Nightside condensation of iron in an ultrahot giant exoplanet

Abstract

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.

ARES. II. Characterizing the Hot Jupiters WASP-127 b, WASP-79 b, and WASP-62b with the Hubble Space Telescope*

“…A recent study from Ehrenreich et al (2020) found atomic iron (Fe) in the dayside of the planet WASP-76 b and not in the terminator, concluding that Fe is condensing on the nightside, then falling into deeper layers of the atmosphere. Furthermore, the results in Pluriel et al (2020) and Caldas et al (2019) have investigated how the 3D structure of the atmosphere biases the abundances retrieved with typical retrieval codes, as there is a chemical dichotomy between the day-and nightside that is not considered in a 1D treatment of transit geometry.…”

“…Examining the atmospheres of exoplanets further unveils their planetary properties, with their study made possible by various methods, including transit spectroscopy (e.g., Tinetti et al 2007;Swain et al 2008). Facilities such as the Hubble Space Telescope (HST) and the Spitzer Space Telescope, as well as some ground-based observatories, have provided constraints on these properties for a limited number of targets and, in some cases, have identified the key molecules present in their atmospheres while also detecting the presence of clouds and probing their thermal structure (e.g., Brogi et al 2012;Majeau et al 2012;Stevenson et al 2014;Fu et al 2017;Hoeijmakers et al 2018;Tsiaras et al 2018Tsiaras et al , 2019Pinhas et al 2019;Edwards et al 2020a;Ehrenreich et al 2020).…”

ARES I: WASP-76 b, A Tale of Two HST Spectra*

“…These metallicity ranges provide H 2 O abundances which are similar to those recovered here (10 −3 -10 −4 ). The models from Parmentier et al (2018) suggest that, for WASP-121 b, ∼70% of the H 2 O in the 1.4 μm photosphere should be dissociated, compared to~50% in WASP-76 b. Additionally, the metallicity (Fe/H) of WASP-76 is greater than that of WASP-121 and both are above solar, at 0.366 and 0.12 respectively (Ehrenreich et al 2020;Mikal-Evans et al 2019). We could therefore expect WASP-76 b to have slightly more H 2 O than WASP-121 b but, while the bestfit solution agrees with this prediction, the 1σ errors on the abundance are too large to be conclusive.…”

“…However, the best-fit inclinations differed between the transit and eclipse light curve and so the orbital parameters were set to values from Ehrenreich et al (2020). The limb-darkening coefficients were selected from the best available stellar parameters using values from Claret et al (2012Claret et al ( , 2013 and using the stellar parameters from Ehrenreich et al (2020). We did not fit for the limb-darkening coefficients, as they are degenerate with other parameters, particularly given the periodic gaps in the HST data.…”

“…Two transits of WASP-76 b were observed using the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations instrument on the Very Large Telescope (VLT) and used to reveal an asymmetric absorption signature which was attributed to neutral iron (Fe, Ehrenreich et al 2020). The signature was blueshifted on the trailing limb, proving evidence for strong day-to-night winds and an asymmetric dayside.…”