Transmission Spectroscopy Along a Bright Star occultation by Jupiter: A Proxy for Exoplanets Atmospheric Characterization

In this work, we retrieved transmission spectra of Jupiter's atmosphere, using high resolution observations performed with the iSHELL infrared high resolution spectrograph at the IRTF telescope (Maunakea observatory). In this context, we have applied transmission spectroscopy techniques to the observations obtained. We took advantage on previous studies in this scope in order to prepare our research protocol (Drossart et al. 2000, Raynaud et al., 2003, Pasachoff et al. 2011, Tanga et al. 2011, Widemann et al. 2012). Our aim was to produce a case study where we compare the retrieved spectroscopic transmission observables on Jupiter with the datasets obtained by space probes and ground-based observations, which will consist in a valuable template and unique calibration opportunity.

The transmission spectra obtained took advantage on several high-resolution spectra in the infrared along ingress and egress of the bright star that was occulted by Jupiter. Due to Jupiter’s brightness and our desire to detect the occultation through the higher layers of Jupiter’s atmosphere (French & Souami, 2023), we chose to use an H2 filter (centered on 1.65 microns), where there are significant methane bands to suppress Jupiter’s brightness.

On the October 13th 2025 a rare, time-critical, stellar occultation by Jupiter was visible from IRTF during night time. This was the brightest star (Kmag = 5.13) to be diametrically occulted by Jupiter, as seen from Earth, until 2031 (French & Souami, 2023). This unique stellar occultation could enable a study of Jupiter's atmosphere analogous to transit spectroscopy studies, a crucial method to study exoplanet atmospheres using transmission spectroscopy techniques for probing the atmosphere's upper layers and as a proxy for exoplanets atmosphere characterization - in direct synergy with the upcoming ESA Ariel space mission. While stellar occultation studies are relatively standard in spacecraft missions to other planets, much rarer ground-based observations of stellar occultations remain the sole way of observing these events in high spectral resolution.

Hence, this consisted of a remarkable opportunity - unique for the remainder of this decade - to calibrate this technique in a solar system target using High Resolution spectroscopy in Infrared wavelengths. This is because the entire duration of this upcoming stellar occultation by Jupiter was visible from MaunaKea Observatory, enabling the use of IRTF/iSHELL high resolution spectrograph to observe this time critical event. We successfully used IRTF/iSHELL to observe the occultation ingress and the Occultation Egress of the star, for a total of 90 minutes centered in the predicted moment of the occultation start, in order to get baseline measurements of both the star and Jupiter within and outside of the occultation, providing valuable transmission spectroscopy data for the scientific community preparing the upcoming ESA Ariel space mission.

The observation periods: Occultation Ingress: 11:35 to 13:05 of October 13, 2025, in UTC, Occultation Egress: 14:45 to 16:15 of October 13, 2025, in UTC. Stellar occultation details: star occulted: HIP 37442, Star Coordinates (ICRS at J2000 epoch): RA: 07 41 12.72000; DEC: +21 26 47.82082, Star Apparent Magnitudes: Kmag = 5.13, Hmag = 5.32, Vmag = 7.85, Jupiter Center Geocentric Coordinates: RA: 07 41 12.70846; DEC: +21 26 46.74017 Mid Occultation Time: 2025-10-13 14:00:32.12 (UTC) Occultation Time: 2h48min (168 min).

Transmission spectroscopy probes the atmospheric limb of a transiting planet. In this Jupiter observational project our goal was to retrieve the transmission spectra coming from the high layers of Jupiter's atmosphere high layers and detect chemical compounds on from it, as we did in our previous paper using on this subject related with high resolution spectra along the 2012 Venus' transit (Branco et al., 2024). We focussed our research in the retrieval of: NH3, C2H2, H20, but we also explored the presence of other atmospheric minor compounds. We applied exoplanets tools to this observation, such as on this case as the cross-correlation function (CCF) and line-by-line techniques.

In fact the relevance of this project is twofold: one related with the rare possibility of calibrating exoplanets' tools for atmospheric research, since we could retrieve the same observables we obtained in this project from previous measurements with other techniques; and on the other hand, for Jupiter's itself atmospheric research, since we used Cross Correlation Functions (CCFs) that allowed us to look for minor compounds in the Jupiter' atmosphere upper layers.

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