Exoplanets, -moons, -comets

Spinning Out Of Focus: The Challenge Of Rotational Line Broadening In Exoplanet Reflection Spectroscopy

By Keith Cowing

Status Report

astro-ph.EP

May 22, 2026

Filed under astro-ph.EP, astrochemistry, astronomy, CARMENES, exoplanet, imaging, KELT-9, Spectroscopy

Spinning Out Of Focus: The Challenge Of Rotational Line Broadening In Exoplanet Reflection Spectroscopy — Declination of the 50 targets presenting the largest RSM′ values as a function of their respective score. The ten most favourable targets are annotated with their names. The areas of the data points scale with the planetary radii. For reference, an exemplary data point corresponding to 1 RJup is shown in the bottom right. The colour of a given data point indicates the respective planetary equilibrium temperature. Since non-transiting planets do not have any measured equilibrium temperatures listed in the NASA exoplanet archive (Akeson et al. 2013), they are shown as empty data points. Planets that had previously been targeted for reflection signature searches are marked by dotted crosshairs (see text for references). The latitudes of two exemplary high-resolution spectrographs, CARMENES and ESPRESSO, are visualised as horizontal dashed lines, the declination ranges within which targets culminate at airmasses lower than 1.5 are shaded in the respective colour. — astro-ph.EP

Detecting light reflected off the dayside of an exoplanet in high-resolution spectroscopic data has proved to be a notoriously difficult endeavour. Despite several attempts, the faint signal has yet to be detected.

We present a new effort at finding reflection signatures and show how a strong rotational broadening of the reflected spectrum can complicate this objective. We introduce a new figure of merit that quantifies the favourability of different systems for a reflection study, the reflection spectroscopy metric.

Applying this metric, we identify the KELT-9 system, which features a highly misaligned, rapidly rotating host star, as the target for a case study based on a spectroscopic time series obtained by CARMENES. We also perform an injection-recovery test to determine the detectability of the signal in our data and demonstrate its sensitivity to rotational line broadening.

The search for a genuine reflection signal in our data resulted in a non-detection. The injection-recovery test puts this finding into context by revealing the critical importance of taking rotational broadening into account when dealing with systems featuring rapidly rotating stars and large spin-orbit misalignments.

The case study presented here underscores the need to incorporate stellar rotation and spin-orbit misalignment into assessments of a given planet’s favourability to reflection studies.

T. O. Winterhalder, K. Molaverdikhani, D. Cont, F. Yan, E. Nagel, A. Kaminski, L. Nortmann, P. J. Amado, V. J. S. Béjar, G. Bergond, J. A. Caballero, S. Czesla, Th. Henning, G. Morello, D. Montes, E. Palle, A. Quirrenbach, A. Reiners, I. Ribas, A. Schweitzer

Comments: Accepted for publication in Astronomy and Astrophysics
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2605.16491 [astro-ph.EP] (or arXiv:2605.16491v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2605.16491
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Submission history
From: Thomas Winterhalder
[v1] Fri, 15 May 2026 18:00:01 UTC (4,987 KB)
https://arxiv.org/abs/2605.16491
Astrobiology, exoplanet,

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

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