Sky & Telescope - December 2024 - 35

SAYOSTUDIO
UNDER CONSTRUCTION
The earliest exoplanet spectra from the James
Webb Space Telescope are extraordinary -
and challenging. Astronomers' toolkits need a
major upgrade in order to interpret the data.
L
uis Welbanks tingled with anticipation. It was July
2022, and his team had been eagerly waiting to see
their data from the newly launched James Webb Space
Telescope (S&T: Nov. 2022, p. 12).
For roughly eight hours, JWST had observed starlight as
it filtered through the atmosphere encircling the distant
gas giant WASP-39b, which lies about 700 light-years away.
Astronomers often gather exoplanet data in this manner,
relying on the host star to reveal the invisible: Although
scientists typically cannot image the world directly, the alien
air still leaves its mark by absorbing specific wavelengths of
starlight that correspond to specific molecules in the exoplanet's
atmosphere.
These so-called transmission spectra appear as nothing
more than a series of wiggles and bumps. Yet they have the
power to unlock crucial details - potentially even signs of life
- on worlds that remain mostly invisible to our greatest telescopes.
And JWST will give astronomers the best picture yet.
With so much at stake, it's no wonder that Welbanks (Arizona
State University) was nervous. But the spectrum from
WASP-39b did little to relieve his anxiety. As both observers
and theorists gathered around a laptop with the new
spectrum on its screen, their reactions played out in stark
contrast. The observers beamed with happiness, overjoyed
by the clarity of the spectrum and the details that quickly
emerged. But the theorists, Welbanks included, whipped out
their phones, worry lines on their faces.
A small bump was visible around 4 microns that Welbanks
did not recognize. No theorist did. So he and his colleagues
began frantically searching through database after database,
trying to determine the source.
" The data were so great and the observers were so happy
with it, but the modelers were freaking out a little, " Welbanks
says. " The journey for us had just begun. "
It took several days (and half a year before any published
results) to determine the source of the bump. It was sulfur
dioxide. But the original computer models used to interpret
the data didn't include the molecule - in part because the
calculations didn't account for the myriad of chemical reactions
that light from the planet's host star can drive in the
upper atmosphere.
Now scientists understand that ultraviolet photons break
up water molecules in WASP-39b's atmosphere, allowing the
free hydrogen to interact with hydrogen sulfide and eventually
create sulfur dioxide. But that answer was not obvious
at first. In fact, the detection marked the first time sulfur
dioxide was identified in any exoplanet atmosphere.
WASP-39b is just one example of the challenges that
arise when scientists cross a frontier in our knowledge. No
previous observatory has measured the subtle differences in
so many individual wavelengths across the 3- to 5.5-micron
infrared range to the precision necessary for compounds
like sulfur dioxide to pop out in an exoplanet's transmission
spectrum. " JWST revolutionized the field, " Welbanks
says. " But with better data, we need better models to explain
these subtleties. "
The problem is that astronomers can't simply pull up a
complete list of all the spectral lines created in nature, compare
it to the complex transmission spectrum they measured,
and match up the lines. First, that master list is unknown.
Second, the real spectral lines will change depending on the
conditions in the alien atmosphere.
So astronomers create a series of models with every new
dataset. Essentially, they run computer codes to build several
synthetic planets and generate the predicted transmission
spectrum of each hypothetical planet. They then compare
those mockups to the actual one. The best match helps place
constraints on the actual planet, allowing them to argue that
the detected planet has a certain temperature, composition,
or - the true holy grail - life.
But the models need work. Many, like the ones originally
built for WASP-39b, lack crucial details about the planet's
atmosphere, the star itself, and even the physics behind how
light and matter interact. The difference between current
models and reality is akin to the difference between a sketch
and a photograph.
Luckily, theorists across the globe are already hard at work
filling in the details of their planetary portraits.
A Huge Experiment
Welbanks has only recently finished analyzing the spectrum
of WASP-39b. Through JWST's Early Release Science Program,
he led a team of roughly 50 scientists in scrutinizing
the planet with different methods. The goal was to provide
astronomers with an instruction manual for how to handle
future JWST observations.
But the results were alarming. Different astronomers,
using different models, found wildly different answers for
the properties of the exoplanet's atmosphere. " We had a
sk yand tele scope .o r g * DECEMBER 2024 35
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