One of the greatest mysteries in exoplanet science is why hot Jupiters exist. These strange worlds, like their namesake in our own solar system, are giant balls of gas many times the mass of Earth. But unlike Jupiter, which orbits far from our Sun, hot Jupiters have tight orbits, some with years lasting just days or even hours, giving them scorching hot temperatures of up to 1,700°C (3,100°F).
Hundreds of hot Jupiters have been identified since the first, 51 Pegasi b, was discovered in 1995. Yet astronomers still haven’t figured out how they end up so close to their star. “We weren’t expecting hot Jupiters,” said Laura Flagg, an astronomer at Cornell University in New York. “They’ve been something of a conundrum since they’ve been found.”
Ignacio Mendigutía, an astronomer at the Spanish Astrobiology Center in Madrid, and colleagues think the star’s magnetic field may be involved.
Studying data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and ESA’s Gaia star-mapping spacecraft and focusing on 47 intermediate-mass stars 1.5 to 3 times the mass of our Sun, the group saw a limit to how close hot Jupiters orbit their star. They believe the magnetic field of the star may be helping to hold the planets at bay. Their research was published in Astronomy and Astrophysics.
Planets are born around young stars inside protoplanetary disks—wide disks of dust and gas that persist for several million years. In one theory of hot Jupiter formation, the interaction of this disk with a newly formed gas giant causes the planet to migrate inward toward the star as the gas and dust push against the planet. “The disk material alters the orbit of the planet, which begins to spiral towards the star,” Mendigutía said.
The protoplanetary disk does not extend all the way to the star; the star’s outward temperature vaporizes the dust when it gets too close, and its magnetosphere halts the gas, funneling it to the star’s poles when it gets close enough.
“We found it is the absence of gas, not the absence of dust, that is important.”
For stars like our Sun, the difference between the gas and dust barriers is hard to spot, Mendigutía said. But for hotter stars like the intermediate-mass stars the team studied, that difference can be much more pronounced because the higher temperature vaporizes the dust farther out. “The gas barrier could be at 2 times the stellar radius, while the dust barrier could be at 8 times,” Mendigutía said.
The TESS and Gaia data showed that all the hot Jupiters they studied appeared to stop their inward migration at the gas barrier. This result suggests that this line, dictated by the star’s magnetosphere, is where hot Jupiters cannot get any closer to their star and would explain why we find so many in these tight orbits. “From the moment there is no more material in the disk, migration ceases,” Mendigutía said. “We found it is the absence of gas, not the absence of dust, that is important.”
Eating Planets
Flagg, who was not involved in the study, said the idea was enticing. “There has to be a mechanism to stop that migration. Otherwise, they would all go into their stars,” she said. “This seems like a really logical way to stop the migration.” However, she noted that the authors assumed that protoplanetary disks cause the inward migration of hot Jupiters, but there are other possibilities. The migration could be caused by gravitational interactions with other planets or stars—something believed to have happened with Jupiter as its orbit shifted early on. Or the planets could form in place on their tight orbits around the stars.
The idea can be tested by studying Herbig stars, which are 3 to 4 times the mass of our Sun, Mendigutía said. These stars are known to lack magnetospheres, and if they also lack hot Jupiters, that could suggest that any gas giants kept migrating inward until they were eaten by the star. For now, however, hunting for planets around such stars is difficult because of their size and brightness.
“They are a mystery, and mysteries are always fun.”
Further observations from TESS could be useful too, Flagg said. Astronomers could look for hot Jupiters in young star systems that still have a protoplanetary disk. “If we see a lot of really young hot Jupiters, it’s really likely migration is disk driven,” she said.
Finding out for certain would give us insight into this rare and unusual class of planets that although not present in our own solar system, is a window into the characteristics of many others. “It’s possible that a system where we find a potential for life has a hot Jupiter,” said Flagg. “They are a mystery, and mysteries are always fun.”
—Jonathan O’Callaghan (@astro_jonny), Science Writer