A Pileup of Perpendicular Planets

Illustration of WASP-79b
Artist’s illustration of WASP-79b, an instance of an exoplanet that circles its star on a polar orbit.
ESO / B. Addison

Much of science includes looking for patterns and traits in information. Patterns within the universe — preferences for sure shapes, areas, alignments, and many others. — can usually reveal hidden underlying physics that drives nature to take a non-random course. This implies that patterns and traits continuously present the important thing to understanding how the universe works.

Exoplanet populations are an particularly intriguing place to search for traits. In current years, our pattern of noticed exoplanets has grown massive sufficient that we are able to now begin to do helpful statistical evaluation — and there’s rather a lot we are able to hope to be taught from this concerning the formation and evolution of planetary programs.

Illustration of a protostar
A protostar lies embedded in a disk of gas and dirt on this visualization. Since stars and their planets kind from the identical cloud, it could make sense for his or her rotations to be aligned.
NASA’s Goddard Space Flight Center

One explicit curiosity amongst exoplanets: a planet’s orbital path shouldn’t be at all times aligned with its host star’s spin path. Since a star and its planets all kind out of the identical rotating cloud of gas and dirt, conservation of angular momentum ought to produce planet orbits and stellar spins which can be aligned. But, whereas we see a big inhabitants of well-aligned programs, we additionally see a smaller inhabitants of misaligned programs.

What causes planets to change into misaligned with their stars? A new examine led by Simon Albrecht (Aarhus University, Denmark) examines patterns in a inhabitants of noticed star–planet programs to search out out.

A Polar Population

A diagram illustrating the angle between the sky-projected stellar spin and planetary orbit and the actual 3D angle between the spin and orbit
Diagram illustrating the angle between the sky-projected stellar spin and planetary orbit (λ) and the precise 3D angle between the spin and orbit (Ψ). The tilt of the star relative to the observer line of sight is marked by i.
Albrecht et al. 2021

Albrecht and collaborators explored a beneficial pattern of 57 star–planet programs. For the bulk of planetary programs with noticed spin/orbital instructions, we are able to solely measure the angle between the sky-projected orbital and spin axes. But for the pattern that Albrecht and collaborators used, now we have unbiased measurements of the inclination angle of the star relative to our line of sight. Thus, for these 57 programs, the authors have been capable of establish the precise angle in 3D space between the planets’ orbital axes and the stars’ spin axes.

The consequence? Albrecht and collaborators discover that almost all of the programs are aligned, as anticipated. But the 19 misaligned programs do not have misalignments which can be distributed randomly by way of all angles. Instead, nearly all of the misalignments cluster round 90° (starting from 80°–125°) — that means that the planet orbits the poles of the star, perpendicular to the path that the star spins.

Two graphs comparing the distribution of orbits in misaligned systems that the scientists' expected and the actual distribution
Left: The angle between the sky-projected orbital and spin axes (λ) for the authors’ pattern. Right: The precise angle between the axes (Ψ). The precise angles present two clusterings: one close to zero (aligned), and one round 90° (perpendicular). Click to enlarge.
Adapted from Albrecht et al. 2021

What might trigger this polar pileup? The authors suggest a number of theoretical prospects that embody dynamical interactions between the planet and the star, or between the planet and a further unseen, distant companion physique. But, as we’ve seen, nature has a thoughts of its personal — and there could also be a number of mechanisms at work! We don’t but have sufficient info to resolve this puzzle with certainty, however a continued seek for patterns is certain to level us in the correct path finally.


“A Preponderance of Perpendicular Planets,” Simon H. Albrecht et al 2021 ApJL 916 L1. doi:10.3847/2041-8213/ac0f03

This publish initially appeared on AAS Nova, which options analysis highlights from the journals of the American Astronomical Society.


Leave a Comment

Your email address will not be published. Required fields are marked *

Shopping Cart