![Planets that will be observed with MIRI in transit, in JWST Cycles 1-5. As mentioned in section 5, we see a wide variety of planet sizes and equilibrium temperatures. The currently published planets are shown in black dots, and listed in Table 1 and Figure 1. We have ommitted the white dwarf planet WD 1856 b (Teq = 160K, Rp = 10.2R⊕, cycle 4), as well as the disintergrating planets K2-22 b (see section 2.1) and BD+05 4868 A b (Teq = 1820K, unknown Rp, cycle 4). -- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/09/A-Homogeneous.png?fit=1200%2C676&ssl=1)
[astro-ph.EP] The launch of JWST has enabled unprecedented atmospheric characterization of exoplanets. In particular, the JWST-MIRI instrument in the mid-infrared enables probing of molecular species that are harder to detect in the near-infrared.
This has come to the fore with the first MIRI-LRS observation of a candidate hycean world, K2-18 b, with potential signs of dimethyl sulfide in its atmosphere.
The statistical significance of this result has been debated in subsequent works, arguing instead for the possibility of random noise, instrumental systematics, or a different molecule. The same applies to similar spectra reported for several other exoplanets using MIRI-LRS and interpreted with similar retrieval architectures.
In this work, we distinguish between these possibilities with a homogeneous survey of MIRI-LRS transmission spectra of 10 diverse exoplanets, from temperate sub-Neptunes to hot rocky planets and gas giants.
We first perform a comparative assessment of the spectra, considering fits with featureless models and pairwise correlations, followed by extensive atmospheric retrievals. With the retrievals, we search for over 150 trace species in the atmosphere of each planet.
We find hints of complex molecules in the temperate sub-Neptunes and candidate hycean worlds K2-18 b, TOI-732 c and TOI-270 d, similar to previous works, but not in the other seven planets. Our results indicate that the observed spectral features are likely due to molecular absorption rather than instrument systematics or noise, and underscore these temperate sub-Neptunes as a unique class of planets.
More observations are required to robustly identify the specific molecules and constrain the underlying chemical processes, biotic or abiotic.
Martin Binet, Nikku Madhusudhan, Måns Holmberg, Subhajit Sarkar
Comments: Accepted for publication in ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2609.05264 [astro-ph.EP] (or arXiv:2609.05264v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.05264
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Submission history
From: Madhusudhan Nikku
[v1] Fri, 4 Sep 2026 15:23:39 UTC (3,188 KB)
https://arxiv.org/abs/2609.05264
Astrobiology,
