Research Highlight #8
The Orbital Architecture of Qatar-6: A Fully Aligned 3-Body System?
(Rice, Wang, Gerbig, et al. 2023 AJ 165, 65)
If a planet is born in a binary star system — that is, in a system with two stars — how does that affect its orbital evolution? What is the role of stellar multiplicity in flipping orbits, or in pushing them back toward alignment?
These questions provide motivation for the case study examined here: the Qatar-6 AB b three-body system, which includes two stars and a warm Jupiter companion orbiting the primary star. This is the fourth result from the Stellar Obliquities in Long-period Exoplanet Systems (SOLES) survey, which is designed to constrain the evolutionary pathways that produce diverse transiting exoplanet systems.
We began by measuring the Rossiter-McLaughlin (RM) signal of the Qatar-6 A b warm Jupiter. The RM signal encodes which direction the planet orbits relative to the host star's spin. Our symmetric signal, shown in the figure below, indicates that the system is well-aligned with its host star's equator within the sky-plane.
Figure 1: Rossiter-McLaughlin observation and model fit for the warm Jupiter Qatar-6 A b. The system is consistent with exact alignment in the sky-plane.
Combining this result with additional constraints, we found that the true, 3D obliquity of the system is also consistent with near-exact alignment (within a few degrees). Long-term "secular" interactions with stellar companions are often quoted as a way to produce misalignments, so this was already interesting — no evidence for misalignments was found, suggestive of the tendency toward alignment previously noted in single-star warm Jupiter systems.
Investigating the system further, we next looked at how the orientation of the stellar binary orbits might teach us about the system's past. Surprisingly, we found that the binary star system was extremely close to edge-on, just like the transiting exoplanet orbit!
Figure 2: Geometry of the edge-on Qatar-6 AB stellar binary system.
All of the collected evidence so far points toward the 3 bodies in the system being both aligned and collinear: the planet (Qatar-6 A b) orbits in the same direction that its host star (Qatar-6 A) spins, and the orbits of the planet (Qatar-6 A b) and the binary star system (Qatar-6 AB) are both edge-on.
Figure 3: Selection of 1,000 accepted solutions for a binary orbit fit to Qatar-6 AB.
There are certainly caveats — we don't know which way the planet is orbiting within the sky-plane, for example. So, the system could still harbor a misalignment that we can't see within the current dataset. With that said, all existing constraints so far point toward Qatar-6 being a great example of a pristine, fully quiescent system: the system formed with all of its components aligned (or was pushed to alignment over relatively short timescales), and it stayed that way.
If the alignment is truly 3D, it likely originated at the protoplanetary disk phase: precession and dissipation of energy pushes disks towards an aligned state, and gravitational star-disk coupling and/or strong magnetic fields in the young system can keep the planet aligned with its host star.
Is this "normal" for warm Jupiters? What factors decide whether a system maintains such a distinct alignment, versus the many sideways-orbiting and otherwise misaligned systems? Case studies like the Qatar-6 system will enable us to better understand the regimes in which spin-orbit misalignment does (or doesn't!) operate, distinguishing the range of evolutionary pathways for exoplanet systems.