When we think of Jupiter-like planets, we usually picture massive, cloud-covered worlds orbiting far from their stars. This distance prevents their volatile gases from being evaporated by the heat of the stars, much like what happens in our solar system. So why do so many exoplanets, known as “hot Jupiters,” orbit very close to their stars? That's the question astronomers ask as they study these extreme worlds more closely.
It turns out that hot Jupiters don't start their lives in such close proximity. Instead, they form much farther from their stars in the protoplanetary nebula. This raises the question: How did they migrate inward? The planetary scientists' answer was “we're not sure.” But astronomers at MIT, Penn State University, and a number of other institutions believe they have a better answer. They have found a hot Jupiter “progenitor,” a young version of a Jovian world that is slowly transitioning from cold to hot. The clues lie in its orbit and could provide insight into how other planets evolve.
Introduction of a Proto Hot Jupiter
This new world is called TIC 241249530 b, and it lies about 1,100 light-years away from us. Instead of orbiting its star in a nearly circular elliptical orbit (like our Jupiter does around the Sun), this world is in a highly elliptical orbit. This squashed “egg-shaped” orbit takes it very close to its star (about 10 times closer than Mercury's orbit). Then it moves out to about the distance that Earth is from the Sun. Not only is that a weird orbit, but it gets weirder. The orbit is “retrograde.” That is, its direction of motion is opposite to the star's rotation. Think of it this way: The star rotates in one direction, and the planet orbits it in the opposite direction.
Both the highly elliptical orbit and the retrograde orbit tell planetary scientists that the formerly “cool,” Jupiter-like world is evolving into one of those hot Jupiters. And as if that wasn't weird enough, the star the planet orbits is actually a binary star. That is, it has a stellar companion. Over time, successive interactions between the two orbits – the planet's and its star's – force the planet to move ever closer to its star. This forces its elliptical orbit to change into a tighter, more circular one. This will take about a billion years, and then the planet will have fully evolved into a hot Jupiter.
An orbital comparison of this evolving hot Jupiter if it existed in our solar system. Courtesy of NOIRLab.
How do hot Jupiters fit into the formation theory?
The standard theory of planet formation usually assumes that rocky worlds form closer to their stars than gas and ice giants. This is because the heat of the newborn star evaporates any “volatile” gases like hydrogen and keeps them away from newly forming planets. Worlds with lots of these volatiles are more likely to form in cooler places where these gases don't evaporate.
Artist's impression of early planet formation from gas and dust around a young star. Planets with large amounts of volatile elements (such as hydrogen) require cooler environments much farther from their stars to preserve their volatile elements. So-called “hot Jupiters” can form farther away but then migrate closer to their stars. Image credit: NASA/JPL-Caltech
So does this new world fit that theory? According to MIT's Sarah Millholland, it does. “This new planet supports the theory that high eccentricity migration should explain some of the hot Jupiters,” Millholland said. “We think this planet must have been a frigid world when it formed. And because of the dramatic orbital dynamics, in about a billion years it will become a hot Jupiter with temperatures of several thousand Kelvin. So it's a huge shift from where it started.”
So this hot Jupiter (and many others observed in exoplanet studies) started out farther from its star. Then it came closer through orbital interactions. This could explain many of the hot Jupiters observed in exoplanet discoveries.
Simulations of orbital dances
“It's really difficult to catch these hot Jupiter progenitors 'red-handed' during their extremely eccentric phases, so it's very exciting to find a system that goes through this process,” says Smadar Naoz, a professor of physics and astronomy at the University of California, Los Angeles, who was not involved in the study. “I believe this discovery opens the door to a deeper understanding of the birth configuration of the exoplanet system.”
Of course, tracking changes in exoplanets' orbits can take a long time, so Millholland and her colleagues ran computer simulations that allowed them to model how this particular hot Jupiter might have evolved. The team's observations, as well as their simulations of the planet's evolution, support the theory that hot Jupiters may form through high-eccentricity migration, a process in which a planet gradually moves into place over time through extreme changes in its orbit.
“It's clear not only from this but from other statistical studies that high eccentricity migration should explain some of the hot Jupiters,” Millholland said. “This system shows how incredibly diverse exoplanets can be. They are mysterious other worlds that can have wild orbits that tell a story about how they got that way and where they're going. For this planet, its journey isn't quite over yet.”
For more informations
Astronomers discover a highly eccentric planet on its way to becoming a hot Jupiter
A hot Jupiter progenitor on a supereccentric retrograde orbit
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