Detection of a Westward Hotspot Offset in the Atmosphere of a Hot Gas Giant CoRoT-2b

TL;DR

This study reports the first robust detection of a westward hotspot offset on the hot Jupiter CoRoT-2b, challenging existing models of heat distribution and atmospheric circulation on tidally locked exoplanets.

Contribution

It presents thermal phase observations revealing a westward hotspot offset, suggesting alternative atmospheric dynamics or magnetic effects, and raises questions about tidal locking assumptions.

Findings

Detected a 23 ± 4 degree westward hotspot offset

Suggests possible non-synchronous rotation or magnetic effects

Indicates incomplete understanding of exoplanet atmospheric circulation

Abstract

Short-period planets exhibit day-night temperature contrasts of hundreds to thousands of degrees K. They also exhibit eastward hotspot offsets whereby the hottest region on the planet is east of the substellar point; this has been widely interpreted as advection of heat due to eastward winds. We present thermal phase observations of the hot Jupiter CoRoT-2b obtained with the IRAC instrument on the Spitzer Space Telescope. These measurements show the most robust detection to date of a westward hotspot offset of 23 $\pm$ 4 degrees, in contrast with the nine other planets with equivalent measurements. The peculiar infrared flux map of CoRoT-2b may result from westward winds due to non-synchronous rotation magnetic effects, or partial cloud coverage, that obscures the emergent flux from the planet's eastern hemisphere. Non-synchronous rotation and magnetic effects may also explain the planet's anomalously large radius. On the other hand, partial cloud coverage could explain the featureless dayside emission spectrum of the planet. If CoRoT-2b is not tidally locked, then it means that our understanding of star-planet tidal interaction is incomplete. If the westward offset is due to magnetic effects, our result represents an opportunity to study an exoplanet's magnetic field. If it has Eastern clouds, then it means that our understanding of large-scale circulation on tidally locked planets is incomplete.