Most planets like Jupiter that orbit dangerously close to their stars — a class of scorching gas giants called "hot Jupiters" — are thought to be tidally locked, always showing the same face to their star, the same way the Moon always shows the same face to Earth.
This video illustrates how each time the Moon completes a turn, it also orbits the Earth once, keeping its far side perpetually hidden. Sizes and distances are not to scale. A smiling face has been added to make it easier to see the Moon’s rotation. [Credit: NASA/Vi Nguyen]
That locked synchronous rotation should produce a predictable pattern: powerful eastward winds that push each planet's hottest point slightly east of the point receiving the most starlight. Nearly every hot Jupiter studied so far follows this rule... except the young hot Jupiter CoRoT-2b.
A decade-old mystery
However, when Assistant Professor Lisa Dang studied CoRoT-2b with the Spitzer Space Telescope as part of her PhD work, her observations revealed something strange: its hottest region appeared to be shifted westward, making it one of the most unusual hot Jupiters known. Dang proposed three possible hypotheses to explain the unusual position of the hotspot: patchy clouds obscuring our view, complex magnetic field interactions, or a planet rotating more slowly than expected (Detection of a westward hotspot offset in the atmosphere of hot gas giant CoRoT-2b | Nature Astronomy). Nearly a decade on, no single explanation had been confirmed.
Step one: a more detailed atmosphere
That began to change with a new study led by Dang's own PhD student at WCA, Ying Shu, using the IGRINS spectrograph on the Gemini South telescope to take the closest look yet at CoRoT-2b's atmosphere. Shu, Dang and collaborators made the first high-resolution detections of water vapour and carbon monoxide in the planet's atmosphere, showing it was far more chemically complex than earlier; lower-resolution observations had suggested. The paper also laid the groundwork for the next step: phase-resolved observations that could start to test the three competing explanations for the westward hot spot.
Step two: a leading suspect emerges
In a recently published paper this week, Dang and her collaborators have revisited CoRoT-2b. Combined with additional new spectroscopic data from CRIRES+ on the Very Large Telescope at the European Southern Observatory, they investigated the atmosphere and rotation of CoRoT-2b.
Instead of taking only a single snapshot, the researchers observed the planet at different points in its orbit, before and after it passed behind its star. This allowed them to compare different portions of the planet's dayside atmosphere and track how it changed across its surface. The observations confirmed the presence of a westward shifted hotspot and were used to try to differentiate between Dang’s three hypotheses.
Using the spectral data, they measured a rotational velocity of about 2.2 km/s, while a tidally locked planet of this size and orbit should rotate at roughly 4.4 km/s. In more familiar terms, one day on CoRoT-2 b is equivalent to about three Earth days, but one year on CoRoT-2 b is half of that—about 1.5 days. That means the hot Jupiter will have orbited its host star twice by the time it completes one rotation on its axis, or one CoRoT-2 b day.
Challenging the rulebook
Dang and collaborators conclude that the strongest explanation is that CoRoT-2b may be rotating more slowly than expected. Instead of being perfectly tidally locked to its star, the planet may exhibit sub-synchronous rotation, meaning it turns on its axis more slowly than its orbital motion would predict. This slower rotation could alter the planet's global wind patterns and naturally produce the observed western hotspot.
This animation shows how CoRoT-2 b is rotating slower and in the opposite direction of typical hot Jupiters, which are usually tidally locked. The planet models are based off work from Hayley Beltz, Emily Rauscher, and their collaborators, which predicts what winds may look like on a slowly rotating planet. Credit: Keith Miller (Caltech/IPAC - SELab).
CoRoT-2b appears to challenge one of the standard assumptions about hot Jupiters: that they are all tidally locked and therefore share similar atmospheric circulation patterns. "If CoRoT-2b really is sub-synchronous, detailed follow-up observations could shed light on the physical processes at play before a hot Jupiter becomes tidally locked," Dang said.
As promising as the result is, Dang and her collaborators are careful not to call the case closed. The team would like to see the system observed independently, ideally with a different instrument and a separate group of researchers, before treating sub-synchronous rotation as the final answer.
That's the next step for Dang and her collaborators: confirming the finding, and, if it holds, understanding why CoRoT-2b is rotating sub-synchronously in the first place.