The first discovery of extraterrestrial life will almost certainly NOT be when it visits us,
nor when we visit it. It won’t be when we intercept it’s stray embarassing TV signals. It’ll be in the
excruciatingly faint changes in the color of alien sunsets glimpsed hundreds of light
years away. Today we’re going to talk about the first such hint, why it's probably not aliens,
but why there’s still a tiny chance that it still might not not be aliens.
We have a long and storied history of almost certainly not finding aliens when some people
thought maybe we had. From the WOW signal to Oumuamua, claims of alien origins have always
given way to natural explanations once we thought harder about the phenomena. And the latest case
of a just maybe but really very much probably not detection of life in the universe is this
guy—K2-18b—the extra-solar mini-neptune whose atmosphere probably doesn’t, but just
may not not have a tiny bit of a molecule-dimethyl sulphide-that on Earth is produced only by life.
Why is this case more exciting than the other nearly detected aliens? Well it's because the method by which we did this is probably
how we’re actually going to find alien life in the first place — by observing the subtle effect of living
organisms on the starlight that sifts through the alien atmosphere as the hopefully-inhabited
planet passes across the face of its home star. That’s right seeing alien sunsets. This method and the
machine behind the current observations - the James Webb Space Telescope - might well be the
key to our first detection of extraterrestrial life, even if K2-18b proves to be barren. And let me
add that an even better reason to talk about this planet is that it's very
interesting in its own right. But I will lead with aliens because that’s what the kids seem to want.
Let’s get to know K2-18b a bit better. It was discovered as part of the Kepler
mission. Which discovered 1000s of extra-solar planets - exoplanets - by watching for the rare cases where
these planets happen to pass across the face of their home star from our point of view.
The K2-18 system was identified in 2015. It’s relatively close by, at a mere 124 lightyears.
The home star—K2-18a—is a dim red M-dwarf, almost exactly half the mass and radius of our sun,
with a surface temperature of about 3500 Kelvin, probably between 2 and 3 billion
years old. Orbiting close to this star are two known planets, K2-18b and K2-18c.
We know quite a bit about these planets just from the Kepler observations. By
measuring the dimming of the star during a transit, we get the radius of the planet. By
measuring the time between transits, we get the size and duration of its orbit.
To get the mass of the planet itself we need an extra observation to see how the planet’s
gravity tugs ever so slightly on its home star, causing it to wobble—an effect only
visible in the tiny back-and-forth Doppler wavelength shift of the star’s spectrum.
From all this we know that K2-18b 8.6 times the mass of the earth,
and 2.6 time its diameter. It orbits closer to its star than Mercury does to our sun. Its planetary neighbor,
KB-18c, orbits even closer and is a bit smaller.
Now a planet’s proximity to its star tells us whether water can reasonably exist in its
liquid form given the amount of light it gets from its star. We’ve done an episode on why
water is the best stuff in the universe in which to brew and sustain life—so for today let’s just
accept that when we find a planet it the so-called habitable zone we have cause to be interested.
And K2-18b is I’d say marginally within its stars habitable zone. Although it’s orbiting much
closer, that star is much fainter than our sun. So there might be H2O in liquid form which means their might be oceans.
OK, so how do we tell what this thing is made of? If we combine the planet’s mass with its size we get
its density. Sometimes that gives us a really good idea of what the planet is made of. K2-18b
has a density less that half that of Earth. That means it's not all rock. It probably has a rocky
interior, but there’s some sort of fluffier stuff on top of that—material that puffs up the planet’s
size so it creates a bigger eclipse crossing its star, but which doesn’t weigh as much as rock.
The two broad alternatives could be a massive planet-wide ocean or a huge amounst of gas or
perhaps both. We’ll come back to the details on this. Now planets within this mass range are sometimes called
super-Earths, sometimes mini-Neptunes—depending on whether they’re more rocky or more gassy.
Given the possibility of liquid water, astronomers were excited to see what
K2-18b is really made of. In 2019, we pointed the Hubble space telescope at
the planet to find out what its atmosphere is made of. Now keep in mind that Hubble can in
no way see this planet. Its faint home star is still vastly brighter than the planet. But we
can still see the signature of the planet’s atmosphere in the light of the star itself.
When you see our Sun’s light we're seeing it though the filter of our own atmosphere.
A lot of the shorter, bluer wavelengths of light are scattered away making it
more yellow. If you could tune your eyes to scan through infrared wavelengths,
you’d see that the sun goes dark over certain wavelength ranges where much of its light
is blocked by molecules in the atmosphere. Molecules like H20 and C02 tend to vibrate
at particular frequencies. When a photon in these frequency ranges passes near such
a molecule it’s very likely to get sucked up to power one of these vibrations. The
result is that the spectrum of sunlight viewed from the Earth’s surface has these deep dark gaps.
So if we can do this for Earth then why not for other planets? Well,
because we aren’t on the surface of those planets. But remember, these Kepler planets
by definition pass right in front of their star. Most of the photons that the star shoots at us
miss that planet entirely. A small fraction of them are completely blocked by the planet,
which is how we know the planet is there. And a yet much smaller fraction skim the edge of the planet,
passing through its atmosphere and traveling to us. And a yet tinier fraction of those photons still try to pass
through the atmosphere but happen to have just the right frequency to be blocked by
molecules in that atmosphere. So by comparing the starlight that passes through the atmosphere to
the starlight that doesn’t, we can figure out what molecules this alien atmosphere contains.
So yeah, although we don’t see this directly we’re looking at the rim of reddened light sifting
through the planetary atmosphere—the sum of all its alien sunsets and sunrises at that moment.
This is not exactly a straightforward process. On one of K2-18b’s transits,
the Hubble Space Telescope observed this spectrum. By comparing this to how the star
looks when the planet isn’t in the way it was possible to identify some divots—absorption
features—caused by some of the light getting trapped as it filtered through the planet’s
atmosphere. The researchers noticed a signature consistent with the atmosphere
containing water vapor. And this catapulted K2-18b to the then-most-famous exoplanet in the universe.
I mentioned that this planet had an ambiguous composition—perhaps it was a super-Earth—a large,
rocky world with a giant ocean. Or perhaps a mini-Neptune—in which case a lot more of its
mass would be in an enormous atmosphere dominated by hydrogen gas. The former would perhaps be more
exciting for the alien hunters because it’s closer to the one place in the universe that
we know life did form—Earth. And the tentative detection of water made that a bit more likely.
But this water detection was indeed tentative. Hubble is only able to see a little way into the
infrared spectrum, and so doesn’t catch the full range of absorption features of water.
Enter the James Webb Space Telescope, which, by design, is sensitive to
wavelengths 10 times longer than Hubble—it takes us deep into the infrared spectrum,
and gives us access to a much broader range of molecular signatures.
In September of this year, astronomers reported that they had made their first measurement of the
atmosphere of K2-18b using the JWST. Using two of the infrared spectrometers, the Near InfraRed
Imager and Slitless Spectrograph (NIRISS) and the Near-InfraRed Camera (NIRCam), astronomers found
an abundance of methane and carbon dioxide in the atmosphere of K2-18b. Those aren’t too surprising,
there’s plenty of CO2 on Mars and Venus and we’ve seen methane on Saturn’s moon Titan.
But what about the water? Well, it turns out that there isn’t any. At least JWST found no evidence
of water in the upper atmosphere of K2-18b. The absorption features detected by Hubble turned out
to be probably methane. Now before you roll our eyes and click to the next video, we’re not done with this planet yet.
We’ll come back to the water issue. But first - the reason K2-18b really got a bunch of media
attention is that JWST saw a hint of something potentially even more interesting than water.
Webb may also have spotted a funny little molecule called dimethyl sulfide—DMS. It’s basically a
couple of methanes stitched together with a sulfur atom. It smells like cabbages. In fact, it’s the
molecule produced when you cook cabbage—although currently civilization of cabbage fanatics is currently not
the favoured model for DMS in K2-18b. This molecule is also a byproduct of bacterial
and phytoplankton metabolism. On Earth there are no significant non-biological sources of dimethyl
sulfide—so if its present on our exo-world, perhaps we have our first alien biosignature.
Now before you crack open that when-we-find-aliens bottle of expensive champagne that you put away
years ago, let me add a note of extreme caution. The detection of dimethyl sulfide is at the
1-sigma level. Normally to be confident in a detection we’d want 5-sigma - which would mean
that the detection is clear enough that it could only arise by chance fluctuations in the data 1
in 50,000 times. For a 1-sigma detection that’s a 32% chance of that the signal could arise from random
fluctuations. 1-sigma detections of very unlikely things show up all the time, and most of them
turn out to be false positives. That’s the case here. There probably isn’t any DMS, but there also
might be some. And the latter is exciting enough that we should keep looking. And exciting enough for me to dig
a little deeper into exo-atmospheric chemistry to understand what K2-18b might really be like.
Now we should also do this because K2-18b’s atmosphere is interesting in its own
right. Observing molecules at the top of an atmosphere is only the first step in
understanding its nature. We can make models of an atmosphere that include the chemical composition,
density, pressure, and temperature all the way from the surface to space,
plus the effect of varying radiation from the star plus interactions with the surface,
and so on. With a given model we can compute the chemical reactions that can occur at different
layers in the atmosphere, and how material gets transported between them. So if we observe, say,
a bunch of methane and CO2 in the upper atmosphere as we have in K2-18b, then the atmospheric
model we come up with has to tell us how those molecules got there in the measured quantities.
Many atmospheric models were tried for K2-18b, and it so happens that the one that seems to best
match the methane and CO2 abundances includes … wait for it … a planet-wide ocean of liquid
water beneath an atmosphere of mostly hydrogen. In that case, we don’t necessarily expect H2O to
make it to the upper atmosphere where we can see it. It’s reasonable to expect that it condenses
and rains out in deeper layers leaving the upper atmosphere dry, just like the stratosphere on earth.
Another interesting piece of evidence to do with what we didn’t see with the Webb
spectrum. We didn’t see any ammonia. Ammonia is expected to be found in hydrogen rich
atmospheres of planets like this if the atmosphere is actually big and fluffy. If
there’s a huge ocean then we might expect less ammonia because there would be less atmosphere
to produce that ammonia in, and because the ocean can extract ammonia from the atmosphere.
But remember, these are only models. Some researchers claim that a water world beneath
a hydrogen atmosphere is the best fit to the observations, but there are other models that
could also fit the data, albeit not as neatly, and we also can’t know if we’ve exhausted the space of
models—the range of possible atmospheres that could have led to the observations.
Regardless, astronomers have came up with a name for this type of world—an ocean world beneath
a hydrogen atmosphere—we call these now Hycean planets. And K2-18b’s status as the best observed candidate
for this new planet type makes it exciting regardless of whether it harbors life.
Now that we have an idea of what the atmosphere of the planet might be like,
let’s think about whether it’s the sort of place we can even expect life.
As you travel down in an atmosphere both the pressure and temperature get higher,
and on K2-18b the pressure might get so high that it’s beyond the critical point of water - the
point where the liquid and gas phases are no longer distinct. So depending on the mass of
the atmosphere, there might even be no phase transition between the atmosphere and ocean,
just a smooth increase in density as you go down.
These hydrogen-atmosphere’d worlds, especially those with giant oceans,
are expected to reach pretty extreme pressures down deep. But if you put water under too much
pressure it’ll form ice at any temperature. A sufficiently deep ocean on a hycean world should
transition to a layer of solid ice before you get to the rocky interior. And that forms a hard block
between the water and the planet, preventing the exchange elements with the upper layers. The
pressure of the ocean alone could lock down plate tectonics. Together, these effects would stop
the planet from having the robust carbon cycling processes needed to support a stable biosphere.
It’s also hard to imagine where on such a world life would first develop.
On Earth the favoured candidates are deep-sea hydrothermal vents,
or perhaps tidal pools on the rocky ocean shore. Basically, a stable interface between
solid and liquid with a strong energy gradient and ample raw material resources. But the “best
fit” model of the interior of K2-18b doesn’t have these ideal cauldrons for brewing up life.
Okay, so what next?
The first and obvious thing is to stare at it more with JWST. The
current observations, with the NIRISS and NIRCam instruments, only cover near-infrared wavelengths
of about 1 to 5 microns. JWST’s other instrument, the Mid-Infrared Interferometer - MIRI - goes all
the way to 28 microns, the exact part of the spectrum it needs to see to disentangle the
spectra of methane from dimethyl sulfide, and should be expected in the fairly near
future. We just need to wait for it to pass in front of its home star one or two more time. By next year some time,
it should be clear whether or not K2-18b has DMS, and who knows what other surprise
molecules might show up in the mid-infrared. If we do confirm the DMS then we need to work a
lot harder to rule out all non-biological processes that could have produced it.
Even if K2-18b doesn’t turn out to harbour life, this is a really exciting time for
exoplanetary astronomy. We now have a way to probe planetary atmospheres with more
sensitivity than ever before. With thousands of known exoplanets and the best infrared telescope
ever built now working on them, we might be close to answering fundamental questions
about the types of planets and planetary systems that populate our galaxy. And,
perhaps, about the formation and abundance of life in the universe. It’s never aliens,
but we’ll keep looking until it is. At which point you can pop your champagne and order your new “ok,
it was aliens” t-shirt, which we hope will one day grace the merch store for PBS Space Time.