Sky & Telescope - December 2024 - 39

of exoplanet atmospheres but also spinning galaxies, stellar
nurseries, and even the earliest structures of the universe.
But in reality, it is not so simple. The spectral lines can
change shape depending on the temperature of the gas and
even the surrounding molecules. Moreover, not all molecules
have been studied extensively.
To date, most of the research has been Earth-centric,
meaning that scientists have mapped molecules common on
Earth and at temperatures we normally encounter. But these
same molecules act differently at temperatures of thousands
of degrees, like those found within the atmospheres of hot
Jupiters. Here, electrons are more likely to jump between
energy levels, and the molecules are more likely to rotate and
vibrate in different ways, creating more complex spectra.
Water, for example, might have 320,000 transitions at room
temperature, but it has roughly 5 billion at high temperatures.
Needless to say, it is challenging to map the absorption
spectrum for every possible atom and molecule at every possible
temperature.
To make matters more difficult, it is hard to recreate these
conditions in the lab. Some of the gases, like phosphine -
which was recently detected in Venus's atmosphere - are
both poisonous and explosive at high temperatures.
" You cannot find many people who
would want to do experiments like that, " says
Iouli Gordon (Center for Astrophysics, Harvard
& Smithsonian).
So scientists often have to rely on theoretical
calculations to determine how molecules
absorb light in different environments. But
those calculations need new generations of
computing facilities and can take impossibly
long to run. Right now, scientists are trying
to balance the two, using some lab data to
provide key results that inform computational
work (S&T: May 2021, p. 34).
Scientists also worry about how a molecule's
spectrum will shift when it's surrounded
by other, different molecules. Water,
for example, has been mapped extensively
on Earth - that is, in a nitrogen-dominated
atmosphere. But if it is surrounded by hydrogen
instead, like in a hot Jupiter, it will behave
slightly differently. And so if astronomers try
to interpret a water feature using data derived
on Earth, their understanding will be inexact.
" It would almost be like using a Rosetta
Stone with modern Greek instead of ancient
Greek, " says Julien de Wit (Massachusetts
Institute of Technology). " We'd get some
things right. But some other things would be
slightly off. That's the issue with the opacity
challenge. "
De Wit, Gordon, and their colleagues
recently analyzed synthetic data (similar to
what JWST will see) with different spectral-line lists built
using different methods. They found that the derived planetary
properties - such as composition - could change by
an order of magnitude for some planets, depending on their
assumptions. Welbanks and his team encountered a similar
challenge in analyzing WASP-39b's spectrum.
To accurately gauge these planets, scientists need accurate
opacity models.
Enter HITRAN (High-resolution Transmission Molecular
Absorption Database), a repository of both experimentally
derived and calculated lists for a wide variety of different
molecules. Originally built by the U.S. Air Force in the 1960s,
the database initially included only seven molecules. It is now
managed by the Center for Astrophysics and has expanded to
include dozens of molecules in variant forms, including gases
that are not necessarily present in the terrestrial atmosphere.
Gordon, the current director of the HITRAN project, is working
hard to ensure that expansion continues - acting as a
liaison between the exoplanet community and the theorists
who can provide new, improved spectra.
The database will inform a wide array of applications.
Climate prediction models for Earth utilize HITRAN. So does
work in the medical field. One research group
even used it to determine the best glass shape
from which to drink champagne and beer.
A World Made of Cheese
The worst-case scenario is that, due to model
assumptions, astronomers may reach an
egregious conclusion. " If we're not careful, it
is a matter of time before someone puts out a
press release that says, 'Astronomers Find that
the Moon Is Made out of Cheese,' " Welbanks
says. This result could happen if astronomers
tried to compare the composition of the Moon
with a cheese model and a sponge model. " Both
models are wrong - but when we only have
those two models, and we compare only those
two models, one of them is going to win by
definition. "
That, of course, is an extreme case. But it
does demonstrate how scientists might argue a
planet has life because that is the best match to
the data, only to be woefully wrong.
Despite the obstacles, many theorists are
optimistic. " These are challenges, but also
opportunities, " de Wit says. " As we're forced
to improve our models of each of these components
- stellar models, opacity models,
planetary models - we are going to gain new
insights. We'll get access to new physics, not
only when we do the theoretical work, but also
when we compare the theoretical work to the
data that we get access to with James Webb. It's
a new frontier. "
sk yand tele scope .o r g * DECEMBER 2024 39
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