More Questions About Life on Exoplanet K2-18b

Whenever scientists present new research showing potential biosignatures on an exoplanet, follow-up articles spread like ripples on a pond. Mainstream media usually runs with it, which shows how the issue captures people's attention. The issue of life on other worlds is a compelling one. This is what happened recently with the exoplanet K2-18b.

Science works when a community of people embracing intellectual integrity gathers, assesses, and discusses evidence. The discussion consists of papers published in peer-reviewed journals, where researchers test each other's ideas, ask new questions, and present new evidence. The adage that extraordinary claims require extraordinary evidence applies here.

Since the announcement that K2-18b showed signs of chemical biosignatures in its atmosphere, follow-up papers have questioned the idea. Professor Nikku Madhusudhan from Cambridge was the lead author of the paper that presented evidence of dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) in the planet's atmosphere.

"This is the strongest evidence yet there is possibly life out there. I can realistically say that we can confirm this signal within one to two years," Madhusudhan told the BBC. Madhusudan also pointed out that more work is needed to understand what the JWST detected in K2-18b's atmosphere.

Madhusudan's recent paper isn't the only one to present evidence of biosignatures at K2-18b. He and his colleagues published an earlier study that also detected carbon dioxide and methane on the planet.

The graph shows the observed transmission spectrum of the habitable zone exoplanet K2-18 b using the JWST MIRI spectrograph. The image behind the graph is an illustration of a hycean planet orbiting a red dwarf star. The spectrum shows tantalizing hints of DMS and DMDS, but it's complicated. Image Credit: A. Smith, N. Madhusudhan (University of Cambridge)

The newest response in the K2-18b conversation comes from a new paper titled "Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b. From a joint analysis of JWST NIRISS, NIRSpec, and MIRI observations." The lead author is Rafael Luque from the Department of Astronomy & Astrophysics at the University of Chicago. The paper has been submitted to Astronomy and Astrophysics and is available on arxiv.org.

The paper adds to the chorus of skeptical responses to Madhusudhan's initial paper.

"We found the data we have so far is much too noisy for the proof that would be needed to make that claim," said lead author Luque. "There's just not enough certainty to say one way or the other."

Peeling back the layers of this scientific issue exposes a deeper truth about science, the JWST, and current limitations in exoplanet science that aren't always discussed in the mainstream media. An important thing to understand is that molecules like DMS don't jump out of the data.

This planet is 124 light-years away, and this is where things get dicey. Even though we routinely hear of distances in the tens of thousands of light-years or more in astronomy, 124 light-years is still an extremely long distance. The JWST is extraordinarily powerful, but it's not magic.

The JWST simply gathers data agnostically, and scientists use models to find patterns in the data. There are multiple models, each with different strengths and weaknesses. The previous image of the JWST's spectra from K2-18b makes this clear. It shows the data as yellow points and the model used to explain it in blue. There's no exact match.

Detecting DMS in this case really means detecting hints of carbon and hydrogen, as study co-author Michael Zhang explains in a press release. "Anything with a carbon bonded to three hydrogens will show up at a particular wavelength," he said. "That’s what dimethyl sulfide has. But there are countless other compounds that contain a carbon and three hydrogens, and would exhibit similar features in Webb's data. So, even with much better data, it'll be hard to be sure that dimethyl sulfide is what we're seeing."

Organic compounds like methane, ethane, and propane contain carbon and three hydrogen atoms. So do compounds like methylamine, dimethyl ether, and many others.

Ethane drew the researchers' attention because it's been found in Neptune's atmosphere, where we can authoritatively rule out life. Occam's razor tells us to choose the simplest explanation over the more complex one, and in this case, ethane could be the simplest explanation for the hints of DMS in K2-18b's atmosphere.

Dimethyl sulphide (left) and ethane (right) both have carbon atoms bonded to 3 hydrogen atoms, and it's difficult to tell them apart in the data.

"Ethane has a very similar chemical structure to DMS," the authors write. They explain that it's a common byproduct of methane photolysis and is "the most abundant hydrocarbon predicted by the abiotic photochemistry simulations of K2-18 b" in other research.

"We should only introduce exotic molecules in the interpretation after ruling out molecules that we would expect to be in the atmosphere," said study co-author Caroline Piaulet-Ghorayeb.

Another criticism aimed at the detection of DMS at K2-18b concerns the number of observations. Madhusudhan's paper is based on only one set of observations. But the new paper from Luque and co-authors casts its net wider.

"We test the robustness of the proposed DMS/DMDS detections by simultaneously modelling the NIRISS and NIRSpec observations jointly with the MIRI spectrum, considering different data reductions and modelling choices," the authors write.

They also point out that previous observations with the Hubble and with the JWST, when combined with the more recent observations, weaken the case for DMS.

"We find that other generic hydrocarbons can provide an alternative explanation for the putative DMS/DMDS spectral features proposed by <previous researchers>, as demonstrated with the case of ethane (C2H6) in this work," the authors write in their paper's conclusion.

The JWST would need to observe K2-18b for several years to more definitively detect DMS. Image Credit: NASA.

The researchers say that to achieve a robust spectral detection of DMS, the JWST would need to observe K2-18b for 26 transits. Unfortunately, that could take more than six years. "We note that due to the star's position in the sky and the long orbital period of the planet, there are typically only four transit opportunities observable with JWST each year," they write.

K2-18b is an interesting planet, but we also need to remember we aren't even certain what type of planet it is. It's a sub-Neptune by mass, but may be a hycean world or a mini-Neptune with no habitable surface.

We all crave certainty, but it can be a long time coming. A definitive detection of life elsewhere would be a defining moment for humanity. The scientific debate over K2-18b shows us we're not close yet, but it also shows us the path forward.

"Answering whether there is life outside the solar system is the most important question of our field. It is why we are all studying these planets," Luque said. "We are making enormous progress in this field, and we don’t want that to be overshadowed by premature declarations."