Hot Jupiters reveal new insights into seasons on exoplanets

Observations of hot Jupiter exoplanets could advance our knowledge of seasons on planets outside our solar system as well as their planet origins and evolution.

Winds that are so strong that they move at the speed of sound is just one of the aspects of the atmosphere on XO-3b, one of a class of exoplanets, known as hot Jupiters. The eccentric orbit of the planets also results in seasonal variations that are hundreds of times stronger than what we experience on Earth.

In a recent paper, a McGill-led research team provides new insight into what seasons appear to be like on a planet outside of our solar system. The researchers also suggest that the oval orbit, extremely high surface temperatures (2,000 degrees C- hot enough to vaporise rock) and ‘puffiness’ of XO-3b reveal traces of the planet’s history. The findings will potentially advance both the scientific understanding of how exoplanets form and evolve, as well as give some context for planets in our own solar system.

Characteristics of a hot Jupiter

Hot Jupiters are gigantic, gaseous planets like Jupiter, that orbit closer to their parent stars than Mercury’s proximity to the Sun. Though not present in our own solar system, they appear to be common throughout the galaxy. Despite being the most studied type of exoplanet, important questions remain about how they form, such as if there could be a subclass of hot Jupiters with different formation stories.

For example, do these planets take shape far from their parent stars at a distance where it is cold enough for molecules such as water to become solid, or much closer? The first scenario seems more plausible with theories about how planets in our own solar system are born, but the question of what would drive these types of planets to migrate so close to their parent stars remains unclear.

To test those theories, the authors of a recent McGill-led study utilised data from NASA’s retired Spitzer Space Telescope to analyse the atmosphere of exoplanet XO-3b. They observed eccentric seasons and measured wind speeds on the planet by obtaining a phase curve of the planet as it completed a full rotation about its host star.

Looking at atmospheric dynamics and interior evolution

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

The eccentric orbit of the hot Jupiter also leads to seasonal variations hundreds of times stronger than what we experience on Earth. Nicolas Cowan, a McGill professor commented: “The entire planet receives three times more energy when it is close to its star­­ during a brief sort 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 that the possible source of this heating could be due to leftover nuclear fusion.

Excess warmth and puffiness could be due to tidal heating

Observations by Gaia, an ESA (European Space Agency) mission, discovered that the planet is puffier than expected which indicates its interior may be particularly energetic. Observations from Spitzer also suggest that the planet produces much of its own heat as XO-3b’s excess thermal emission is not seasonal, it is instead observed throughout the year on XO-3b. It is possible that the excess warmth is coming from the planet’s interior, through a process called tidal heating. 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 examine ideas about which formation processes may producer certain characteristics in these exoplanets. For instance, could tidal heating in other hot Jupiters also be a sign of recent migration? XO-3b alone won’t unlock the mystery, but it serves as an important test for emerging ideas about these blistering giant planets.

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