Cool It Down: Warm Jupiters Are Better Aligned Than Hot Jupiters

Although some planets calmly follow the lead of their host stars and always stay more or less aligned with the star’s equator, others are more unruly and are found circling their parent in any direction they choose. A recent study adds evidence to the claim that “warm Jupiters”, or massive planets that are a little farther from their host star than their hot-Jupiter cousins, are almost always well-behaved no matter what else is going on around them.

Disorderly and Dramatic

Hot Jupiters, or massive planets that orbit incredibly close to their host stars, have pretty tough lives. Thanks to a ceaseless stellar blowtorch constantly scorching one hemisphere, their atmospheres can be hot enough to melt steel. Even among this tortured group, some have it worse than others. While some stars, like our sun, are relatively cool, others are more massive and therefore hotter. Hot Jupiters around these hot stars must tolerate even more extreme conditions than their (relatively) cooler counterparts.

Perhaps, then, we can forgive this charred group for acting out. In recent years, astronomers have noticed that hot Jupiters around hot stars tend to orbit within planes that are severely misaligned to the one set by their stars’ equators. Some of these planets have been found on nearly polar orbits, meaning they move nearly perpendicularly to the direction of the star’s spin. Their counterparts around cooler stars, however, seem to mostly follow the rules and stay well aligned with their star.

This pattern invites some obvious questions: Why the difference? Is there something about a star’s temperature that would determine the geometry of its planets’ orbits?

Cool and Collected

To answer these and others, a team of astronomers have spent the past few years measuring the alignments of “warm Jupiters”, or planets about as massive as hot Jupiters but slightly farther from their host stars. The latest study in this effort, led by Xian-Yu Wang of Indiana University, brings the total sample of measured warm Jupiters up to 23 planets. None of these worlds are misaligned; in other words, warm Jupiters, even when around hot stars, never set off on their own.

This consistency allows the team to put together a self-consistent story that not only explains their data, but also the outstanding mystery of how hot Jupiters ended up on such inhospitable orbits. In their model, most planets form in quiet disks that are aligned with the star’s spin. In some systems, the planets jostle one another around, sending one or more careening toward the star on wild, eccentric orbits. These unlucky worlds will eventually end up on tight, circular, but tipped-over orbits thanks to tidal interactions and gravitational dynamics: in other words, they’ll become misaligned hot Jupiters. As for the hot Jupiters we see that are aligned, that’s where the stellar temperature comes into play. Cool stars will slowly, over time, wrestle any misaligned planets onto lower-inclination orbits, again thanks to tides, while hot stars are powerless to alter the orbits of their planets.

Through this combination of new observations and intensive modeling of planetary dynamics, astronomers continue to build a fuller picture of planetary formation, and ultimately, how any planets got to where they are today.

Citation

“Single-star Warm-Jupiter Systems Tend to Be Aligned, Even around Hot Stellar Hosts: No Teff–λ Dependency,” Xian-Yu Wang et al 2024 ApJL 973 L21. doi:10.3847/2041-8213/ad7469