Research

JWST-TST DREAMS: Non-Uniform Dayside Emission for WASP-17b from MIRI/LRS

We present the first spectroscopic characterisation of the dayside atmosphere of WASP-17b in the mid-infrared using a single JWST MIRI/LRS eclipse observation. From forward-model fits to the 5-12 μm emission spectrum, we tightly constrain the heat redistribution factor of WASP-17b to be 0.92±0.02 at the pressures probed by this data, indicative of inefficient global heat redistribution. We also marginally detect a supersolar abundance of water, consistent with previous findings for WASP-17b, but note our weak constraints on this parameter. These results reflect the thermodynamically rich but chemically poor information content of MIRI/LRS emission data for high-temperature hot Jupiters. Using the eclipse mapping method, which utilises the signals that the spatial emission profile of an exoplanet imprints on the eclipse light curve during ingress/egress due to its partial occultation by the host star, we also construct the first eclipse map of WASP-17b, allowing us to diagnose its multidimensional atmospheric dynamics for the first time. We find a day-night temperature contrast of order 1000 K at the pressures probed by this data, consistent with our derived heat redistribution factor, along with an eastward longitudinal hotspot offset of 18.7+11.1°−3.8, indicative of the presence of an equatorial jet induced by day-night thermal forcing being the dominant redistributor of heat from the substellar point. These dynamics are consistent with general circulation model predictions for WASP-17b. This work is part of a series of studies by the JWST Telescope Scientist Team (JWST-TST), in which we use Guaranteed Time Observations to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS).

Eclipse Mapping with Ariel: Future Prospects for a Population-Level Mapping Survey

Eclipse mapping is a powerful tool for measuring 3D profiles of exoplanet atmospheres. To date, only JWST has been capable of widely applying this technique, but as a general observatory, it is too time-limited to conduct population-level mapping studies. Ariel, on the other hand, is a dedicated exoplanet mission set to observe 1000 transiting exoplanets, making it a natural candidate for this. To assess Ariel’s mapping potential, we quantitatively benchmark its abilities against those of JWST using a simulation-and-retrieval framework with existing JWST eclipse maps as test cases. We find that for high-ranking targets, Ariel will be able to derive qualitatively similar maps to JWST using the same amount of observations; for mid-ranking targets, Ariel will be able to compete using as few as 3x as many observations; and for lower-ranking targets, the use of phase curves overcomes the need for an impractical number of repeated eclipse observations. We find that while Ariel is unlikely to have extensive latitudinal mapping abilities, it will have wide-ranging longitudinal abilities, from which the first-order atmospheric dynamics can be constrained. Using an analytically-derived metric, we determine the best eclipse mapping targets for Ariel, finding that it will be able to map nearly 100 targets using full phase curves in only quarter of its lifetime. This would be the largest mapping survey to date, and have enormous ramifications for our demographic understanding of exoplanet atmospheric dynamics. Finally, we rank all the best mapping targets for both JWST and Ariel in order to encourage future eclipse mapping studies.

JWST GO 5687: Unlocking New Dimensions in Eclipse Mapping with KELT-8b

Eclipse mapping is currently the only method capable of measuring 2D (latitude-longitude) profiles of exoplanet atmospheres. The method has been used to accurately map the longitudinal profiles of a handful of exoplanets, allowing us to characterise their atmospheric dynamics using key tracers such as the longitudinal offset of the hotspot (the hottest point on the planet) due to supersonic jets. However, no exoplanet has yet been adequately mapped in latitude because there is a limited parameter space in which we are capable of observing such signals. Latitudinal hotspot offsets can be induced by a marginal misalignment between the magnetic field of an exoplanet and its spin axis, with the scale of the offset related to the field strength and angle of misalignment. Characterising this offset would thus allow us to probe these parameters for planetary-mass objects for the first time, which is only possible via eclipse mapping because it is the only method capable of mapping these objects in latitude as well as longitude.

KELT-8b’s 1675 K equilibrium temperature make it ideally susceptible to a magnetically-induced latitudinal hotspot offset, and its 0.741 impact parameter and 45 minute ingress/egress duration make this offset optimally observable. We will use MIRI/LRS to observe two eclipses of KELT-8b, using a unique observing strategy designed to give the best constraints on both its longitudinal and latitudinal profile. This will allow us to construct the most informative eclipse map of an exoplanet to date, from which we will be able to extract a wealth of information, including the atmospheric dynamics, heat redistribution, wind speeds, and magnetic field geometry.