HiDef Neighbors: Solar System objects as exoplanet analogs

I. Gaseous objects

¹ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
³ Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA 02139, USA
⁴ School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 June 2025
Accepted: 2 January 2026

Abstract

Context. Intermediate-resolution spectral observations are vital tools for characterizing the composition and physical properties of Solar System bodies in detail. At present, the planets and moons of our Solar System remain the only planetary environments for which spatially resolved high-quality spectroscopic data are obtainable. Observations that provide such data can advance knowledge of the Solar System and establish reference points for future exoplanet observations with next-generation telescopes.

Aims. We present a library of intermediate-resolution (R ≈ 10000) spectra, with this first paper of the HiDef Neighbors project focusing on Saturn, Uranus, Neptune, and Titan’s atmospheres. We provide homogeneous, high S/N (≈250) data for comparative studies and modeling. While other projects have presented high S/N broad-wavelength data of individual objects, they vary widely in resolution, calibration standards, and observing strategies, limiting their comparability. We used one instrument, consistent calibrations, and a single reduction pipeline, with images obtained near opposition to ensure consistent phase geometry. We also address how the methane distribution differs between gas and ice giants within our data. We provide constraints on the methane distribution, as our dataset allows for direct comparison of the Solar System gas giants and serves as a benchmark for future atmospheric studies.

Methods. We obtained spectra using X-shooter in the Integral Field Unit mode, covering a spectral range of 0.3 μm to 2.1 μm. The data were integrated, cleaned of telluric absorption features using MOLECFIT, and refined by removing solar absorption lines and applying Doppler corrections.

Results. We present the resulting spectra for the selected Solar System objects and a number of equivalent widths for methane absorptions. These spectra are publicly available and will be a valuable resource for future atmospheric investigations and comparative planetology studies.