The Chaotic History of the Retrograde Multi-planet System in K2-290A Driven by Distant Stars

Abstract

The equator of star K2-290A was recently found to be inclined by 124° ± 6° relative to the orbits of both its known transiting planets. The presence of a companion star B at ∼100 au suggested that the birth protoplanetary disk could have tilted, thus providing an explanation for the peculiar retrograde state of this multi-planet system. In this work, we show that a primordial misalignment is not required and that the observed retrograde state is a natural consequence of the chaotic stellar obliquity evolution driven by a wider-orbit companion C at ≳2000 au long after the disk disperses. The star C drives eccentricity and/or inclination oscillations on the inner binary orbit, leading to widespread chaos from the periodic resonance passages between the stellar spin and planetary secular modes. Based on a population synthesis study, we find that the observed stellar obliquity is reached in ∼40%–70% of the systems, making this mechanism a robust outcome of the secular dynamics, regardless of the spin-down history of the central star. This work highlights the unusual role that very distant companions can have on the orbits of close-in planets and the host star’s spin evolution, connecting four orders of magnitude in distance scale over billions of orbits. We finally comment on the application to other exoplanet systems, including multi-planet systems in wide binaries.

Stellar Obliquities in Exoplanetary Systems

“…A stellar companion, K2-290 B (projected separation ≈110 au), was also detected and appears capable of tilting the protoplanetary disk but not capable of tilting the system today. This system is probably the best known candidate for a primordial misalignment, although a recent study by Best & Petrovich (2022) suggested an alternative possibility: The third known star in that system, K2-290 C (projected separation ≈2500 au) might have been responsible for the misalignment.…”

NEID Reveals That the Young Warm Neptune TOI-2076 b Has a Low Obliquity

“…To explain the misalignments, for the K2-290 system it has been suggested that the outer star K2-290 B (projected separation of 110 au) could have tilted the protoplanetary disk of K2-290 A, causing K2-290 b and c to form coplanar in an initially misaligned disk (Hjorth et al 2021). An alternative formation scenario was suggested by Best & Petrovich (2022), in which the third star in the triple system, K2-290 C (projected separation of 2500 au), could be responsible for the misalignment of both planets through gravitational perturbations at much longer timescales (typically 100 Myr). For the HD 3167 system, Bourrier et al (2021) suggest that the perpendicular architecture likely arose from the outer planet being tilted through gravitational interactions with a possible outer companion, while the inner ultrashort period planet likely retained a low obliquity due to tight tidal-coupling with the host star.…”

Companion mass limits for 17 binary systems obtained with binary differential imaging and MagAO/Clio

“…The presence of an additional giant planet can further induce secular resonances (Bazsó & Pilat-Lohinger 2020) or high-e migration (Hamers 2017;Hamers & Tremaine 2017) interior to the giant planet, planet-planet scattering, and push the surviving planet into high-eccentricity orbits which could be further boosted by Kozai-Lidov cycles from the stellar companion (Mustill et al 2021). Wide-orbit stellar companion(s) can also be sufficient to explain stellar spin-planetary orbit misalignment even if the companion is orders of magnitude more distant, including inducing retrograde obliquities (Best & Petrovich 2022).…”