The process of finding thousands of planets around other stars has been one of having expectations overturned again and again. Nevertheless, even when expecting the unexpected, the planetary system HD 3167 is a shock – with one planet behaving normally, a second orbiting at right angles, while a third does something else again.
As we understand it, planets form with orbits roughly in line with the equator of their parental star. That's certainly the case for our Solar System (demoting Pluto had the advantage of removing the one so-called planet that wasn't obeying the rules). We don't always know where stars' equators are. In the cases we can determine them, the majority of star systems are similar, but there are more exceptions than anyone expected.
Astronomers think planets in orbits that take them over their star's poles (or stranger still, the reverse direction to the star's spin) were disrupted by the gravity of some larger object, be it a giant planet or star.
All of this makes the case of HD 3167, reported in Astronomy and Astrophysics particularly puzzling. This system has one planet in an ordinary orbit closely aligned with the plane of the star's equator, and another whose orbit passes almost over the poles. We know less about a planet at an intermediary distance, but its thought its orbit is also close to polar.
All three lie very close to their star, with orbits lasting just 23 hours, 8.5 days, and 29.8 days. Confusingly, in order outwards, their names are HD 3167b, d, and c. HD 3167s doesn't transit across its star's face as seen from Earth and was therefore discovered later, with less known about it.
The authors explain these observations by concluding that an as yet unseen object shifted the orbits of the outer two worlds, more thoroughly in c's case than d's. The closer a planet is to its star, the more tightly bound its orbit, so HD 3167b's orbit apparently wasn't changed much.
Although the explanation is simple, the physics is not. There is probably quite a narrow window of forces strong enough to disrupt the outer two worlds, but barely change the inner one. Moreover, once in their new orbits, the three planets would be engaging in a slow gravitational tug of war trying to get their orbits to align. With c and d having more mass than HD 3167b, its persistence is unexpected.
On the assumption the disruptive force in the system is a planet, Professor Vincent Bourrier of l’Université de Genève and co-authors of the paper have begun the search for it, hoping its size and orbit will answer some questions. Alternatively, chaos may have been caused by a close encounter with another system, something far harder to track.
The planets of the HD3167 system themselves seem rather ordinary, even if their orbits are strange. HD3167c and d both appear to be somewhat smaller and lighter than Neptune, while HD3167b is a so-called “super-Earth”, with a mass five times our home. Even though HD3167 is an orange dwarf half as bright as the Sun its innermost planet is close enough to be horrendously hot. HD3167 c and d, while cooler, would also be far too hot for life, even if they were rocky.