A New Angle on Forming Misaligned Stellar Systems

In mystery thriller books, the authors always lead you to suspect that the culprit is someone outside the group: the gardener or the locksmith, perhaps. Sometimes, however, the answer is right in front of you, and the perpetrator is in the inner circle. A group of astronomers has recently reached the same conclusion: that an ongoing orbital alignment mystery seen in some stellar systems isn’t caused by disruptions from the outside, but rather comes from within the stellar systems themselves. 

A cartoon of a protoplanetary disk: the star is at the middle surrounded a small gap and then by the inner disk, there's another gap, and then the outer disk is present. The angular momentum vectors all point up but at slightly different angles.

A schematic of the geometry of a misaligned system, showing the angular momentum direction of the star, the inner disk, and the outer disk. [Epstein-Martin et al. 2022]

A Mystery Arises 

When a stellar system forms, everything is thought to be aligned: the star forms, it ignites nuclear fusion, and all of the leftover gas and dust orbit in a single plane and in the same direction that the star spins. This theoretical picture initially seemed to fit the planetary systems we had found…. that is, until recent space missions started uncovering thousands of new planets and began to tilt this theory on its head. All of a sudden, astronomers were discovering stars whose spin axes were misaligned with the orbits of their planets. But how does this situation arise? Shouldn’t the angular momentum of the system extend to the stellar spin axes, and everything should be aligned? According to recent exoplanet discoveries, apparently not!  

These questions remain a hot topic in the field of planet formation. One proposed explanation is that an external companion star could exert a torque on a star-forming region and misalign everything. In counterpoint, a team of astronomers led by Marguerite Epstein-Martin (California Institute of Technology and Columbia University) posit that the troublemaker was instead internal: forces within the disk itself.

A plot of stellar age [years]on the x-axis (going from ~10^5 to 10^7) against angular momentum (in AU^2 solar masses/yr) on the y-axis (going from ~10^-3 to ~10^1). The star is the bottom curve then the inner disk (A_in = 5 - 10 AU, a shaded region), and the outermost disk (A_out = 40 - 90 AU, a shaded region). All slowly slopes downward to the right.

The angular momentum ranges for the three components in the authors’ modeled system of a star and its surrounding disk. The yellow line shows a solar-mass star, the red region shows the inner disk, and the purple region shows the range of values for an outer disk. This figure clearly shows the hierarchy of angular momentum in the system. [Epstein-Martin et al. 2022]

An Unexpected Suspect 

Protoplanetary disks are typically modeled as rigid objects. However, recent observations show ~85% of the disks that can be resolved contain gaps with misalignments between the inner and outer disks, which result from the formation of massive planets or the presence of a stellar binary companion. The team theorized that in these cases of an inner and outer disk misalignment, the outer disk can play the role of a stellar companion and influence the dynamics of the inner disk–star orientation. From there, the team used equations to model these systems and found that there’s a hierarchy to the angular momentum within the system: the outer disk has the largest angular momentum and will apply a torque on the inner disk, which will itself exert a torque on the star. This is analogous to the dynamics between a star, an unbroken disk, and an outside companion that caused the misalignment in previous theories.  

Identification of the Culprit 

By using a series of complex equations that represent the dynamics in these misaligned systems, the team determined that, given the timescale of the contraction of the star and the lifetime of the disk, there would be just enough time for the disruptions to take place that cause the star’s misalignment with its eventual planets’ orbits. Though this does fit observations, the team notes that they employed several simplifications and assumptions. Overall, this study provides a new avenue for the formation of disks misaligned from the spins of their host star. So next time, don’t get distracted by the outside characters, because the culprit could come from within! 


“Generating Stellar Obliquity in Systems with Broken Protoplanetary Disks,” Marguerite Epstein-Martin et al 2022 ApJ 931 42. doi: 10.3847/1538-4357/ac5b79