[astro-ph.EP] Observations of temperate sub-Neptunes with JWST have ushered in a new era for atmospheric characterization of small exoplanets.
In particular, the MIRI instrument provides a unique opportunity to search for molecules that are not easily accessible in the near-infrared, as demonstrated by recent mid-infrared observations of K2-18 b.
In this work, we present the first mid-infrared transmission spectrum of TOI-270 d observed using the JWST MIRI LRS (5-12 μm) instrument. By leveraging archival MIRI LRS data, we establish a new empirical relation between the detector settling timescale and the flux, which helps accurately model the spectral light curves and improve the precision of the transmission spectrum.
We find that there is notable evidence of molecular features in the MIRI transmission spectrum of TOI-270 d, favouring the presence of atmospheric absorption at lnB = 2.8-5.3 when comparing physically plausible atmospheric models with and without molecular line absorption. The data show excess absorption beyond what could be attributed to CH4 and CO2 detected previously, in line with recent near-infrared results.
Through an agnostic search for 203 species, we identify several candidate trace molecules, most of which are complex molecules, evaluate their physical plausibility, and compare them against inferences from near-infrared observations.
We also compare the MIRI spectrum of TOI-270 d to that of K2-18 b and find that random or systematic noise is unlikely to explain these observations. Future follow-up observations are necessary to definitively identify the additional absorbers beyond CH4 and CO2. These observations demonstrate the unique capability of JWST MIRI for atmospheric characterisation of temperate sub-Neptunes.
Måns Holmberg, Nikku Madhusudhan, Martin Binet, Subhajit Sarkar, Frances E. Rigby, Julianne I. Moses
Comments: Accepted for publication in ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2607.00950 [astro-ph.EP] (or arXiv:2607.00950v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2607.00950
Focus to learn more
Submission history
From: Madhusudhan Nikku
[v1] Wed, 1 Jul 2026 13:49:02 UTC (5,652 KB)
https://arxiv.org/abs/2607.00950
Astrobiology, exoplanet,
