Climate of an ultra hot Jupiter

Spectroscopic phase curve of WASP-18b with HST/WFC3^★

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
e-mail: J.Arcangeli@uva.nl
² Atmospheric, Ocean, and Planetary Physics, Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, OX1 3PU, UK
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Department of Astronomy & Astrophysics, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA
⁵ School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287, USA
⁶ Center for Astrophysics | Harvard and Smithsonian, Cambridge, MA 02138, USA
⁷ Harvard Society of Fellows, 78 Mt. Auburn St., Cambridge, MA 02138, USA
⁸ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁹ Department of Planetary Sciences and Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA

Received: 15 December 2018
Accepted: 3 April 2019

Abstract

We present the analysis of a full-orbit, spectroscopic phase curve of the ultra hot Jupiter (UHJ) WASP-18b, obtained with the Wide Field Camera 3 aboard the Hubble Space Telescope. We measured the normalised day-night contrast of the planet as >0.96 in luminosity: the disc-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 <32 ppm or 1430 K at the 3σ level. We also find that the peak of the phase curve exhibits a small, but significant offset in brightness of 4.5 ± 0.5° 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 recirculation 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 because of the high-temperature chemistry in the atmosphere of WASP-18b. Additionally, we compared 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.