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 , submitted for publication 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 for all types of exoplanets."
A cosmic tango
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 imagine walking from the hot dayside, through a permanent sunset zone, and over to the cool nightside. But for gas giant planets like hot Jupiters, tidal locking produces more tangled results.
Tidally locked hot Jupiters still exhibit a hot dayside and a cooler nightside, but their swirling atmospheres blur the boundaries between these zones. They end up with large hot spots on the dayside, shifted slightly towards the direction of their rotation and their orbit around the host star.
"The conditions for tidal locking are important for astronomers to understand because the habitable zone for planets around M dwarfs is within the tidal locking zone, where we expect tidal locking to happen pretty quickly," said Kesseli. Low-mass stars, also called M dwarfs, are the most common stars in the universe, and likely host a significant population of exoplanets. "The way that a planet rotates greatly affects how the planet distributes its heat, and therefore affects its habitability, so for a planet that is tidally locked, the temperatures, winds, and climates are going to look completely different than those of a planet that is not tidally locked."
Oddball CoRoT-2 b defies this pattern and previous assumptions made in models of hot Jupiters, with its hot spot in the opposite direction of its orbit. This was first discovered by Dang in 2018, and in her paper, she outlined three possible hypotheses as to why this planet resists the norm: there could be clouds obscuring our view, there are complex magnetic field interactions, or the planet could be rotating slower than it is orbiting the host star.
Kesseli measured the planet's velocity in the paper, and with a back-of-the-envelope calculation, she found that 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.
"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!' Seeing the data pretty clearly pointing towards one of them was just really exciting," said Kesseli.
But Kesseli and other astronomers are still not quite sure what is causing this slower rotation speed, and they're looking forward to gathering more data on these fiery gas giants.
"Hot Jupiters are the first type of planet where we have been able to really explore and refine our models of their climates," said Kesseli. "With the next generation of telescopes like the Habitable Worlds Observatory and the Extremely Large Telescope, we'll be able to do more in depth measurements across more planets, maybe even potentially habitable ones."
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