Climate of an Ultra hot Jupiter: Spectroscopic phase curve of WASP-18b with HST/WFC3

Abstract

We present the analysis of a full-orbit, spectroscopic phase curve of the ultra hot Jupiter WASP-18b, obtained with the Wide Field Camera 3 aboard the Hubble Space Telescope. We measure the planet's normalized day-night contrast as >0.96 in luminosity: the disk-integrated dayside emission from the planet is at 964+-25 ppm, corresponding to 2894+-30 K, and we place an upper limit on the nightside emission of <32ppm or 1430K at the 3-sigma level. We also find that the peak of the phase curve exhibits a small, but significant offset in brightness of 4.5+-0.5 degrees eastward. We compare the extracted phase curve and phase resolved spectra to 3D Global Circulation Models and find that broadly the data can be well reproduced by some of these models. We find from this comparison several constraints on the atmospheric properties of the planet. Firstly we find that we need efficient drag to explain the very inefficient day-night re-circulation observed. We demonstrate that this drag could be due to Lorentz-force drag by a magnetic field as weak as 10 Gauss. Secondly, we show that a high metallicity is not required to match the large day-night temperature contrast. In fact, the effect of metallicity on the phase curve is different from cooler gas-giant counterparts, due to the high-temperature chemistry in WASP-18b's atmosphere. Additionally, we compare the current UHJ spectroscopic phase curves, WASP-18b and WASP-103b, and show that these two planets provide a consistent picture with remarkable similarities in their measured and inferred properties. However, key differences in these properties, such as their brightness offsets and radius anomalies, suggest that UHJ could be used to separate between competing theories for the inflation of gas-giant planets.

Accepted for publication in A&A

References in corpus (21)

• Detection of Thermal Emission from an Extrasolar Planet
• Thermal structure of an exoplanet atmosphere from phase-resolved emission spectroscopy
• Inflating Hot Jupiters With Ohmic Dissipation
• Inverting Phase Functions to Map Exoplanets
• Spitzer Phase Curve Constraints for WASP-43b at 3.6 and 4.5 microns
• The Atmospheric Circulation of the Hot Jupiter WASP-43b: Comparing Three-Dimensional Models to Spectrophotometric Data
• Hot Nights on Extrasolar Planets: Mid-IR Phase Variations of Hot Jupiters
• Inflating and Deflating Hot Jupiters: Coupled Tidal and Thermal Evolution of Known Transiting Planets
• Magnetic Effects in Hot Jupiter Atmospheres
• Variability in the Atmosphere of the Hot Giant Planet HAT-P-7 b
• The Impact of Non-Uniform Thermal Structure on the Interpretation of Exoplanet Emission Spectra
• Evidence for a Dayside Thermal Inversion and High Metallicity for the Hot Jupiter WASP-18b
• Estimating the magnetic field strength in hot Jupiters
• Detection of a Westward Hotspot Offset in the Atmosphere of a Hot Gas Giant CoRoT-2b
• Ohmic Dissipation in the Interiors of Hot Jupiters
• Sparkling nights and very hot days on WASP-18b: the formation of clouds and the emergence of an ionosphere
• The Very Low Albedo of WASP-12b From Spectral Eclipse Observations with $\text{Hubble}$
• High-precision photometry by telescope defocussing. VII. The ultra-short period planet WASP-103
• Phase Offsets and the Energy Budgets of Hot Jupiters
• Revisiting the Energy Budget of WASP-43b: Enhanced day-night heat transport
• Shallow-water Magnetohydrodynamics for Westward Hotspots on Hot Jupiters

Cited by in corpus (34)

• The Atmospheric Circulation of Ultra-hot Jupiters
• Hot Jupiters: Origins, Structure, Atmospheres
• Decomposing the Iron Cross-Correlation Signal of the Ultra-Hot Jupiter WASP-76b in Transmission using 3D Monte-Carlo Radiative Transfer
• Five key exoplanet questions answered via the analysis of 25 hot Jupiter atmospheres in eclipse
• Evidence for H${}_2$ Dissociation and Recombination Heat Transport in the Atmosphere of KELT-9b
• Detection of Fe and evidence for TiO in the dayside emission spectrum of WASP-33b
• A transition between the hot and the ultra-hot Jupiter atmospheres
• Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b
• TESS unveils the phase curve of WASP-33b. Characterization of the planetary atmosphere and the pulsations from the star
• 2D Retrieval Frameworks for Hot Jupiter Phase Curves
• A JWST NIRSpec Phase Curve for WASP-121b: Dayside Emission Strongest Eastward of the Substellar Point and Nightside Conditions Conducive to Cloud Formation
• $TESS$ Phase Curve of the Hot Jupiter WASP-19b
• Transmission spectroscopy for the warm sub-Neptune HD3167c: evidence for molecular absorption and a possible high metallicity atmosphere
• H- and Dissociation in Ultra-hot Jupiters: A Retrieval Case Study of WASP-18b
• Phase curve pollution of exoplanet transmission spectra
• The stable climate of KELT-9b
• Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b II. Mapping the effects of gas kinetics
• The Equatorial Jet Speed on Tidally Locked Planets: I -- Terrestrial Planets
• On spectroscopic phase-curve retrievals: H2 dissociation and thermal inversion in the atmosphere of the ultra-hot Jupiter WASP-103 b
• A new approach to spectroscopic phase curves: The emission spectrum of WASP-12b observed in quadrature with HST/WFC3
• A survey of exoplanet phase curves with Ariel
• A retrieval challenge exercise for the Ariel mission
• The THOR+HELIOS general circulation model: multi-wavelength radiative transfer with accurate scattering by clouds/hazes
• Spitzer thermal phase curve of WASP-121 b
• Constraints on TESS albedos for five hot Jupiters
• Effects of Thermal Emission on the Transmission Spectra of Hot Jupiters
• Hotter Than Expected: HST/WFC3 Phase-resolved Spectroscopy of a Rare Irradiated Brown Dwarf with Strong Internal Heat Flux
• The curse of clouds
• ARES VI: Viability of one-dimensional retrieval models for transmission spectroscopy characterization of exo-atmospheres in the era of JWST and Ariel
• Is binning always sinning? The impact of time-averaging for exoplanet phase curves
• The Hubble PanCET program: Emission spectrum of hot Jupiter HAT-P-41b
• Reflected-light Phase Curves with PICASO: A Kepler-7b Case Study
• Spitzer phase curve observations and circulation models of the inflated ultra-hot Jupiter WASP-76b
• Cloudy and Cloud-free Thermal Phase Curves with PICASO: Applications to WASP-43b