Oddball exoplanet challenges what it means to be a hot Jupiter

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New research led by a scientist at IPAC—a science and data center for astrophysics and planetary science at Caltech—studying the hot Jupiter CoRoT-2 b has settled on one of the three leading hypotheses explaining why its atmosphere has a hot spot in the opposite direction from that seen on all other exoplanets of this type.

This work was presented during a press conference at the 248th meeting of the American Astronomical Society in Pasadena, California, on June 16, 2026.

In a blazing orbit—in both definitions of the word—of somewhere between one and 10 days, tightly wound around its host star, hot Jupiters are some of the most extreme planets beyond our solar system. Because of their large size (similar to or larger than Jupiter) and close proximity to their host stars, it is easier to collect lots of different data on a single object, making them rich test subjects for planetary formation theories.

Aurora Kesseli, a staff scientist at the NASA Exoplanet Science Institute (NExScI) at IPAC, dove into one particularly unique hot Jupiter in her paper, published in The Astronomical Journal. Using new spectroscopic data from the Very Large Telescope at the European Southern Observatory, Kesseli and her collaborators followed up on three proposed hypotheses from previous work led by her collaborator Lisa Dang, a professor at the University of Waterloo.

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

Kesseli lands on one likely answer that challenges all previous assumptions about hot Jupiters: This planet is not tidally locked.

"Now we can see that a one-size-fits-all model does not work, even for planets that we've been studying for a long time," said Kesseli. "Every time we look at another hot Jupiter, we learn something new to help refine our models, which are useful for understanding not only hot Jupiters, but all types of exoplanets."

When a planet is tidally locked to its host star, the same side always faces the star, like how we always see the same side of the moon from Earth. Almost like friction, a body becomes tidally locked when its host star gravitationally tugs at the planet's rotation until it has slowed to zero relative to its orbit. Hot Jupiters orbit so closely to their host stars that astronomers assume they are always tidally locked.

For rocky planets, we can imag