For years, astronomers have been studying giant planets known as hot Jupiters, as they offer a rare chance to observe extreme worlds up close.
These massive gas giants orbit so close to their stars that a year can last just a few days. Because of their high temperatures and rapid orbits, they rank among the most unusual planets ever discovered.
Most hot Jupiters appear to behave in a predictable manner. One side constantly faces the star, creating a bright dayside and a colder nightside. Powerful winds transport heat across the planet, producing a hot spot that usually shifts slightly in the direction of the planet’s orbit.
But one world has puzzled scientists for years, since its hot spot is located in the wrong place. New research has uncovered the most likely explanation. The planet, named CoRoT-2 b, may not be tidally locked at all.
The study was led by Aurora Kesseli from NASA’s Exoplanet Science Institute at IPAC, a research center at the California Institute of Technology. The findings were presented at the 248th meeting of the American Astronomical Society (AAS).
Using fresh spectroscopic observations from the Very Large Telescope of the European Southern Observatory, Kesseli and her colleagues revisited a mystery that has challenged planetary scientists since 2018.
“I really enjoy studying unusual objects—finding planets that don’t fit the standard picture—and solving puzzles,” said Kesseli.
CoRoT-2 b stands out because its hottest region is located in the opposite direction from what is seen on other hot Jupiters. Earlier studies proposed three possible explanations. Clouds might be hiding part of the atmosphere. Magnetic fields could be influencing how heat spreads across the planet. Or the planet might be rotating more slowly than expected. The new data strongly point to the third possibility.
Tidal locking occurs when a planet’s rotation synchronizes with its orbit. The Moon is a well-known example. We always see the same side because the Moon rotates once on its axis for every orbit around Earth.
Scientists have long assumed that hot Jupiters become tidally locked because they orbit very close to their stars. Powerful gravitational forces should gradually slow their rotation until the same side permanently faces the star.
For gas giants, tidal locking creates a complex weather system. Their dense atmospheres constantly move heat around the planet. Still, most hot Jupiters show a similar pattern: their hottest area is slightly shifted in the direction of motion.
“The astronomical community needs to understand the conditions of tidal locking, since the habitable zone for planets around M-dwarf stars lies within the tidal locking zone, where we expect tidal locking to happen fairly quickly,” said Kesseli.
Low-mass M-dwarf stars are the most common type of star in the universe. Many potentially habitable planets discovered in the future might orbit these stars, making tidal locking a key factor in understanding alien climates.
“A planet’s rotation strongly affects how heat is distributed on it, and therefore its habitability. So on a tidally locked planet, the temperature, winds, and climate would be completely different from those on a planet that is not tidally locked,” noted Kesseli.
To test the competing ideas, Kesseli measured the planet’s velocity and estimated its rotation speed. The results were unexpected. One day on CoRoT-2b lasts about three Earth days, while its year is only 1.5 days long. In other words, the planet completes two orbits around its star before finishing one full rotation.
“I was pleasantly surprised when I tried several methods and thought, ‘Aha! It turns out to be one of the three hypotheses!’ Seeing data that quite clearly pointed to one of them was really exciting,” said Kesseli.
If confirmed, this discovery challenges the long-held assumption that all hot Jupiters eventually become tidally locked. Instead, some may have a more complex rotational history than astronomers previously thought.
Researchers still do not know why CoRoT-2b rotates so slowly. Interactions with the star, internal planetary processes, or other factors might be involved. Additional observations will be needed to determine the cause.
This finding highlights a broader lesson in exoplanet science. Over 5,000 planets have already been confirmed beyond our solar system, and each new observation reveals just how diverse these worlds can be. Patterns that seem universal often have exceptions.
“We now see that the universal model doesn’t work, even for planets we’ve been studying for a long time,” said Kesseli. “Every time we study another hot Jupiter, we learn something new that helps refine our models, useful not only for understanding hot Jupiters but for all types of exoplanets.”
Astronomers are optimistic about the potential of future observatories, hoping they will provide clearer answers. Upcoming instruments will be able to study exoplanet atmospheres in greater detail. They will measure winds and temperature more accurately. These tools will also allow exploration of worlds that may be much more Earth-like than the giant gas planets currently dominating the field.
“Hot Jupiters are the first type of planet for which we were truly able to study and refine our models of their climates,” said Kesseli. “With next-generation telescopes like the Habitable Worlds Observatory and the Extremely Large Telescope, we’ll be able to conduct more in-depth measurements on more planets, perhaps even on potentially habitable ones.”
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