Journal Article

Nightside condensation of iron in an ultra-hot giant exoplanet

Santos, Pedro, Hara, Nathan, Zerbi, Filippo, Sousa, Sérgio, Conconi, Paolo, Fragoso, Ana, Lavie, Baptiste, Bianco, Andrea, Tabernero, Hugo, Iwert, Olaf, Lizon, Jean-Louis, Martins, C. J. A. P., Cabral, Alexandre, Pariani, Giorgio, Rebolo, Rafael, D’Odorico, Valentina, Knudstrup, Jens, Oliveira, António, Santos, Ricardo Génova, Mehner, Andrea, Pasquini, Luca, Tenegi, Fabio, Molaro, Paolo, Spanò, Paolo, Aliverti, Matteo, Ségransan, Damien, Benz, Willy, Genoni, Matteo, Sosnowska, Danuta, Cupani, Guido, Figueira, Pedro, Abreu, Manuel, Bolmont, Émeline, Riva, Marco, Zanutta, Alessio, Modigliani, Andrea, Osorio, María Rosa Zapatero, Oggioni, Luca, Ehrenreich, David, Cumani, Claudio, Alves, David, Santin, Paolo, Sozzetti, Alessandro, Curto, Gaspare Lo, Dumusque, Xavier, Poretti, Ennio, Kerber, Florian, Landoni, Marco, Adibekyan, Vardan, Pepe, Francesco, Marcantonio, Paolo Di, Mégevand, Denis, Seidel, Julia V., Genolet, Ludovic, Cirami, Roberto, Manescau, Antonio, Broeg, Christopher, Hernández, Jonay I. González, Müller, Eric, Coelho, João M. P., Hughes, Ian, Nunes, Nelson, Demangeon, Olivier, Prieto, Carlos Allende, A

2020

Overview

Ultra-hot giant exoplanets receive thousands of times Earth’s insolation1,2. Their high-temperature atmospheres (>2,000 K) are ideal laboratories for studying extreme planetary climates and chemistry3–5. Daysides are predicted to be cloud-free, dominated by atomic species6 and substantially hotter than nightsides5,7,8. Atoms are expected to recombine into molecules over the nightside9, resulting in different day-night chemistry. While metallic elements and a large temperature contrast have been observed10–14, no chemical gradient has been measured across the surface of such an exoplanet. Different atmospheric chemistry between the day-to-night (“evening”) and night-to-day (“morning”) terminators could, however, be revealed as an asymmetric absorption signature during transit4,7,15. Here, we report the detection of an asymmetric atmospheric signature in the ultra-hot exoplanet WASP-76b. We spectrally and temporally resolve this signature thanks to the combination of high-dispersion spectroscopy with a large photon-collecting area. The absorption signal, attributed to neutral iron, is blueshifted by −11±0.7 km s-1 on the trailing limb, which can be explained by a combination of planetary rotation and wind blowing from the hot dayside16. In contrast, no signal arises from the nightside close to the morning terminator, showing that atomic iron is not absorbing starlight there. Iron must thus condense during its journey across the nightside.