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Making Misaligned Planets

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illustration of a planet orbiting two stars
This artist's impression shows TOI 1338 b, the first exoplanet orbiting two stars to be discovered by the Transiting Exoplanet Survey Satellite (TESS) mission. [NASA's Goddard Space Flight Center]

Can planets form in a tilted disk around a pair of stars? New simulations explore what happens in off-kilter planetary systems.

Seeking Skewed Systems

radio observations of the disk around a protostellar binary system

Radio observations revealed that the young binary system L1551 NE has a misaligned disk. [Adapted from Takakuwa et al. 2017]

Thanks to exoplanet-hunting spacecraft like Kepler, we’ve discovered more than a dozen planets orbiting two stars rather than one. Though all of the circumbinary planets discovered so far orbit close to the same plane as their host stars, theory suggests they could be found perpendicular to the plane of the binary system, and observations of tilted disks around young stars suggest that a wide range of inclination angles is possible.

The lack of planets seen in highly misaligned orbits might mean these planets are rare, but it could also mean that these planets are simply difficult to detect. With misaligned circumbinary systems still hard to come by, we must turn to models to explore what happens in these systems — what determines whether a planet orbiting two stars does so in the same plane as the binary system, perpendicular to it, or somewhere in between?

Modeling Misalignment

Anna Childs and Rebecca Martin (University of Nevada, Las Vegas) approached this question by modeling how the eccentricity of a binary system and the initial tilt of the surrounding disk affect the final orbital parameters of a planetary system. Childs and Martin investigated binary systems with perfectly circular orbits (e=0) and disks inclined by 30 and 60 degrees, as well as binary systems with very elongated orbits (e=0.8) and disks tilted by 60 degrees. In each case, the team initiated their simulations late in the process of planet formation, when the planetary systems are mostly free of gas and contain a host of Moon- and Mars-sized planetesimals.

example simulation results

Simulation results at three points in time for a circular binary system with a 30° inclined disk (top) and an eccentric binary system with a 60° inclined disk (bottom). Click to enlarge. [Childs & Martin 2022]

Their results showed that certain initial configurations generate only coplanar planets, while others churn out only perpendicular planets. In circular binary systems, disks that are tilted by 30 degrees tend to form planets that stick close to the plane of the binary, but if the initial tilt of the disk is cranked up to 60 degrees, collisions and gravitational interactions kick out 84% of the planet-forming material. In contrast, in eccentric binary systems, planets can still form in an extremely tilted disk — but in this case, the resulting planets orbit perpendicular to the binary system.

Giant Planets in the Mix

plot of simulation results

Degree of misalignment from a coplanar or polar orbit for simulations without (left) or with (right) giant planets introduced. The simulations shown are circular and inclined by 30 degrees (C30), circular and inclined by 60 degrees (C60), and eccentric and inclined by 60 degrees (P60). The symbol size and color varies with the planet mass. Click to enlarge. [Adapted from Childs & Martin 2022]

The authors also introduced Jupiter and Saturn analogs into some of their simulations to understand how the presence of giant planets affects the formation of planets in a misaligned disk. Giant planets tended to increase the amount of material kicked out of the planetary systems, but they also increased the rate at which the planetesimals collided, so the few planets remaining at the end of the simulations tended to be more massive.

Notably, almost all of the simulated planets fell into coplanar or perpendicular orbits. If future observations reveal planets at intermediate inclinations, this might mean that those planets followed a different formation pathway from the one explored in this study. Hopefully, it’s just a matter of time before we detect circumbinary planets in misaligned orbits and put our theories to the test!

Citation

“Misalignment of Terrestrial Circumbinary Planets as an Indicator of Their Formation Mechanism,” Anna C. Childs and Rebecca G. Martin 2022 ApJL 927 L7. doi:10.3847/2041-8213/ac574f