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A Pileup of Perpendicular Planets

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Illustration of a planet orbiting a star in an ellipse that traces above the poles of the star rather than in the equatorial plane.
Artist's illustration of WASP-79b, an example of an exoplanet that circles its star on a polar orbit. [ESO/BAddison]

In some planetary systems, the direction that a star spins and the direction its planets orbit don’t always line up. A new study explores what we can learn from these nonconformists.

Nature Is Trending

Much of science involves searching for patterns and trends in data. Patterns in the universe — preferences for certain shapes, locations, alignments, etc. — can often reveal hidden underlying physics that drives nature to take a non-random course. This means that patterns and trends frequently provide the key to understanding how the universe works.

illustration of dust and gas swirling around a bright, newly forming star.

A protostar lies embedded in a disk of gas and dust in this visualization. Since stars and their planets form from the same cloud, it would make sense for their rotations to be aligned. [NASA’s Goddard SFC]

Exoplanet populations are an especially intriguing place to look for trends. In recent years, our sample of observed exoplanets has grown large enough that we can now start to do useful statistical analysis — and there’s a lot we can hope to learn from this about the formation and evolution of planetary systems.

One particular curiosity among exoplanets: a planet’s orbital direction is not always aligned with its host star’s spin direction. Since a star and its planets all form out of the same rotating cloud of gas and dust, conservation of angular momentum should produce planet orbits and stellar spins that are aligned. But, while we see a large population of well-aligned systems, we also see a smaller population of misaligned systems.

diagram labeling various angles on a sphere relative to the line of sight.

Diagram illustrating the angle between the sky-projected stellar spin and planetary orbit (λ) and the actual 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]

What causes planets to become misaligned with their stars? A new study led by Simon Albrecht (Aarhus University, Denmark) examines patterns in a population of observed star–planet systems to find out.

A Polar Population

Albrecht and collaborators explored a valuable sample of 57 star–planet systems. For the majority of planetary systems with observed spin/orbital directions, we can only measure the angle between the sky-projected orbital and spin axes. But for the sample that Albrecht and collaborators used, we have independent measurements of the inclination angle of the star relative to our line of sight. Thus, for these 57 systems, the authors were able to identify the actual angle in 3D space between the planets’ orbital axes and the stars’ spin axes.

Two plots showing the two measured angles for the population of 57 planetary systems.

Left: The angle between the sky-projected orbital and spin axes (λ) for the authors’ sample. Right: The actual angle between the axes (Ψ). The actual angles show two clusterings: one near zero (aligned), and one around 90° (perpendicular). Click to enlarge. [Adapted from Albrecht et al. 2021]

The result? Albrecht and collaborators find that the majority of the systems are aligned, as expected. But the 19 misaligned systems do not have misalignments that are distributed randomly through all angles. Instead, almost all of the misalignments cluster around 90° (ranging from 80°–125°) — meaning that the planet orbits the poles of the star, perpendicular to the direction that the star spins.

What could cause this polar pileup? The authors propose several theoretical possibilities that include dynamical interactions between the planet and the star, or between the planet and an additional unseen, distant companion body. But, as we’ve seen, nature has a mind of its own — and there may be multiple mechanisms at work! We don’t yet have enough information to solve this puzzle with certainty, but a continued search for patterns is sure to point us in the right direction eventually.

Citation

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