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.