine a sample of simulated exoplanet
atmosphere created in the chamber behind them. Credit: Will

“The fundamental question for this paper was: Which of these
gas mixtures – which of these atmospheres – will we expect to
be hazy?” said Hörst.

The researchers found that all nine variants made haze in
varying amounts. The surprise lay in which combinations made
more. The team found the most haze particles in two of the
water-dominant atmospheres. “We had this idea for a long time
that methane chemistry was the one true path to make a haze,
and we know that’s not true now,” said Hörst, referring to
compounds abundant in both hydrogen and carbon.

Furthermore, the scientists found differences in the colors of
the particles, which could affect how much heat is trapped by
the haze. “Having a haze layer can change the temperature
structure of an ,” said Hörst. “It can
prevent really energetic photons from reaching a surface.”

Like the ozone layer that now protects life on Earth from
harmful radiation, scientists have speculated a primitive haze
layer may have shielded life in the very beginning. This could
be meaningful in our search for external life.

For Hörst’s group, the next steps involve analyzing the
different hazes to see how the color and size of the particles
affect how the particles interact with light. They also plan to
try other compositions, temperatures, energy sources and
examine the composition of the haze produced.

“The production rates were the very, very first step of what’s
going to be a long process in trying to figure out which
atmospheres are hazy and what the impact of the particles is,” Hörst said.

Explore further:

Pluto’s hydrocarbon haze keeps dwarf planet colder than

More information: Sarah M. Hörst et al, Haze production
rates in super-Earth and mini-Neptune atmosphere experiments,
Nature Astronomy (2018). DOI: 10.1038/s41550-018-0397-0

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