It seems natural that stars would spin in the same direction as, and nicely aligned with, their planets. Recent work, however, shows that might not be the case if the star in question is particularly hot.
Toppled-Over Stars
When we picture planets moving around their host stars, we usually imagine them as perfect spheres traveling along perfect circles aligned within a perfect plane. Reality, unsurprisingly, is messier than this ideal image. Some objects, like Mars, maneuver on orbits that look more like ovals than circles, while others, most famously Pluto, drift above and below everyone else on orbits that are tipped over relative to the bulk of the solar system. The closer you look, the more jumbled everything appears: while that’s maybe disappointing from an aesthetic point of view, astronomers thrive in this chaos and latch onto every irregularity they can in order to explain how a given system came to be.
One property astronomers love to measure is the angle between a star’s spin axis and the orbits of its planets, also known as its obliquity. Our Sun sets a good example as a mostly-aligned host star: its north pole is tipped over a modest 7 degrees from the rest of the solar system, a slant larger than that of the leaning tower of Pisa but still respectively close to upright. About a decade ago, however, astronomers began to realize that not all stars were as well behaved. While in general stars are more likely to be aligned with their planets’ orbits than not, a good fraction of the hotter stars seemed to be nearly randomly oriented without any care for what’s circling them.This was an intriguing but difficult to confirm finding, since the traditional ways of measuring obliquity required a tricky procedure and lots of telescope time. Researchers came up with an alternative, faster way to approximate the measurement, but progress was still slow and it was difficult to assemble an unbiased sample of stars. Since it was easier to measure the obliquities of cool stars with large planets, astronomers simply hadn’t attempted to analyze many hot stars with small companions.
New Observations
That’s where Emma Louden (Yale University) and collaborators came in. Over the course of several months, Louden and the team periodically commandeered the High Resolution Echelle Spectrometer (HIRES) instrument on the Keck I telescope to collect spectra of nearly 500 stars.
for the analysis. Since the team was aiming to fill in gaps of previous catalogs, they went after numerous hot stars and stars with planets smaller than Jupiter. After vetting their candidates and cleaning up their data, they were left with a sample that had 500% more hot stars than any previous attempt, and 50% more cool stars for good measure too. The team found that the trend noticed years ago does indeed seem to hold up to this stronger statistical pressure test: hot stars, regardless of the size of their planets, tend to be more misaligned than cool stars. Going further, they also took advantage of their larger sample and looked for trends with other properties, finding weak evidence that planet period might also correlate with stellar misalignment. However, even though their sample was larger than any previously assembled, it isn’t large enough to do more than hint at this additional find.Still, knowing that hot stars seem to misbehave in almost all circumstances does put some pressure on our theories of planetary formation and evolution. While there is still no clear-cut winner, and as the authors note, likely more than one mechanism is at play, we’re beginning to understand what’s more and what’s less likely to have happened to these tipsy, boiling stars and their planets.
Citation
“A Larger Sample Confirms Small Planets around Hot Stars Are Misaligned,” Emma M. Louden et al 2024 ApJL 968 L2. doi:10.3847/2041-8213/ad4b1b