'Hot Jupiter': What an enormous planet's powerful seasons could tell us about our universe

XO-3b is a gas giant with nearly 12 times the mass of Jupiter, with surface temperatures hot enough to turn stone to steam and winds so powerful Earth’s hurricanes seem nothing but a gentle breeze by comparison.

The planet's powerful seasons are partially caused by its peculiar orbit: a fact experts believe could indicate how some solar systems come to look the way they do.

Among these experts is Lisa Dang, a Ph.D. candidate at McGill University in Montreal.

Leading a team of researchers from across North America, Dang authored a paper in last month’s Astronomical Journal providing further insight into the curious nature of XO-3b, a so-called “Hot Jupiter,” and its extreme seasons.

Using data from NASA’s retired Spitzer telescope, Dang’s team took a closer look at the planet’s seasons, measuring the heat and wind speeds on its surface.

In doing so, researchers used XO-3b as a “planetary lab” of sorts, uncovering information with the potential to help us better understand our universe.

A SIGN OF MIGRATION

Discovered in Hawaii in 2007, XO-3b is what Dang calls one of the most “astonishing” finds of the astronomical community.

“It forced us to revisit our theories of how planets form, because these planets are not theorized to form here,” she told CTV News.

The planet is considered a “Hot Jupiter” because it’s, well, hot — a fact which has forced experts to rethink how some planets evolve.

Gas giants like Jupiter and XO-3b are believed to form in only the coldest of conditions, past what is commonly referred to as the “snow line”: far enough from its central star for compounds such as water and methane to form into ice grains.

Over time, these solid grains clump together and become the cores of gaseous planets.

But XO-3b is even closer to its parent star than Mercury is to ours. With such snug proximity to a heat source, how did it come to be?

Many have speculated that the planet originally formed past the snow line, only to migrate closer to its star as time went on.

“They might actually not stay where they formed, but they might move around,” said Dang.

Why and how these planets move, however, remains a mystery.

This migration theory is supported by XO-3b’s unique orbital pattern, says Dang: rather than orbiting its star in a circle, as we do here on Earth, XO-3b moves in an oval-like shape, suggesting the planet has only recently travelled towards its star.

Top-view of XO-3b’s orbit (2014).The gray dots represent the planet’s position at 1 hour intervals and the planet revolves counterclockwise. The shaded area represents our line of sight. (Lisa Dang et. al/The Astronomical Journal)

“One of the mechanisms by which planets move around is a mechanism called ‘eccentricity tidal migration,’” Dang explained. “They start up in a very eccentric orbit. And eventually with tides and interaction with the star, the orbit sort of shrinks” and becomes circular.

The fact that XO-3b is still orbiting in an oval shape indicates that it’s “currently on its way in.”

ECCENTRIC ORBIT, EXTREME SEASONS

XO-3b’s eccentric orbit can also explain its extreme seasons, at least partially.

“Because it’s an oval, the distance between the star and the planet changes,” said Dang. “There's moments in the year when it’s hotter, and moments where it's colder.”

The orbit itself is incredibly brief, with a year on XO-3b lasting no more than 3.2 days.

Using data from the Spitzer telescope, Dang and her team measured the planet’s brightness throughout its three-day orbit, allowing them to determine its varying temperatures.

NASA's Spitzer telescope was retired in 2020 after 16 years studying the universe in infrared light. (NASA/JPL-Caltech)

“We see seasons on the planet, with variations that are hundreds of times [stronger than] the seasons that we have on Earth.”

This includes winds so powerful they travel at nearly the speed of sound.

But something wasn’t adding up; the temperatures recorded were too hot to be chalked up to XO-3b’s orbital behaviour alone.

“We calculated how hot the planet should be, depending on how much heat it's receiving from the star. And then we [saw] that the planet is hotter than how much heat it’s receiving,” Dang said.

So how is this possible?

“We think that actually, what is happening is that the planet is generating a lot of its own energy.”

To get to the bottom of the issue, researchers obtained data from Gaia, a mission led by the European Space Agency, to take a closer look at the radius and mass of XO-3b.

“What we saw is that the planet is a little puffy,” said Dang. “We think there's some mechanism inflating the planet.”

XO-3b’s eccentric orbit may once again be a key contributing factor to this phenomenon.

“When you're on an uncentered orbit, there's this weird, gravitational interaction that makes the planet sort of squish and oscillate. And that makes the planet produce some energy,” said Dang.

A BROWN DWARF?

Evidently, XO-3b is one hot planet. But what if it isn’t a planet to begin with?

“Because it's so massive, we think that it might actually pass the threshold of not being a regular planet anymore,” said Dang. “It’s sort of in-between being a planet and a star.”

Researchers believe XO-3b might fall beneath the "brown dwarf" umbrella, objects that begin their life the same way stars do, but don’t have enough mass to sustain nuclear fusion.

Eventually, fusion stops and the object cools down to become a regular, large planet. Dang says this may be the case for XO-3b: a possible brown brown dwarf that is “still burning nuclear fuel.”

To uncover more about XO-3b and similar mysterious planets, Dang hopes to make use of the recently-launched James Webb telescope.

She will be one of the first Canadian astronomers to be granted access to the new technology in 2022 and will first use it to explore the Neptune-like exoplanet K2-141.

“We’re getting into more interesting climate phenomenon on these planets that we are going to be able to study with Webb,” she said. “We are super excited.”