Our galaxy hosts a diverse array of planetary systems—some stars with many worlds, others with none. Typically, planets orbit in the flat equatorial plane of their host star, much like in our Solar System. Yet exceptions exist, such as the intriguing HD 3167 system, located 150 light-years from Earth.
Precise measurements of star-planet spin-orbit alignments are crucial for unraveling exoplanet formation and migration. Disk-driven migration generally preserves alignment between the protoplanetary disk's angular momentum and planetary orbits. However, various factors can cause misalignments between a star's rotation and its planets' paths.
Mapping the orbital architecture of multi-planet systems helps distinguish formation and migration scenarios. A new study in Nature Astronomy, led by astronomers from the University of Geneva (Switzerland), unveils details of one such exceptional system.
In 2016, astronomers identified two super-Earths—planets between Earth and Neptune in size—orbiting HD 3167, 150 light-years distant. These worlds, HD 3167 b and c, complete orbits every 30 days. A third planet, HD 3167 d, was found in 2017, with an 8-day orbit.
This system stood out early on. Observations revealed that the outer planets, HD 3167 c and d, follow polar orbits, circling above and below the star's poles rather than its equator, unlike Earth and its siblings.
Polar orbits aren't unheard of for individual exoplanets, but this system's oddity deepened. Using the ESPRESSO spectrograph on ESO's Very Large Telescope in Chile, researchers measured the innermost planet HD 3167 b's orbit for the first time. It lies in the star's equatorial plane—perpendicular to those of c and d.
“It was clearly a surprise,” says lead author Vincent Bourrier. “It's something radically different from our own solar system. If you had a telescope and looked at the trajectory of the other planets in the system, they would move vertically across the sky.”
The team suggests this misalignment stems from an unseen massive companion at the system's outer edge. Shweta Dalal from the University of Exeter (UK) points to evidence for a Jupiter-sized planet orbiting every 80 days. Its gravity may have tilted the outer planets' paths, while the tightly bound HD 3167 b remained aligned with the star.
In contrast, our Solar System's inner planets avoided such disruption thanks to their wider spacing. Here, all known planets huddle within Mercury's orbit, intensifying gravitational interactions.
