Consider yourself in a location where the winds are so strong that they move at the speed of sound. That’s just one aspect of the atmosphere on XO-3b, one of the hot Jupiters, a class of exoplanets (planets outside our solar system). The planet’s eccentric orbit also causes seasonal variations hundreds of times stronger than what we experience on Earth.
A McGill-led research team provides new insight into what seasons look like on a planet outside our solar system in a recent paper. The oval orbit, extremely high surface temperatures (2,000 degrees C- hot enough to vaporize rock), and “puffiness” of XO-3b may also reveal traces of the planet’s history, according to the researchers. The findings could help scientists understand how exoplanets form and evolve, as well as provide context for planets in our own solar system.
Hot Jupiters are massive, gaseous worlds similar to Jupiter that orbit their parent stars closer than Mercury does the Sun. Though they do not appear to be common in our solar system, they do appear to be common throughout the galaxy. Despite being the most studied type of exoplanet, major questions about how they form remain unanswered. Could there be hot Jupiter subclasses with different formation stories? For example, do these planets form far from their parent stars at a distance where molecules such as water can solidify or closer?
This planet is an extremely interesting case study for atmospheric dynamics and interior evolution because it lies in an intermediate regime of planetary mass where processes normally ignored for less massive hot Jupiters may come into play.
Lisa Dang
The first scenario is more consistent with theories about how planets in our own solar system are formed, but what drives these planets to migrate so close to their parent stars is unknown.
To put those theories to the test, the authors of a recent McGill-led study examined the atmosphere of exoplanet XO-3b using data from NASA’s retired Spitzer Space Telescope. They measured wind speeds and observed eccentric seasons on the planet by obtaining a phase curve of the planet as it completed a full revolution around its host star.
Looking at atmospheric dynamics and interior evolution
“This planet is an extremely interesting case study for atmospheric dynamics and interior evolution because it lies in an intermediate regime of planetary mass where processes normally ignored for less massive hot Jupiters may come into play,” says Lisa Dang, a PhD student at McGill University’s Department of Physics and the first author of a paper published recently in The Astronomical Journal. “XO-3b’s orbit is oval, as opposed to the circular orbits of almost all other known hot Jupiters. This indicates that it has recently migrated toward its parent star; if so, it will eventually settle into a more circular orbit.”
The planet’s eccentric orbit causes seasonal variations hundreds of times stronger than what we experience on Earth. According to Nicolas Cowan, a McGill professor, “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 recalculated the planet’s mass and radius and discovered that it was surprisingly puffier than expected. They propose that the source of this heating could be leftover nuclear fusion.
Excess warmth and puffiness due to tidal heating?
Gaia, an ESA (European Space Agency) mission, discovered that the planet is puffier than expected, indicating that its interior may be particularly energetic. Spitzer observations also suggest that the planet generates a significant amount of its own heat, as XO-3b’s excess thermal emission is observed all year. It’s possible that the extra heat is coming from the planet’s interior, via a process known as tidal heating. The star’s gravitational pull on the planet oscillates as the planet’s oblong orbit takes it farther, then closer to the star. Heat is produced as a result of the changes in interior pressure.
Dang sees this unusually hot Jupiter as an opportunity to test theories about which formation processes might result in certain characteristics in these exoplanets. Could tidal heating in other hot Jupiters, for example, be a sign of recent migration? XO-3b alone will not solve the mystery, but it will serve as an important test for new theories about these scorching giants.
