Ultra-hot giant exoplanets receive thousands of times Earth’s insolation 1 , 2 . Their high-temperature atmospheres (>2,000 K) are ideal laboratories for studying extreme planetary climates and chemistry 3 – 5 . Daysides are predicted to be cloud-free, dominated by atomic species 6 and substantially hotter than nightsides 5 , 7 , 8 . Atoms are expected to recombine into molecules over the nightside 9 , resulting in different day-night chemistry. While metallic elements and a large temperature contrast have been observed 10 – 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 transit 4 , 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 dayside 16 . 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.

Context. ESPRESSO is the new high-resolution spectrograph of ESO's Very Large Telescope (VLT). It was designed for ultra-high radial-velocity (RV) precision and extreme spectral fidelity with the aim of performing exoplanet research and fundamental astrophysical experiments with unprecedented precision and accuracy. It is able to observe with any of the four Unit Telescopes (UTs) of the VLT at a spectral resolving power of 140 000 or 190 000 over the 378.2 to 788.7 nm wavelength range; it can also observe with all four UTs together, turning the VLT into a 16-m diameter equivalent telescope in terms of collecting area while still providing a resolving power of 70 000. Aims. We provide a general description of the ESPRESSO instrument, report on its on-sky performance, and present our Guaranteed Time Observation (GTO) program along with its first results. Methods. ESPRESSO was installed on the Paranal Observatory in fall 2017. Commissioning (on-sky testing) was conducted between December 2017 and September 2018. The instrument saw its official start of operations on October 1, 2018, but improvements to the instrument and recommissioning runs were conducted until July 2019. Results. The measured overall optical throughput of ESPRESSO at 550 nm and a seeing of 0.65 exceeds the 10% mark under nominal astroclimatic conditions. We demonstrate an RV precision of better than 25 cm s −1 during a single night and 50 cm s −1 over several months. These values being limited by photon noise and stellar jitter shows that the performance is compatible with an instrumental precision of 10 cm s −1. No difference has been measured across the UTs, neither in throughput nor RV precision. Conclusions. The combination of the large collecting telescope area with the efficiency and the exquisite spectral fidelity of ESPRESSO opens a new parameter space in RV measurements, the study of planetary atmospheres, fundamental constants, stellar characterization, and many other fields.

The acronym ESPRESSO stems for Echelle SPectrograph for RockyExoplanets and Stable Spectroscopic Observations; this instrument will be the next VLTh igh resolution spectrograph. The spectrograph will be installed at the CombinedCoudéL aboratory of the VLTa nd linked to the four 8.2 mU nit Telescopes (UT) through four optical Coudét rains. ESPRESSO will combine efficiencya nd extreme spectroscopic precision. ESPRESSO is foreseen to achieve ag ain of twomagnitudes with respect to its predecessor HARPS, and to improve the instrumental radial-velocity precision to reach the 10 cm s −1 level. It can be operated either with as ingle UT or with up to four UTs, enabling an additional gain in the latter mode. The incoherent combination of four telescopes and the extreme precision requirements called for many innovative design solutions while ensuring the technical heritage of the successful HARPS experience. ESPRESSO will allowtoexplore newfrontiers in most domains of astrophysics that require precision and sensitivity.The main scientific drivers are the search and characterization of rockyexoplanets in the habitable zone of quiet, nearby GtoM-dwarfs and the analysis of the variability of fundamental physical constants. The project passed the final design reviewinMay 2013 and entered the manufacturing phase. ESPRESSO will be installed at the Paranal Observatory in 2016 and its operation is planned to start by the end of the same year. 1I ntroductionHigh-resolution spectroscopy provides physical insights in the study of stars, galaxies, and interstellarand intergalactic medium.B esides the importance of observing fainter and fainter objects by increasing the photon collecting area by making bigger telescopes, the importance of high-precision has emerged in recent years as ac rucial element in spectroscopy. In manyi nvestigations repeatable observations overl ong temporal baseline are needed. Fori nstance, the Corresponding author: molaro@oats.inaf.it HARPS spectrograph at the ESO 3.6-m telescope is ap ioneering instrument for precise radial-velocity (RV) measurements (Mayor et al. 2003). The search for terrestrial planets in habitable zone is one of the most exciting science topics of the next decades and one of the main drivers for the newg eneration of Extremely Large Telescopes. The need for as imilar instrument on the VLTh as been emphasized in the ESO-ESA working group report on extrasolar planets. In October 2007 the ESO STC recommended the development of additional second-generation VLTi nstruments, and this proposal wasendorsed by the ESO Council in December of the same year.Among the recommended instru-

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