Observations of a hot Jupiter could also advance our understanding of the origins and evolution of planets

Imagine being in a place where the winds are so strong that they move at the speed of sound. This is just one aspect of the atmosphere on XO-3b, one of a class of exoplanets (planets outside our solar system), known as hot Jupiters. The planet’s eccentric orbit also results in seasonal variations hundreds of times stronger than what we experience on Earth. In a recent paper, a McGill-led research team provides new insight into what the seasons look like on a planet outside our solar system. The researchers also suggest that XO-3b’s oval orbit, extremely high surface temperatures (2,000 degrees C – hot enough to vaporize rock) and “bulges” reveal traces of the planet’s history. The discoveries will potentially advance both scientific understanding of the formation and evolution of exoplanets and provide some context for planets in our own solar system.

Hot Jupiters are massive, gaseous worlds like Jupiter, which orbit closer to their parent stars than Mercury does to the Sun. Although they are not present in our own solar system, they seem to be common throughout the galaxy. Although they are the most studied type of exoplanet, major questions remain about their formation. Could there be subclasses of hot Jupiters with different formation histories? For example, do these planets form far from their parent stars – at a distance where it is cold enough for molecules such as water to become solid – or closer. The first scenario fits better with theories about the birth of planets in our own solar system, but what would cause these types of planets to migrate so close to their mother stars remains unclear.

To test these ideas, the authors of a recent McGill-led study used data from NASA’s retired Spitzer Space Telescope to observe the atmosphere of exoplanet XO-3b. They observed eccentric seasons and measured wind speeds on the planet obtaining a phase curve of the planet as it made a full revolution around its host star.

A look at atmospheric dynamics and inner evolution

“This planet is an extremely interesting case study for atmospheric dynamics and interior evolution, as it sits in an intermediate planetary-mass regime where processes normally overlooked for less massive hot Jupiters may come into play,” says Lisa. Dang, the first author of an article recently published in The Astronomical Diary, PhD student in the Department of Physics at McGill University. “XO-3b has an oval orbit rather than the circular orbit of nearly all other known hot Jupiters. This suggests that it has recently migrated to its parent star; if so, it will eventually settle on a more circular orbit.”

The planet’s eccentric orbit also results in seasonal variations hundreds of times stronger than what we experience on Earth. Nicolas Cowan, professor at McGill, explains: “The whole planet receives three times more energy when it is close to its star during a brief kind of summer, than when it is far from the star.

The researchers also re-estimated the planet’s mass and radius and found that the planet was surprisingly puffier than expected. They suggest and that the possible source of this heating could be due to remnants of nuclear fusion.

Excessive heat and pockets due to the warming of the tides?

Observations from Gaia, an ESA (European Space Agency) mission, have revealed that the planet is more puffy than expected, indicating that its interior could be particularly energetic. Spitzer’s observations also suggest that the planet produces much of its own heat because the excess thermal emission from XO-3b is not seasonal – it is observed throughout the year on XO-3b. It is possible that the excess heat is coming from inside the planet, through a process called tidal warming. The star’s gravitational squeeze on the planet oscillates as the oblong orbit takes the planet farther and then closer to the star. The resulting changes in interior pressure produce heat.

For Dang, this unusual hot Jupiter provides an opportunity to test ideas about the formation processes that might produce certain features of these exoplanets. For example, could the warming tides in other hot Jupiters also be a sign of recent migration? XO-3b alone won’t solve the mystery, but it serves as an important test for emerging ideas about these burning giants.

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