WINERED Detects a Strong Atmospheric Outflow on the Sub-Neptune GJ 3090b

Abstract

Sub-Neptunes are the most common short-period (P<100 d) planets known. A major goal for the field is to build a comprehensive understanding of their compositions and evolutionary histories, and an important tool for achieving this goal is metastable helium (He^*) transmission spectroscopy, which traces atmospheric escape. In this paper, we report the detection of He^* in the atmosphere of the sub-Neptune GJ 3090b at high spectral resolution. We observed two consecutive transits of this planet with Magellan II/WINERED, and detected peak He^* absorptions during transit of 2.033±0.086% (23.6σ) and 1.619±0.122% (13.3σ), respectively. The first transit was affected by a stellar flare near egress while the second transit was unaffected. The signal is variable in amplitude and deeper than anticipated by the JWST/NIRISS observations of Ahrer et al. (2025). We do not observe a significant Doppler shift, and the signal has a FWHM of $25$ km s⁻¹, implicating photoevaporation. Our outflow modeling suggests that Zatm≲100× solar, as the observed He^* amplitudes are challenging to achieve in a metal-rich atmosphere. To match the muted transmission spectra from JWST and VLT/CRIRES+, GJ 3090b's lower atmosphere is likely covered by aerosols. Finally, we found that GJ 3090b has an atmospheric lifetime of ∼300 Myr, much shorter than previously reported. This adolescent (∼1 Gyr) sub-Neptune is currently in a transformative phase of photoevaporative evolution.

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BibTeX RIS

BibTeX

@misc{vissapragada2026winered,
  title = {{WINERED Detects a Strong Atmospheric Outflow on the Sub-Neptune GJ 3090b}},
  author = {Shreyas Vissapragada and Zifan Lin and Annabella Meech and Collin Cherubim and Leonardo A. Dos Santos and Aaron Householder and Krishna Kanumalla and Mercedes Lopez-Morales and Andrew McWilliam and William Misener and Ava Morrissey and Morgan Saidel and Jessica J. Spake and Johanna Teske and Nicole L. Wallack},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.14704},
  eprint = {2609.14704},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.14704},
}

RIS

TY  - UNPB
TI  - WINERED Detects a Strong Atmospheric Outflow on the Sub-Neptune GJ 3090b
AU  - Shreyas Vissapragada
AU  - Zifan Lin
AU  - Annabella Meech
AU  - Collin Cherubim
AU  - Leonardo A. Dos Santos
AU  - Aaron Householder
AU  - Krishna Kanumalla
AU  - Mercedes Lopez-Morales
AU  - Andrew McWilliam
AU  - William Misener
AU  - Ava Morrissey
AU  - Morgan Saidel
AU  - Jessica J. Spake
AU  - Johanna Teske
AU  - Nicole L. Wallack
PY  - 2026
DA  - 2026/09/13
PB  - arXiv
DO  - 10.48550/arxiv.2609.14704
UR  - https://arxiv.org/abs/2609.14704
AB  - Sub-Neptunes are the most common short-period (P<100 d) planets known. A major goal for the field is to build a comprehensive understanding of their compositions and evolutionary histories, and an important tool for achieving this goal is metastable helium (He^*) transmission spectroscopy, which traces atmospheric escape. In this paper, we report the detection of He^* in the atmosphere of the sub-Neptune GJ 3090b at high spectral resolution. We observed two consecutive transits of this planet with Magellan II/WINERED, and detected peak He^* absorptions during transit of 2.033±0.086% (23.6σ) and 1.619±0.122% (13.3σ), respectively. The first transit was affected by a stellar flare near egress while the second transit was unaffected. The signal is variable in amplitude and deeper than anticipated by the JWST/NIRISS observations of Ahrer et al. (2025). We do not observe a significant Doppler shift, and the signal has a FWHM of $25$ km s^−1, implicating photoevaporation. Our outflow modeling suggests that Z_atm≲100× solar, as the observed He^* amplitudes are challenging to achieve in a metal-rich atmosphere. To match the muted transmission spectra from JWST and VLT/CRIRES+, GJ 3090b's lower atmosphere is likely covered by aerosols. Finally, we found that GJ 3090b has an atmospheric lifetime of ∼300 Myr, much shorter than previously reported. This adolescent (∼1 Gyr) sub-Neptune is currently in a transformative phase of photoevaporative evolution.
ER  -

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