Astronomers Find Evidence That Exoplanet CoRoT-2 b Rotates Slower Than Other Hot Jupiters

The Mystery of a Slow-Spinning Exoplanet: Westward Winds and a 3-Day Day

New data from NASA’s Exoplanet Science Institute (NExScI) has revealed that the hot Jupiter CoRoT-2 b has a day that lasts three Earth days—twice as long as its year. This peculiar rotation may finally explain why its atmospheric hot spot is shifted in the "wrong" direction.

Most hot Jupiters are predictable. These gas giants orbit so close to their stars that they become tidally locked, meaning one side permanently faces the star. This extreme heating generates powerful eastward winds, shifting the planet's hottest point to the east of the star-facing point. CoRoT-2 b, however, has always been an outlier.

When astronomers first observed its hot spot shifting westward in 2018, it contradicted every model. Three hypotheses were proposed to explain this: obscuring clouds, magnetic field interactions, or a surprisingly slow planetary rotation.

"I really like looking at the weird ones—finding planets that don't fit the standard picture—and doing some mystery solving."
— Aurora Kesseli, NExScI

Presenting data at the 248th meeting of the American Astronomical Society, the NExScI team revealed new velocity measurements of CoRoT-2 b. The results point definitively to the slow rotation hypothesis.

• Rotation vs. Orbit: The planet rotates once every three Earth days.
• Orbital period: It circles its star once every 1.5 Earth days.

This means CoRoT-2 b rotates slower than it orbits. This is the opposite of the tidal locking seen in other hot Jupiters, which typically rotate synchronously with their orbit. The slower spin rate directly explains the westward-shifted hot spot, supporting one of the three hypotheses proposed by a team including University of Michigan’s Emily Rauscher.

The slow rotation disrupts the standard model of atmospheric circulation on hot Jupiters. Instead of the intense, uniform heating that drives eastward winds, the slower spin allows for a different atmospheric flow pattern, pushing the heat westward.

"I find the data really compelling. We could be wrong—that always happens in science—but this actually does make sense."
— Emily Rauscher, University of Michigan

While the evidence is strong, the scientific process demands verification.

• The team is calling for confirmation using other instruments, such as the upcoming Extremely Large Telescope.
• Rauscher suggests observing the system with a different team entirely to see if the results can be reproduced.

This discovery turns a mystery into a data point, proving once again that the "weird ones" often teach us the most about planetary physics.