Sky & Telescope - December 2024 - 37

creating distinct spectral features that enable astronomers to
deduce their composition.
Consider the gas giant known as WASP-17b, in a system
some 1,300 light-years from us. When David Grant (University
of Bristol, UK) and his colleagues inspected JWST's
transmission spectrum of this world, they saw an absorption
feature at 8.6 microns, which likely corresponds to pure
quartz crystals swirling in the exoplanet's clouds.
Now that the data actually show absorption features from
clouds themselves, scientists must learn how to identify these
pieces of the crumbled wall. They need much more sophisticated
calculations that incorporate absorption data from both
a wide variety of different cloud compositions and from the
molecules of the gas in the atmosphere, not just the latter.
And the bricks they pull from the rubble will provide telling
hints about the rest of the planet.
The flip side is that if astronomers model the clouds incorrectly,
that error could propagate into other inferences about
the planet - potentially leading to incorrect answers about
the planet's temperature or the abundance of other atmospheric
molecules.
Anna Lueber (Ludwig Maximilian University, Germany)
and her colleagues recently demonstrated this idea by looking
at WASP-39b with two different models - one that assumed
clouds block all wavelengths of light by the same amount
and one that assumed they did not. Her team found that for
certain JWST data, the planet's estimated water abundance
changed by an order of magnitude depending on which
model the researchers used. For a hot Jupiter like WASP-39b,
the amount of water vapor could affect important chemical
processes in the atmosphere.
It's not the first time that astronomers have discovered
that their planetary assumptions are lacking. Even something
as simple as presuming that planets are spherical can cause
trouble. A hot Jupiter - which orbits so close
to its star that it's tidally locked, the same
hemisphere perpetually facing the star - will
have a dayside that is roughly 1000 kelvins
hotter than its nightside, causing it to puff
up and transform the planet into a world
that looks more like an egg than a sphere.
In 2020, Ryan MacDonald (University of
Michigan) and his colleagues found that if
astronomers didn't account for this asymmetry,
they would wildly underestimate the
planet's temperature from the transmission
spectrum, which by nature is a glimpse at
the planet's day-night boundary. In some
cases, the difference might amount to more
than 1000K.
That actually explained a trend that MacDonald's
team had noticed in many scientific
papers, which described exoplanets as being
much cooler than predicted. With updated
models, astronomers can now not only accurately
pinpoint that temperature but also better understand
the circulation of the atmosphere, chemical reactions within
the atmosphere, and even the chemistry of the clouds on
these distant planets. Lueber's recent work on WASP-39b, for
example, showed that the deduced amount of carbon dioxide
changes by almost four orders of magnitude, depending on
whether you assume that the atmosphere's temperature is
constant or not.
Welbanks, too, has found that these factors dramatically
impact their findings from WASP-39b. So do assumptions
made about the star. Transmission spectra, after all,
are obtained when a planet transits in front of its host star,
so the spectra contain information about both the star
and the planet. To obtain information on the planet alone,
astronomers need to subtract the information that is coming
from the star. But in order to do that, they need to correctly
identify which parts of the spectrum are from the star. That
means building not only an excellent planet model but also a
reliable stellar model.
Take GJ 486b, a rocky planet orbiting close to a red dwarf
star 26 light-years away. In 2023, astronomers used JWST
to spy signs of water vapor in its spectrum. It would have
marked the first time scientists had ever managed to discern
an atmosphere on a rocky planet outside our solar system -
and with water vapor to boot!
But an equally likely explanation for the water vapor is
that it originated from the host star, not the planet. Red
dwarfs are much smaller, dimmer, and cooler than our sun.
That makes their star spots especially chilly - so much so
that they can sustain the formation of water vapor. It's still
unclear whether the water detected from the transmission
spectrum originates in the planet's atmosphere or the star's.
The answer - once again - relies on building better models.
A New Rosetta Stone
Underneath these issues is one very basic
assumption: that astronomers understand
how light and matter interact. Opacity is the
measure of how easily photons pass through
a material. They might pass straight through,
be absorbed, or be reflected back, depending
on how they interact with certain molecules
within that material.
Consider a hydrogen atom, with a single
proton in the nucleus and one electron
orbiting the nucleus. The electron can
absorb a photon and jump to a higher energy
level, or it can emit a photon and drop to
a lower energy level. Any undergraduate in
a first-year physics lab has likely peeked at
a hydrogen emission tube and jotted down
the visible wavelengths associated with these
transitions. In fact, such work underpins
much of astronomical research, enabling scientists
to deduce not only the compositions
sk yand tele scope .o r g * DECEMBER 2024 37
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