Astronomers have discovered that the gas giant CoRoT-2 b is likely not tidally locked, challenging long-held assumptions about “hot Jupiters.” Led by Aurora Kesseli at the NASA Exoplanet Science Institute, the research shows the planet rotates once every three Earth days, completing two full orbits before finishing a single spin.
Why is CoRoT-2 b breaking the rules of hot Jupiters?
Most “hot Jupiters”—massive gas giants that orbit extremely close to their stars—behave predictably. Because of their proximity to their suns, these planets are expected to be tidally locked. This means one side constantly faces the star, creating a permanent, scorching dayside and a cooler nightside.
In these standard models, strong winds move heat around the planet, typically shifting the hottest spot slightly in the direction of the planet’s orbit. However, CoRoT-2 b has puzzled scientists since 2018 because its hottest region appears in the “wrong” place, opposite the direction seen on other similar worlds.
Researchers previously considered three explanations for this anomaly: thick clouds masking the atmosphere, magnetic fields disrupting heat movement, or a slower-than-expected rotation. According to the new study, the rotation rate is the culprit.
Tidal locking is the same phenomenon that keeps the Moon facing Earth. Because the Moon’s rotation is synchronized with its orbit, we never see its “far side” from our perspective on Earth.
How did researchers prove the planet’s rotation rate?
Aurora Kesseli, based at the NASA Exoplanet Science Institute at IPAC (a Caltech science and data center), led the team using spectroscopic observations from the European Southern Observatory’s Very Large Telescope. By measuring the planet’s velocity, the team could estimate its actual rotation speed.
The results contradicted the tidal locking theory. The team found that while CoRoT-2 b completes a full orbit in just 1.5 days, it takes about three Earth days to complete one full rotation. This means the planet circles its star twice before it finishes a single spin.
“I was very pleasantly surprised when I tried a bunch of methods, and I was like, ‘Aha! This is actually like one of the three hypotheses!’” Kesseli said regarding the data alignment.
Comparison: Standard Hot Jupiters vs. CoRoT-2 b
| Feature | Standard Hot Jupiter | CoRoT-2 b |
|---|---|---|
| Rotation Style | Tidally Locked (1:1) | Non-Synchronized |
| Rotation vs. Orbit | 1 rotation per 1 orbit | 1 rotation per 2 orbits |
| Hot Spot Position | Shifted with orbit direction | Opposite expected direction |
What does this mean for the search for habitable worlds?
Understanding rotation is vital for the search for life. Many potentially habitable planets are expected to orbit M dwarf stars, which are the most common stars in the universe. Because these habitable zones are close to the star, these planets are also expected to be tidally locked.
The way a planet rotates dictates how it distributes heat, winds, and climate. If a planet isn’t tidally locked, its weather systems will look completely different. Kesseli noted that for planets that are not tidally locked, temperatures and climates will vary significantly from the standard models used by astronomers.
This discovery suggests that the “one-size-fits-all” model for exoplanet climates is insufficient. As researchers move from studying giant gas planets to Earth-like worlds, they must account for more complex rotational histories.
What comes next for exoplanet science?
The research, presented at the 248th meeting of the American Astronomical Society, highlights the diversity of the more than 5,000 confirmed exoplanets. Scientists still don’t know why CoRoT-2 b rotates so slowly, citing potential interactions with its star or internal planetary processes as possible causes.
Future observations will rely on next-generation technology. The Habitable Worlds Observatory and the Extremely Large Telescope are expected to provide much more precise measurements of exoplanet atmospheres, winds, and temperatures. These tools will help refine climate models for both gas giants and potentially habitable Earth-like planets.
Frequently Asked Questions
What is a “hot Jupiter”?
A hot Jupiter is a massive gas giant planet that orbits very close to its host star, resulting in extremely high temperatures and short orbital periods.
What is tidal locking?
Tidal locking occurs when a planet’s rotation speed matches its orbital period, causing one side of the planet to always face its star.
Why is the rotation of CoRoT-2 b important?
It proves that not all close-orbiting planets become tidally locked, which means scientists must rethink how they model climates and habitability for other planets.
What do you think about the discovery of non-locked planets? Could this change our definition of a “habitable” world? Let us know in the comments below or subscribe to our newsletter for more deep dives into space exploration.
