Earth as a transiting exoplanet

SPIRou observation of Earth near-IR transmission spectrum during the November 19, 2021 lunar eclipse^★

¹ Canada France Hawai‘i Telescope Corporation (CFHT), CNRS UAR2208, 65-1238 Mamalahoa Hwy, Kamuela, HI 96743, USA
² Université de Montréal, Département de Physique, IREx, Montréal, QC H3C 3J7, Canada
³ Observatoire du Mont-Mégantic, Université de Montréal, Montréal, QC H3C 3J7, Canada
⁴ LTE, Observatoire de Paris, Université PSL, Sorbonne Université, Université de Lille, LNE, CNRS, 61 Avenue de l’Observatoire, 75014 Paris, France
⁵ Université de Toulouse, CNRS, IRAP, 14 avenue Belin, 31400 Toulouse, France
⁶ Department of Physics and Department of Earth & Planetary Sciences, McGill University, 3600 Rue University, Montréal, QC H3A 2T8, Canada
⁷ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁸ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁹ Aix-Marseille Univ., CNRS, CNES, LAM, 38 Rue Frédéric Joliot Curie, 13338 Marseille, France
¹⁰ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
¹¹ US Geological Survey, 2255 North Gemini Drive, Flagstaff, AZ 86001, USA

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

Received: 28 October 2025
Accepted: 10 April 2026

Abstract

Context. Detecting and characterizing the atmosphere of rocky planets in the habitable zone is a key objective of exoplanet research in the era of JWST and the upcoming E-ELT. Spectroscopy of a planetary transit is the primary method of obtaining a transmission spectrum of an exoplanetary atmosphere. A lunar eclipse is a natural proxy for an Earth transit, offering a unique opportunity to study Earth’s atmosphere as if it were an exoplanet.

Aims. This work aims to derive the empirical, wavelength-dependent effective height of Earth’s atmosphere in the near-infrared, spanning the Y to K bands.

Methods. During a lunar eclipse, the penumbra – the region where the Sun is partially occulted by the Earth – contains the signature of the transmission spectrum of the atmosphere along Earth’s limb. The high-resolution (R=70 000) SPIRou spectrograph at CFHT was used to collect spectra during and after the November 19, 2021 eclipse. Two distinct methods, using either one or two penumbral spectra, were employed to derive the effective height of the atmosphere. Each method was evaluated in the context of distinct astrometric and spectrophotometric approaches.

Results. The derived effective height spans 964 to 2498 nm and shows the absorption bands of H₂O, O₂, CH₄, and CO₂ peaking at up to 29.6 km at a spectral resolution of R=70 000, including a 12.7 km offset defining a reference altitude taken from a transit model. The uncertainty assessed from individual penumbra observations and from uncertainties on telluric lines correction and Moon reflectance is found to be 3σ ≈ 3.6 km. Heights are up to 25.1 km at a convolved resolution of R=1000.

Conclusions. The measured heights are in good agreement with theoretical predictions. The high-resolution wavelength-dependent effective heights are made publicly available to the community.