The Earth as an extrasolar transiting planet

II. HARPS and UVES detection of water vapour, biogenic O₂, and O₃^⋆

¹ Aix Marseille Université, CNRS, OHP (Observatoire de Haute Provence), Institut Pythéas (UMS 3470), 04870 Saint-Michel-l’Observatoire, France
e-mail: Luc.Arnold@osupytheas.fr
² Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland
³ Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis Boulevard Arago, 75014 Paris, France
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, 02138 Cambridge, USA
⁵ Unidad de Astronomía, Facultad de Ciencias Básicas, Universidad de Antofagasta, 601 Avenida Angamos, Antofagasta, Chile
⁶ University of Chile, Department of Electrical Engineering, 2007 Tupper Avenue, Santiago, Chile
⁷ National Institute of Water and Atmospheric Research (NIWA), 1010 Auchland, Lauder, New Zealand
⁸ Deutsches Zentrum für Luft und Raumfahrt e.V. (DLR), Oberpfaffenhofen, 82234 Wessling, Germany
⁹ Institut de Mécanique Céleste et de Calcul des Éphémérides, Observatoire de Paris, Avenue Denfert-Rochereau, 75014 Paris, France
¹⁰ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹¹ CFHT Corporation, 65-1238 Mamalahoa Hwy Kamuela, Hawaii 96743, USA
¹² Astronomy Department, Universidad de Chile, Casilla 36-D Santiago, Chile

Received: 7 November 2013
Accepted: 3 February 2014

Abstract

Context. The atmospheric composition of transiting exoplanets can be characterized during transit by spectroscopy. Detections of several chemical species have previously been reported in the atmosphere of gaseous giant exoplanets. For the transit of an Earth twin, models predict that biogenic oxygen (O₂) and ozone (O₃) atmospheric gases should be detectable, as well as water vapour (H₂O), a molecule linked to habitability as we know it on Earth.

Aims. The aim is to measure the Earth radius versus wavelength λ – or the atmosphere thickness h(λ) – at the highest spectral resolution available to fully characterize the signature of Earth seen as a transiting exoplanet.

Methods. We present observations of the Moon eclipse of December 21, 2010. Seen from the Moon, the Earth eclipses the Sun and opens access to the Earth atmosphere transmission spectrum. We used two different ESO spectrographs (HARPS and UVES) to take penumbra and umbra high-resolution spectra from ≈3100 to 10 400 Å. A change of the quantity of water vapour above the telescope compromised the quality of the UVES data. We corrected for this effect in the data processing. We analyzed the data by three different methods. The first method is based on the analysis of pairs of penumbra spectra. The second makes use of a single penumbra spectrum, and the third of all penumbra and umbra spectra.

Results. Profiles h(λ) are obtained with the three methods for both instruments. The first method gives the best result, in agreement with a model. The second method seems to be more sensitive to the Doppler shift of solar spectral lines with respect to the telluric lines. The third method makes use of umbra spectra, which bias the result by increasing the overall negative slope of h(λ). It can be corrected for this a posteriori from results with the first method. The three methods clearly show the spectral signature of the Rayleigh scattering in the Earth atmosphere and the bands of H₂O, O₂, and O₃. Sodium is detected. Assuming no atmospheric perturbations, we show that the E-ELT is theoretically able to detect the O₂A-band in 8 h of integration for an Earth twin at 10 pc.

Conclusions. Biogenic O₂, O₃, and water vapour are detected in Earth observed as a transiting planet, and, in principle, would be within reach of the E-ELT for an Earth twin at 10 pc.