Cool, Cloudy Nights on an Ultrahot Neptune: A Panchromatic NIRISS/NIRSpec Transmission Spectrum of LTT 9779 b

@misc{stamer2026cool,
  title = {{Cool, Cloudy Nights on an Ultrahot Neptune: A Panchromatic NIRISS/NIRSpec Transmission Spectrum of LTT 9779 b}},
  author = {Sarah Stamer and Diana Dragomir and Annabella Meech and Joshua D. Lothringer and Reza Ashtari and Jonathan Brande and Vivien Parmentier and Ian J. M. Crossfield and Tansu Daylan and Suman Saha and K. Angelique Kahle and Kevin B. Stevenson and David R. Coria and Jared E. J. Splinter and Nicolas B. Cowan and Emma Esparza-Borges},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.36206},
  eprint = {2609.36206},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.36206},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Cool, Cloudy Nights on an Ultrahot Neptune: A Panchromatic NIRISS/NIRSpec Transmission Spectrum of LTT 9779 b
AU  - Sarah Stamer
AU  - Diana Dragomir
AU  - Annabella Meech
AU  - Joshua D. Lothringer
AU  - Reza Ashtari
AU  - Jonathan Brande
AU  - Vivien Parmentier
AU  - Ian J. M. Crossfield
AU  - Tansu Daylan
AU  - Suman Saha
AU  - K. Angelique Kahle
AU  - Kevin B. Stevenson
AU  - David R. Coria
AU  - Jared E. J. Splinter
AU  - Nicolas B. Cowan
AU  - Emma Esparza-Borges
PY  - 2026
DA  - 2026/09/28
PB  - arXiv
DO  - 10.48550/arxiv.2609.36206
UR  - https://arxiv.org/abs/2609.36206
AB  - Studies of planets that reside in the hot Neptune desert allow us to gain insight into how their atmospheres survive the intense flux from their host stars. One such member of that population, LTT 9779 b, is unique in that it has a confirmed atmosphere across various multi-wavelength studies, although results regarding the planet's atmospheric content differ. We present a panchromatic (λ= 0.63 − 5.165 μm) JWST transmission spectrum of LTT 9779 b's atmosphere, including a re-reduction of previously obtained NIRISS/SOSS data and a reduction of recent NIRSpec/G395H data. Through atmospheric retrievals, we find muted spectral features consistent with the presence of clouds and a low terminator temperature (∼560 K for free chemistry, and ∼910−950 K for equilibrium chemistry), with a range of possible atmospheric metallicity values dependent on atmospheric temperature and cloud coverage. Forward modeling suggests that, if clouds are present, small sub-micron particle sizes, moderate-to-high sedimentation efficiency (f_sed = 1 − 3, corresponding to a relatively compact vertical cloud extent), low particle column densities, and high eddy diffusion coefficients (k_zz ≥ 10^10 cm^2s^−1) are preferred. Probing silicate cloud absorption features in the mid-infrared is needed to confirm the cloud species, allowing us to break the cloud-metallicity degeneracy that is present in this panchromatic transmission spectrum.
ER  -