WASP-127b: a misaligned planet with a partly cloudy atmosphere and tenuous sodium signature seen by ESPRESSO^★

¹ Observatoire astronomique de l’Université de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
e-mail: romain.allart@unige.ch
² Anton Pannekoek Institute for Astronomy, University of Amsterdam Science Park 904, 1098 XH Amsterdam, The Netherlands
³ INAF–Osservatorio Astrofisico di Arcetri Largo Enrico Fermi 5, 50125 Firenze, Italy
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, Spain
⁷ Centro de Astrobiología (CSIC-INTA), ESAC, Camino bajo del castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁸ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁹ Institute for Fundamental Physics of the Universe, IFPU, Via Beirut 2, 34151 Grignano, Trieste, Italy
¹⁰ Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain
¹¹ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal
¹² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁴ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁵ Universitat Bern, Physikalisches Institut, Siedlerstrasse 5, 3012 Bern, Switzerland
¹⁶ Faculdade de Ciênçias da Universidade de Lisboa (Departamento de Física), Edificio C8, 1749-016 Lisboa, Portugal
¹⁷ Instituto de Astrofísica e Ciênçias do Espaço, Universidade de Lisboa, Edificio C8, 1749-016 Lisboa, Portugal
¹⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching b. Munchen, Germany
¹⁹ ESO, European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago
²⁰ Institute for Fundamental Physics of the Universe, IFPU, Via Beirut 2, 34151 Grignano, Trieste, Italy
²¹ Fundación G. Galilei – INAF (TNG), Rambla J. A. Fernández Pérez 7, 38712 Breña Baja (La Palma), Spain
²² Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal

Received: 21 August 2020
Accepted: 26 October 2020

Abstract

Context. The study of exoplanet atmospheres is essential for understanding the formation, evolution, and composition of exoplanets. The transmission spectroscopy technique is playing a significant role in this domain. In particular, the combination of state-of-the-art spectrographs at low- and high-spectral resolution is key to our understanding of atmospheric structure and composition.

Aims. We observed two transits of the close-in sub-Saturn-mass planet, WASP-127b, with ESPRESSO in the frame of the Guaranteed Time Observations Consortium. We aim to use these transit observations to study the system architecture and the exoplanet atmosphere simultaneously.

Methods. We used the Reloaded Rossiter-McLaughlin technique to measure the projected obliquity λ and the projected rotational velocity v_eq ⋅sin(i_*). We extracted the high-resolution transmission spectrum of the planet to study atomic lines. We also proposed a new cross-correlation framework to search for molecular species and we applied it to water vapor.

Results. The planet is orbiting its slowly rotating host star (v_eq ⋅sin(i_*) = 0.53_−0.05^+0.07 km s⁻¹) on a retrograde misaligned orbit (λ = −128.41_−5.46^+5.60 °). We detected the sodium line core at the 9-σ confidence level with an excess absorption of 0.34 ± 0.04%, a blueshift of 2.74 ± 0.79 km s⁻¹, and a full width at half maximum of 15.18 ± 1.75 km s⁻¹. However, we did not detect the presence of other atomic species but set upper limits of only a few scale heights. Finally, we put a 3-σ upper limit on the average depth of the 1600 strongest water lines at equilibrium temperature in the visible band of 38 ppm. This constrains the cloud-deck pressure between 0.3 and 0.5 mbar by combining our data with low-resolution data in the near-infrared and models computed for this planet.

Conclusions. WASP-127b, with an age of about 10 Gyr, is an unexpected exoplanet by its orbital architecture but also by the small extension of its sodium atmosphere (~7 scale heights). ESPRESSO allows us to take a step forward in the detection of weak signals, thus bringing strong constraints on the presence of clouds in exoplanet atmospheres. The framework proposed in this work can be applied to search for molecular species and study cloud-decks in other exoplanets.