Possibly the Biggest Discovery Webb Has Ever Made?

Article September 16, 2023
An artist’s illustration of the exoplanet K2-18b with its sun in the background. Credit: NASA/CSA/ESA/J. Olmstead (STScI)/N. Madhusudhan (Cambridge University)
An artist’s illustration of the exoplanet K2-18b with its sun in the background. Credit: NASA/CSA/ESA/J. Olmstead (STScI)/N. Madhusudhan (Cambridge University)

Like this week’s post was going to be about anything else! I was way too excited about this to possibly want to expound upon any other topic. I am personally crossing my fingers that this news will wind up being one of the most significant discoveries in the history of humanity—that we just found our first evidence of life on another planet!

For those who don’t have the name “K2-18b” memorized already, the TL;DR is that on September 11 NASA announced that the James Webb Space Telescope may (or may not) have found evidence of life on a planet with that designation. What it found could be only rather interesting, or it could be one of those moments that goes down in science history along with humans walking on the Moon or figuring out the structure of DNA. Let’s take a look at what Webb saw, how the telescope saw it, and what it might mean.

 

Looking at Planetary Atmospheres

If you happen to think about the James Webb Space Telescope, chances are that what comes to mind are the incredibly lovely images it captures. Personally, I nearly cried when I saw its image of the Pillars of Creation because it was so beautiful (my exact wording when I sent the picture to the Planetarium team was “Webb released the most beautiful picture of space ever taken in the history of telescopes. That’s it, everyone can go home now.”).

But the cameras aren’t the only trick Webb has up its sleeve…er…mirrors. It also has several spectrographs. These are instruments that split light up into its component colors, similar to how sunlight passing through a prism will get broken up into a rainbow—but more complicated than that. All elements and molecules have unique spectrographic signatures. 

The unique spectral patterns produced by hydrogen and helium. Credit: Ranjithsiji
The unique spectral patterns produced by hydrogen and helium. Credit: Ranjithsiji

 If you look at the spectrum of hydrogen, it will produce a pattern of lines that is unique to hydrogen and completely different from the pattern you will see if you look at helium. This is how astronomers can figure out what something is made from just by looking at the light from it.

At a basic level what Webb did with this exoplanet was look at it when it passed in front of its star. Some of the starlight that reached Webb passed through the planet’s atmosphere to reach the telescope. When Webb passed that light through its spectrographs, the imprint of the molecules and elements of the planet’s atmosphere was there.

 

K2-18b

Let’s take a moment to meet the planet that might go down in history. It was discovered by the Kepler Space Telescope (RIP you beautiful planet hunter) in 2015 during the second phase of its operation, which was known as K2. This was the 18th planetary system found during that operation, so the star was designated K2-18 and the planet K2-18b (astronomers, while outrageously creative in some fields, are too practical for their own good in others). 

An artist’s illustration of the Kepler Space Telescope. Credit: NASA
An artist’s illustration of the Kepler Space Telescope. Credit: NASA

The star, K2-18, is a red dwarf, very different from our Sun. It’s under half the Sun’s size and much, much cooler. It can be found about 110 light years away from us in the direction of the constellation Leo. The planet, K2-18b, orbits a mere 13 million miles from the star (which I realize sounds like a big number to us puny humans used to our puny Earth-based distances, but to put it in perspective, Mercury is 29 million miles from the Sun). That is very close for a planet to be to a star, but it’s also a very cool star. That means K2-18b is actually in its sun’s habitable zone—the region around a star where temperatures could be right for liquid water to exist, if all other circumstances allow.

K2-18b is a very different world from Earth. For one thing, it’s a lot bigger, what we in the biz call a super-Earth. It’s twice Earth’s size with over eight times its mass. This is a size of planet is actually very common out in the cosmos but that we happen to lack in our solar system, which can make studying them both extra-interesting and extra-tricky without a local example to consider.

K2-18b first made headlines in the astronomical community back in 2019, when a team used Hubble to take a look at the atmosphere and found evidence of water vapor. At the time this made K2-18b the smallest world we’d managed to find evidence of water vapor on, and being a rocky, habitable zone planet made the presence of water vapor particularly interesting. All this set it up to be a prime observation candidate for Webb and its massive mirrors.

 

The Discovery

When Webb looked at the planet’s atmosphere, it actually didn’t detect water vapor, but that doesn’t mean it didn’t find evidence of water. What it did see was plenty of methane and carbon dioxide. The presence of these two gasses without also finding a lot of ammonia suggests something very specific to planetary scientists. 

The spectrographic pattern of the atmosphere of K2-18b as seen by the James Webb Space Telescope. Credit: NASA/CSA/ESA/R. Crawford (STScI)/J. Olmstead (STScI)/N. Madhusudhan (Cambridge University)
The spectrographic pattern of the atmosphere of K2-18b as seen by the James Webb Space Telescope. Credit: NASA/CSA/ESA/R. Crawford (STScI)/J. Olmstead (STScI)/N. Madhusudhan (Cambridge University)

Folks who study exoplanets like to imagine what kind of planets they may one day be able to discover. One such theoretical type of planet was dubbed “Hycean” (for hydrogen-ocean) by a team from Cambridge back in 2021. Such a planet would likely be a super-Earth with an atmosphere rich in hydrogen covering a global ocean. If you had asked me two weeks ago what an atmosphere with methane and carbon dioxide but no ammonia meant, I would have said smelly poison. It’s still that, but apparently it’s also the exact kind of atmospheric signature you’d find on a Hycean planet. K2-18b, therefore, is likely the first Hycean world we’ve ever confirmed, and it’s probable that its surface is covered in water.

On any other day that would be the most incredible piece of news! Webb discovered an ocean world! Since life as we know it needs liquid water to exist, one of the first steps to finding it in non-terrestrial places has been to search for the water. And here was a habitable zone world suspected to be covered in a global ocean! Seriously, that news alone would normally have me pin-balling obnoxiously off the walls of the Planetarium office (have I mentioned that I have very tolerant co-workers?).

But in this case, that’s not the end of what Webb found.

A model of the molecule dimethyl sulfide, which Webb found hints of in the atmosphere of the exoplanet K2-18b. Credit: Benjah-bmm27
A model of the molecule dimethyl sulfide, which Webb found hints of in the atmosphere of the exoplanet K2-18b. Credit: Benjah-bmm27

Mixed in with the signatures of methane and carbon dioxide in Webb’s atmosphere was a hint of another molecule: dimethyl sulfide, or DMS. And this was where I got stupidly excited, because we also find DMS on Earth, where it can only be produced by life, mostly by phytoplankton in our oceans. We know of no processes on Earth where DMS can be produced without life. This makes DMS a biosignature—scientific evidence of life. And we just detected it in the atmosphere of an exoplanet. An exoplanet with a global ocean of liquid water.

Take that in for a moment.

 

Deep Breath

You might be wondering why astronomers aren’t popping champagne and screaming from rooftops around the world that we just confirmed the presence of life on another planet. Well, it’s because we haven’t. Not yet anyway. My personal god, Carl Sagan, once said that “extraordinary claims require extraordinary evidence”, and the claim of alien life would be one of the most extraordinary ever made. And so far, no one is making it.

Webb may have found evidence of DMS, it was not a strong detection, at least by the standards professional scientists like to use. The team who made the discovery call it “marginal”, and say there’s “a non-negligible chance for DMS being present in the atmosphere”. Suitably cautious language considering the implications of the claim, the fact that the detection was not 100% certain. and the fact that K2-18b’s theoretical ocean could turn out to be hostile to life.

Even so, if you read between the extremely dry and jargon-heavy lines of the discovery paper, you can sense the excitement being determinedly tamped down. Every article I’ve read on the subject from folks in the space news community has been very measured and thoughtful and guarded about how they are covering this news. Most didn’t even mention the DMS detection in the headlines. This is the right and responsible way to do it.

But you know what? This is my blog and we may have just possibly discovered an extraterrestrial biosignature on a water-covered exoplanet, so I’m going to go ahead and scream about it for a while.

 

What’s Next?

Confirming that DMS signature, obviously! Webb is going to be looking at K2-18b again in another wavelength of infrared light to see if it can get a better view of this molecule. Of course even if we can 100% confirm the presence of DMS in the atmosphere, it would not with complete certainty indicate the presence of life on this world. But it sure would be a very strong suggestion that it’s there, and that would be good enough for me.

It could all turn out to be nothing. Or perhaps we’ve finally answered one of humanity’s oldest questions and discovered that we’re not alone in the universe after all.

Take that in for a moment.