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[♪ INTRO]
It’s like a scene out of Romeo and Juliet:
Two planets, both alike in gravity, In the Milky Way where we lay our scene.
Their skies obscure their metallicity But through study their formation can be gleaned
Our protagonists are planets: WASP-67b and HAT-P-38b.
They’re two gas giants orbiting different stars on opposite ends of the galaxy.
You could call them quite literally star-crossed.
Looking at their mass, the stars they orbit, and their orbital distance
from their stars, WASP-67b and HAT-P-38b seem so alike.
But when astronomers compared these two planets in a 2018 study
published in the Astronomical Journal, they found very different atmospheres:
one has clouds where the other seems to have clear skies.
And understanding more about the clouds on these two exoplanets does two things.
First, it reveals that two planets that exist under the same conditions
can “grow up” very differently.
Second, it tells us more about the range of possible atmospheres an exoplanet can have.
That’s useful because we’re just at the beginning
of understanding atmospheres other than our own.
Let’s start, though, with what the planets share.
They’re similar masses, both between a quarter and half as heavy as Jupiter.
They’re also similar sizes, each roughly the diameter of Jupiter.
Their balance of size and mass means that they both have similar gravity,
about the same as Earth, actually.
And because they orbit similar stars at almost exactly the same distance,
their insolation, or how much energy they get from their star,
is almost exactly the same.
If that’s all you knew about these planets,
you would assume that they had the same atmospheres.
That’s because the prevailing wisdom was that the two main drivers
of a gas giant’s cloudiness are its temperature and its gravity.
And studying exoplanet atmospheres is hard.
If finding an exoplanet is like spotting a fly next to a headlight a hundred kilometers
away, then studying its atmosphere is like asking what color the wings are.
To do that, astronomers look for the moment a planet passes in front of its star.
That’s when some of the light that reaches us is filtered through the planet’s atmosphere.
The filtered light will be missing certain colors,
which are absorbed by compounds in the planet’s atmosphere.
That filtered light is referred to as a transmission spectrum.
And by looking at what’s missing, you can work out what compounds blocked it.
It’s hard enough to build telescopes that can reliably measure this spectrum,
without also having to deal with the fact that not all atmospheres
are perfectly uniform and clear.
Clouds scatter the light, obscuring the faint signals of what chemicals are floating around.
When astronomers look at WASP-67B, they don’t see a lot of water in its atmosphere.
They find that clouds best explain its absence.
It’s not necessarily that the clouds are made of water, though they could be.
It’s that the clouds, whether they’re water, methane, ammonia,
or something else, scatter the light.
But when they look at HAT-P-38b they see water, consistent with clear skies.
The other oddity they find is that HAT-P-38b should be more metallic
than it appears to be.
Now, when we say “metallic,” we have to note that to an astronomer,
any element past Helium on the periodic table is a “metal.”
So really that means that the slightly smaller HAT-P-38b should have
more elements like oxygen, nitrogen, and sodium than we observe.
Now that astronomers have figured out that these two star-crossed planets
are different, all that remains to figure out is why.
And there are really two options: It’s either nature or nurture.
Either they were formed under different conditions, meaning the amount
and type of material available in the early stellar environment was different.
Or one, or both, of them had something happen to them along the way.
Gas giant planets are thought to form far away from their stars,
but these two have orbits tighter than that of Mercury around our Sun.
So maybe as these planets fell toward their stars,
they collided with dust, debris, or small planets that changed their composition.
This is exciting in a way, because we just don’t know!
But paired studies like this, just like twin studies in human biology,
are especially useful because they narrow down the variables.
We still don’t know what happened to make the atmospheres of two twin planets
different, but the fact that they are means that our models need updating.
It turns out an atmosphere’s fate isn’t sealed with a given gravity and temperature.
Further study from telescopes like the Webb will allow astronomers to refine the
models even further to find out what other factors can predict a planet’s atmosphere.
And the nature vs nurture debate extends beyond our solar system.
And if you’re curious about how nature and nurture have played out
in your own life, you might be interested in looking into your genealogy.
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