Astronomers have recently found that roughly a third of planet-forming disks around young Sun-like stars are tilted relative to the direction that their star spins.
“All young stars start out with a disk. But the relative orientation between the disk and the star’s spin axis, little was known about that,” explained lead researcher Lauren Biddle, a planetary scientist at the University of Texas at Austin.
This discovery, published in Nature, could help answer the long-standing question of how planets come to orbit their stars at wonky angles: Maybe they were born that way.
Skewed from the Start
There’s a universal truth that when a new star collapses out of a cloud of gas, angular momentum must be conserved. That means that as the nebulous star shrinks in size, it also rotates faster, like when a spinning ice skater draws their arms in and speeds up. The surrounding leftover gas and dust flatten out into a disk that spins in the same direction as the star, and that disk may eventually form planets that spin and orbit in that same direction.
But the universe is rarely so neat and tidy.
Of the thousands of known exoplanets, dozens of them orbit at wonky angles relative to their star’s spin axis. In our own solar system, the plane in which the eight planets orbit is tilted by about 6º from the Sun’s spin axis. Astronomers have theorized that some of these misalignments, or obliquities, result from dynamical events that take place after a planetary system has already formed: A star passes by and disturbs the orbits, or a major collision knocks a planet off course.
Some of those misalignments, however, are baked in from the start. Previous studies have attempted to observe young star systems and their planet-forming disks to see whether those disks start out tilted or aligned. But those studies were limited by the fact that not many protoplanetary disks had yet been discovered, and many of those that were known were part of binary star systems, Biddle explained. Although those studies found some tilted disks, the gravity from the binary star, rather than an intrinsic misalignment, may have been the culprit.
“If systems begin with primordially tilted orbits, then there is no need to invoke other mechanisms—many of which would destabilize neighboring planets—within those systems,” said Malena Rice, a planetary astrophysicist at Yale University in New Haven, Conn. “By understanding the range of primordial tilts and comparing that distribution with more evolved systems, we can piece together the evolutionary sequences of different classes of planetary systems.” Rice was not involved with this study.
“The one-third rate of misalignment stands independent of everything else.”
Biddle and her colleagues compiled a new sample of young star systems by combining observations of protoplanetary disks from the Atacama Large Millimeter/submillimeter Array (ALMA) and measurements of stars’ spin from the Transiting Exoplanet Survey Satellite (TESS) and retired K2 mission. Biddle explained that because ALMA, TESS, and K2 have released such large datasets, her team could curate their sample to look only at Sun-like stars that did not have any binary companions.
They found that 16 of the 49 stars in their sample (about a third) had protoplanetary disks with obliquities of at least 10°, the lower limit of what they could measure. The remaining two thirds of the systems showed no significant evidence of misalignment. This rate of high-obliquity disks is consistent with past studies but more than doubles the number of young, single, Sun-like stars for which astronomers know the degree of disk misalignment.
The 16 stars that host tilted disks did not share any obvious characteristics like mass, temperature, and size, and the disks themselves also had different sizes, masses, and structures.
“We didn’t find any correlation there,” Biddle said. “At this point, independent of other system parameters, the one-third rate of misalignment stands independent of everything else.”
Oblique Across Space and Time
Past studies have suggested that moderate disk obliquities might rise from imperfections in the nebulous cloud that formed the star system: An odd clump in the right spot might create turbulence that grows stronger as the cloud collapses, or the clump might fall onto the disk late and tip it off its axis.
“Moderate misalignments of a few tens of degrees can be produced naturally by either turbulence in the natal molecular cloud, late-stage disk accretion, or some combination of the two,” Rice said.
However, that doesn’t necessarily mean that every misaligned exoplanet, or even a third of them, started out that way.
“It would be great to take a crack at mapping stellar obliquities across space and time.”
“A misalignment between a planet’s orbital plane and its host star’s spin axis can originate in two broad phases: during the star and planet formation stage…[and] later, during the system’s main sequence lifetime,” explained Simon Albrecht, an astronomer at Aarhus University in Denmark who was not involved with this research. “If we can determine the fraction of systems that are already misaligned right after birth, that helps us distinguish between these two broad possibilities.”
Determining how much of a system’s tilt comes early or late and whether that tilt changes over a planetary system’s lifetime will require observing a lot more misaligned planetary systems at all stages of evolution, Biddle said. She added that the upcoming data release from the now-retired Gaia mission will be key to answering both of those questions.
“It would be great to take a crack at mapping stellar obliquities across space and time,” Biddle said. “Being able to fill in that time parameter space will help quantify how important dynamics is for generating that final [obliquity] distribution that we observe in planetary systems.”
—Kimberly M. S. Cartier (@astrokimcartier.bsky.social), Staff Writer
This news article is included in our ENGAGE resource for educators seeking science news for their classroom lessons. Browse all ENGAGE articles, and share with your fellow educators how you integrated the article into an activity in the comments section below.