On Earth, seasons change with regularity, even as climate change is altering their length and texture. But on XO-3b, the seasons come and go with a ferocity and mystery befitting the hot gas giant.
McGill University doctoral student Lisa Dang studied this “hot Jupiter,” a planet like our own gas giant Jupiter but closer to its Sun than Mercury is to ours. There, she discovered seasons, incredible winds, “cryptic heating” and hints regarding the planet’s orbit and formation history. Hot Jupiters are common and well-studied, but this one offers some secrets science is beginning to untangle.
In an interview, Dang said this is the third detection of seasonal change on a world beyond our Solar System.
“The detection of seasons on each of these planets alone is pretty exciting, but now with three planets, we can start comparing the observations of each of these … to find out what drives the climate and seasons on these worlds,” she added.
XO-3b lives fast. Winds blow at 2,400 kilometres per hour, and the planet circles its home star every three days. The planet’s orbit is oval-shaped, with its closest approach to the star being about five million kilometres and its farthest not quite nine million kilometres. That means summer on XO-3b lasts a day and winter just two days. Summer temperatures are about 2,200 Kelvin or 1,900 Celsius, while winter temps are 1,500 K or 1,200 C. The planet gets three times more solar energy in the summer.
The planet is incredibly hot, but the heat is, surprisingly, not just because of the planet’s proximity to the star.
Dang found evidence of “cryptic heating” — that is, something else heating up this puffier-than-expected world.
“The inflated radius suggests the planet is producing a lot of its own heat,” Dang said at a virtual media session last week during a scaled-down American Astronomical Society meeting. Dang’s team thinks there might be some ongoing nuclear fusion.
“XO-3b was thought to be not massive enough to have this deuterium burning, but we are seeing tentative evidence that it might actually generate a lot of its own heat,” she said.
Tidal flexing from the star’s gravity could also be producing this additional energy.
“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,” Dang said in a press release.
It’s unusual for a hot Jupiter to have this kind of orbit so close to its star. The inference is that the three billion-year-old world has taken about that long to migrate from the outer solar system. It has yet to attain the more-usual circular orbit for its class of planets. As it happens, this eccentric orbit is what drives seasons on XO-3b and not the tilt of the axis, as here on Earth.
The planet deserves more attention, in order to understand its orbit, history and heating.
“XO-3b is obviously a strange planet with many interesting characteristics, so we would like to re-visit the system with a complementary observatory to confirm the robustness of internal heating, which would give information about the interior of the planet,” Dang explained to SkyNews. Both the Hubble Space Telescope or the newly launched James Webb Space Telescope could “provide pristine observations allowing us to characterize these seasons: Is it cloudy or rainy during the winter? Do these clouds boil off during the summer?”
It may be some time before we have weather forecasts for XO-3b’s balmy summer days or less-balmy winter nights, but we could be learning more about this oddball world soon enough.
Dang and her team reached their conclusions after compiling data from the retired Spitzer Space Telescope. The work was published in December 2021 in The Astronomical Journal.
Christopher Cokinos’ astronomical articles, essays and poems have appeared in such publications as Scientific American and the Los Angeles Times. He divides his time between northern Utah and Tucson, Arizona, with telescopes in each locale.