Sky & Telescope - December 2024 - 36

Exoplanet Science
Absorption of Light by Hydrogen
Star
Light
Starlight passes
through planet's
atmosphere
121.6
Wavelength
Each electron in an atom or molecule
absorbs specific wavelengths of light,
depending on its energy level. Molecules
can also absorb wavelengths that make
their components vibrate or rotate in
specific ways.
Wavelength
A star's spectrum contains absorption
lines from atoms and molecules in its
atmosphere.
Wavelength
The planet's atmosphere also absorbs
some wavelengths, creating a complex
pattern that astronomers must
disentangle.
TRANSMISSION SPECTRA Astronomers can detect an exoplanet's atmosphere by identifying " extra " absorption lines in the host star's light, left
as chemical fingerprints when the starlight passed through the planet's atmosphere. But correctly identifying the atmospheric composition and conditions
responsible for those chemical fingerprints requires careful work.
$10 billion telescope giving us the best data on the planet, "
Welbanks says. " And even then, we were having challenges in
explaining [the data] with our models. "
So Welbanks and his colleagues began looking at every
detail in the different calculations - from the smallest
assumptions, like which units each team used, to the larger
assumptions about the chemistry and physics at work in
planetary atmospheres. The latter proved to be especially
important.
When building these synthetic planets, astronomers
have long taken shortcuts - assuming, for example, that the
planet has the same composition and temperature everywhere.
That is a far cry from reality.
But while astronomers would love to build the most complicated
models physically plausible, they simply can't. Even
the best computers can only do so much so fast. And scientists
need to run millions of slightly different examples to
find the best match. That means it would take ages to run the
necessary calculations - literally: If theorists wanted to use a
model that takes one month to compute a realistic spectrum,
36 DECEMBER 2024 * SKY & TELESCOPE
they would have had to start back in the last Ice Age to complete
millions by today.
Moreover, a real transmission spectrum doesn't reveal all
the details within a planet's atmosphere: It represents merely
a passing shadow of a world that scientists cannot even see.
So scientists work hard to find the best balance, building
models that are simple enough to run, yet complex enough to
match the data. Now with JWST upping the complexity, that
balance has been upset.
For example, when astronomers observe clouds in visible
wavelengths - like those imaged by the Hubble Space Telescope
- the clouds block all the light that hits them. It's like
the clouds are a solid wall, presenting a blank surface that
reveals little to nothing about its own composition while also
hiding what lies below. " You don't see any more of the gas
absorption, in the same way that you don't see any more blue
sky on Earth, " says Sarah Moran (University of Arizona).
But clouds are not an impenetrable wall in the mid-infrared
wavelengths that JWST detects. In fact, it's just the opposite:
Their molecules absorb starlight at these wavelengths,
Brightness
Brightness
Brightness
BEATRIZ INGLESSIS / S&T, SOURCE: NASA / ESA / LEAH HUSTAK (STSCI)

Sky & Telescope - December 2024

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