Matt Nixon

I had a wonderful time speaking with @startswithabang.bsky.social and @luiswel.bsky.social about biosignatures, exciting sub-Neptune science, and why we should care about studying exoplanets!

Check out our podcast episode here:

Thanks for the shout-out!

A sprawling, textured field of galaxies scattered across the deep black of space. It is filled with the delicate smudges and glowing cores of galaxies of many shapes, sizes and colors, as well as the bright multi-colored points of stars. The image focuses on a collection of interacting galaxies connected by delicate streams of stars. At top center lies a large elliptical galaxy that is dense and smooth, like a polished stone glowing with golden light. Like delicate spider silk or stretched taffy, these stellar bridges link the large elliptical to the few larger galaxies beneath, evidence of past collisions. All throughout the image, thousands of galaxies gather in clusters or are spread throughout, like glittering gems strewn on a table. Some are sharp-edged and spiral, like coiled ribbons; others round and diffuse, like polished pebbles. Still others are just smudges of various colors against the black of space. The background is peppered with pinpoint stars in reds, yellows, and blues, crisp against the velvet black.

A cosmic tapestry of glowing tan and pink gas clouds with dark dust lanes. In the upper right, the Trifid Nebula resembles a small flower in space. Its soft, pinkish gas petals are surrounded by blue gas, and streaked with dark, finger-like veins of dust that divide it into three parts. It radiates a gentle, misty glow, diffuse and soft like the warmth of breath on a cold hand. To the lower left, the much larger Lagoon Nebula stretches wide like a churning sea of magenta gas, with bright blue, knotted clumps sprinkled throughout where new stars are born. Both nebulae are embedded in a soft tan backdrop of gas that is brighter on the left than on the right, etched with dark tendrils of dust and sprinkled with the pinpricks of millions of stars.

A sprawling, textured field of galaxies scattered across the deep black of space. It is filled with the delicate smudges and glowing cores of galaxies of many shapes, sizes and colors, as well as the bright multi-colored points of stars. To the lower left is a region filled with the hundreds of golden glittering gems of a distant galaxy cluster. In the foreground, below and right of center, two blue spiral galaxies look like eyes beneath the entangled mass of a triple galaxy merger in the upper right. A few bright blue points of foreground stars pierce the glittering tapestry. All throughout the image, thousands of galaxies gather in clusters or are spread throughout, like glittering gems strewn on a table. Some are sharp-edged and spiral, like coiled ribbons; others round and diffuse, like polished pebbles. Still others are just smudges of various colors against the black of space. The background is peppered with pinpoint stars in reds, yellows, and blues, crisp against the velvet black.

Introducing...your sneak peek at the cosmos captured by NSF–DOE Vera C. Rubin Observatory!

Can you guess these regions of sky?

This is just a small peek...join us at 11am US EDT for your full First Look at how Rubin will #CaptureTheCosmos! 🔭🧪

#RubinFirstLook
ls.st/rubin-first-look-livestream

I agree with you that saying K2-18 b “can’t” have an ocean or “isn’t” an ocean world is a stretch - we can’t totally rule it out with the present data, but it does appear that Neptune-like or gas dwarf models are consistent with what we know about the planet and require much less fine-tuning

It’s tricky to maintain a thin (~a few bar) H2 atmosphere over Gyr timescales near the intense XUV environment of an M star. This aspect is under-explored as far as I’m aware but is mentioned by e.g. Wogan et al. 2024. You definitely need a lot of model fine-tuning to maintain a thin H2 atmosphere

Planetary albedo is limited by the above-cloud atmosphere: Implications for sub-Neptune climate Energy limits that delineate the `habitable zone' for exoplanets depend on a given exoplanet's net planetary albedo (or `Bond albedo'). We here demonstrate that the planetary albedo of an observed exo...

The planet would also need a high albedo (>~0.6) to prevent a runaway greenhouse scenario where a liquid ocean is unsustainable. Recent analysis of the JWST NIR spectrum suggests that the albedo of K2-18 b is lower than this (arxiv.org/abs/2504.12030, good paper kinda lost in the DMS noise I think)

There are a number of factors which challenge the ocean world hypothesis for K2-18 b. Due to its bulk density, the planet would need to be ~90% H2O by mass to sustain a liquid ocean. Based on analysis of the outer solar system, it’s tricky to see how a planet would form with >>50% H2O

Definitely some kind of space needed, otherwise you’d end up with 51 Pegasib and 55 Cancrie

Congratulations!!! I enjoyed (virtually) attending your thesis defence btw, great talk!

A Systematic Search for Trace Molecules in Exoplanet K2-18 b The first transmission spectrum of the habitable-zone sub-Neptune K2-18 b with JWST has opened a new avenue for atmospheric characterisation of temperate low-mass exoplanets. The observations led to i...

Good to see Pica-Ciamarra, Madhu+ engage with our work and correct claims from last month. Following our recommendation to expand the model space, they now find: ‘No molecule reaches 2σ significance when allowing for an offset…’ 𝐍𝐨 𝐛𝐢𝐨𝐠𝐚𝐬𝐞𝐬 𝐚𝐟𝐭𝐞𝐫 𝐚𝐥𝐥! Sec. 4.4 arxiv.org/abs/2505.10539 🔭 #exoplanets

Image credit: Mark Garlick

Within the context of the interdisciplinary PRELIFE consortium me and Wim van Westrenen are advertising a joint PhD position on the surface water levels and atmospheric composition of the earliest Earth: www.formingworlds.space/phdposition2.... Deadline: 10th June. Please share! 🌋🌏🧬

Thanks so much for this great summary! Also, happy birthday for yesterday!!! Sorry I didn't realise until today 🎂

A real pleasure to chat about our latest work and all things exoplanet atmospheres!

The Challenges of Detecting Gases in Exoplanet Atmospheres Claims of detections of gases in exoplanet atmospheres often rely on comparisons between models including and excluding specific chemical species. However, the space of molecular combinations availabl...

If you're interested in the gory details of all 90 hydrocarbons we tested, or the subtleties of Bayesian statistics, check out the full paper which we have submitted for peer review: arxiv.org/abs/2504.21788

But in the mean time, we need to take a careful approach to interpreting the spectra that we have, and keep in mind the limitations of our models as well as the impact of what possibilities we do and don’t consider. That way, we can make sure we’re building towards truly understanding these worlds.

So, if we can't confirm the presence of propyne, or indeed anything, in the atmosphere of K2-18 b from this spectrum alone, what do we do? Of course, more observations would help, both to make sure there's a real signal from the planet, and to disentangle the different gases that could explain it.

JWST transmission spectrum of K2-18 b, with models including methane, carbon dioxide, and propyne. While this model can explain the data, the signal to noise ratio isn't good enough to uniquely identify these or any other chemical species.

The gas that came closest to explaining the observations out of everything we tested was propyne (C3H4), a hydrocarbon found on Neptune that hadn’t been tested before for K2-18 b. But it still fell far short of what we would consider a detection.

Table of different hydrocarbons, showing how many of them can be "detected" on K2-18 b, but only if you compare them to a limited model with a small number of possible gases.

In fact, by those same Bayesian metrics mentioned earlier, we found that we could “detect” a whole range of gases, as long as you only compared them against a model with most other gases excluded. But if you test all of them together, those supposed detections vanish, even for DMS and DMDS.

JWST transmission spectrum of K2-18 b, with lots of models that can all provide a reasonably good explanation of the data.

We found that, due to the small signal size and high measurement uncertainty, anything from a collection of gases to random noise can explain the data. There just isn’t enough information in the spectrum to reach strong conclusions. But that only becomes clear if you test a large set of models.

Absorption cross-sections of several chemical species, including dimethyl sulphide (DMS), dimethyl disulphide (DMDS), and a number of hydrocarbons.

However, that initial analysis only considered a small set of chemical species in their atmospheric model. So we tried a range of alternative models, and considered dozens of chemical species that weren’t explored in previous work, some of which we might expect to see on this planet, others less so.

JWST transmission spectrum of the sub-Neptune K2-18 b.

As a case study, we investigate recent claims of a potential biosignature detection on K2-18 b. For K2-18 b, previous work found that a model including DMS and DMDS – gases proposed as potential biosignatures - outperformed a model without those chemicals, according to a Bayesian model comparison.

Diagram showing how the apparent significance of a "detection" can change depending on how many alternative explanations you build in to your model.

In exoplanet atmospheres, we often use Bayesian statistics to figure out if we have detected a gas. This is a powerful tool, but it comes with caveats. We can use it to compare the performance of two models, but just because one outperforms the other, doesn’t mean that either is necessarily “right”.

However, we're naturally driven to test the limits of what our telescopes can see. For JWST, this means observing small planets like sub-Neptunes, and trying to spot gases with weaker spectral signatures. When we're close to these limits, we have to be careful in how we interpret tentative signals.

Spectrum of the atmosphere of a giant exoplanet, WASP-39 b, showing a strong carbon dioxide feature.

To start, it’s important to recognise the amazing observations of exoplanet atmospheres we’re getting from JWST. This has allowed us to find strong evidence of several chemical species that were really hard to detect in the past, like methane and carbon dioxide.

Just shared a new paper on the arXiv, led my @luiswel.bsky.social and me, on the challenges associated with detecting gases in exoplanet atmospheres. As the field pushes towards new and exciting opportunities, we thought it was time to talk about what it really means to “detect” something!

Thank you!!!

Thank you very much!!! 🪐🪐🪐

So excited to hang out in the desert and do cool science with @luiswel.bsky.social and @sagnickastro.bsky.social!

Thanks, so excited!!