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The James Webb Space Telescope has found the strongest, most definitive evidence
for carbon dioxide in the atmosphere of an exoplanet 700 light-years away.
WASP-39 b is a gas giant that orbits the sun like
star 700 light years away in the constellation Virgo.
The planet orbits a star at a distance of just 0.05 A.U..
That's well inside Mercury's equivalent orbit around the Sun.
At that distance, the planet's atmosphere is a scorching 1200 Kelvin.
And that's hot enough
to inflate the atmosphere by more than 50% of its normal size.
So even though WASP-39 b has the same mass at Saturn,
its diameter is 30% larger than Jupiter.
That's a big puffy planet.
The planet's orbit is tilted edge on to our line of sight.
And that allows Webb to take its spectrum as a transits in front of its star.
And the result is astonishing!
It's the clearest, obvious, can't be anything else, no kidding
signature of carbon dioxide ever seen in an exoplanet.
The findings were made by the JWST Exoplanets Early
Release Science Team and has been accepted for publication in Nature.
Early Release Science or ERS is designed to do a bunch of science with Webb
right after commissioning and release the data immediately to the public.
That way astronomers can understand just what Webb is really capable of
and use that information to plan out future observations.
So on the one hand, this result provides important insights into the composition
and the formation of WASP-39 b, but it also demonstrates Webb's ability
to characterize the thinner atmospheres of smaller rocky planets.
Webb can do this because its Near-Infrared
Spectrometer has a special observation mode
that was specifically designed to study the atmospheres of transiting exoplanets.
And that allows Webb to produce what's called a "transmission spectrum".
Now, this technique is often described as subtracting the star spectrum
from the star's spectrum when the planet is in front of it.
And whatever remains is the spectrum of the planet's atmosphere.
However, transmission spectroscopy really isn't as simple as that.
The real technique is actually quite different, and it's absolutely fascinating
how they actually do it.
So today we're going to learn how transmission spectroscopy works,
and how the Early Release Science team determined
that WASP-39 b really does gave carbon dioxide in its atmosphere.
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[Ad End] Web observed wasp 39 be on July 10th,
just about a month after its NIRSpec instrument was fully commissioned.
During the transit, WASP-39b
blocks, about two and a half percent of its star's light.
And this shows up as a dip in the star's light curve.
Now, this is the same technique that Kepler and TESS use to detect exoplanets.
But in order to know a planet's atmosphere composition,
you really need to know how different colors - or
wavelengths - of light are being blocked in different amounts.
That way, you can work backward
to figure out which gases have to be present in the atmosphere.
And this is why Webb's NIRSpec instrument features a special observing
mode called Bright Object Time Series, or BOTS.
in BOTS mode, Webb takes a spectrum of the star once per minute.
During the transit, the brightness of the star's spectrum drops,
and that allows NIRSpec to effectively observe the transit
in hundreds of wave bands or "colors" all at once.
Now, because the observations took place over 8 hours,
NIRSpec generated something like 500 spectra.
And that allowed the team to slice ip the spectra into tiny little chunks
or colors, and then plot the changes in those colors over time.
The result is a unique light curve for each of NIRSpec's wave bands.
So here we're looking at some of those light curves, and they've all been
artificially offset from each other so that we can see them individually.
Now, apart from the fact that they are at different wavelengths,
there really isn't any obvious difference in the shapes of the curves.
But that's not really the case.
If we combine
some of these curves together, we can see some interesting differences.
Light at 4.7 microns isn't getting blocked as much,
while the light of 4.3 microns is blocked slightly more.
We can really see the difference when we step through the individual Light curves.
Somewhere around 4.3 microns, we see the strongest dip in brightness.
This is followed by a slight recovery.
If you plot out the amount of light blocked versus
wavelength, the difference becomes even more clear.
The vertical axis represents the percentage of light
that's getting blocked, while the horizontal axis is wavelength.
This is the transmission spectrum of WASP-39 b, and that large bump
right there is due to carbon dioxide absorbing light at 4.3 microns.
It's essentially an inverted absorption spectrum.
Only instead of plotting the amount of star brightness versus wavelength,
we're plotting star blockage versus wavelength.
But still, how do we know that this particular absorption
bump is due to carbon dioxide and not some other gas, or multiple gases
that are combining together to sort of masquerade as CO2?
And that's why the ERs team ran several different algorithms to model
what kinds of gases WASP-39 b would need to have in order
to produce the transmission spectrum we see here.
Now, to see what role carbon dioxide is really playing here,
the models were effectively rerun,
only this time by not including a particular gas.
They found that removing things like water, carbon monoxide, hydrogen
sulfide and methane didn't really change the fit to the large bump.
But removing carbon dioxide effectively erases the bump from the model.
So what the models are saying is, 'hey, if you want to have this bump right here,
then you need to have carbon dioxide in the planet's atmosphere.
And if you want to really fit the spectrum well, you also need water,
carbon monoxide, hydrogen sulfide and a little trace of methane.'
This is how we know that WASP-39 b really has Carbon dioxide in its atmosphere.
And those models are making predictions about the rest of the atmosphere's
composition That can be tested in future observations.
But the models aren't perfect.
The data shows a small but unmistakable bump at around four microns.
And none of the Models are predicting it.
Mo matter how they processed
the data, the ERS team could not seem to get rid of that little bump.
So that suggests that there's some other gas
in the planet's atmosphere That's adding in this extra absorption.
So more work will be needed to tease out what this mysterious absorber is.
Now, to really appreciate just what a remarkable improvement this is.
Take a look at this transmission spectrum of WASP-39 b
that was created by the Spitzer Space Telescope.
Astronomer Hannah Wakeford was Able
to get two measurements across two different wavelength ranges.
And these measurements seem to kind of correlate with a model of carbon dioxide.
But because of the way Spitzer's instruments were set up, it averages
its measurements over those two broad wavelength ranges,
and they're represented here by these two horizontal blue lines.
The vertical bars indicate the uncertainty in the measurements.
Now, Spitzer's measurements do seem to correlate with CO2 absorption,
but it's really hard to draw a firm conclusion.
But now with Webb, we have such precise data
that we can really see a clean transmission spectrum.
It turns out that Spitzer was, in fact, seeing carbon dioxide, only.
there was no way to know for sure until now.
And by the way, this entire spectrum was captured in a single transit!
With other telescopes, there's so much noise in the data
that you typically have to observe several transits over many different days
and even weeks at a time, and then, average all of that data
together to whittle away some of the noise and increase the signal.
Webb took care of business in an afternoon.
Now this is all part of Webb's Early Science program,
so the data is immediately released to the public
so astronomers can plan out those future observations.
And those observations are not going to be limited to gas giants like WASP-39 b.
Webb's first year of observations include transmission spectroscopy
of terrestrial planets, including those of the TRAPPIST-1 system!
Now, naturally, this begs the question as to whether or not Webb could detect
so-called "biomarkers" in a planet's atmosphere?
Well, it certainly is true that on Earth, gases
like molecular oxygen and methane are the byproduct of life.
But those gases don't strictly require life to produce them either.
Still, it will be interesting to see
if Webb can detect any of those gases in the atmosphere of terrestrial planets.
Webb may have better success at detecting biomarkers
in the atmospheres of so-called "hycean" planets.
Hycean planets, are ocean worlds with enormous molecular hydrogen envelopes.
There's some theoretical work that shows that if there was
in fact life swimming around in these global oceans,
they could expel enough biomarkers into the atmosphere for Webb to detect.
I made a video about that, so feel free to check that out when we're done here.
Otherwise, though, it's thought that Webb will probably identify
some exoplanets that could possibly, maybe, host life.
But it's unlikely that Webb should detect unambiguous signs of life.
But then again, I wouldn't be surprised if we were surprised.
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Until next time. Stay curious, my friend.