A yellow and orange star in the center of the image. A vertical line through the center of the star indicates the star’s spin axis, and two white arrows indicate that the star rotates clockwise. An exoplanet transits the star as a dark circle. Its orbit cuts across the star’s surface as a white line with arrows indicating that it moves from top to bottom.
When a planet doesn’t orbit in the same direction its star spins, it tends to be misaligned by about 90°. Credit: Brett Addison, modified from ESO/L. Calçada, CC BY 4.0

Just like the planets of our solar system, most exoplanets tend to orbit their star in the same direction that the star spins. But when they don’t, exoplanet orbits overwhelmingly prefer to be perpendicular. This new understanding of planetary orbits, published in Astrophysical Journal Letters, raises questions about which planets can become misaligned from the direction that their star spins and how the orbits get that way in the first place.

From a Certain Point of View

When seeking to explain strange exoplanet phenomena, the most useful point of comparison is our own solar system. We know more about it than any other of the thousands of planetary systems discovered to date. The dynamics of the solar system are relatively neat and tidy: The orbits of the eight planets all sit very neatly in the same plane, that plane lines up almost exactly with the Sun’s equator, and the whole system rotates in the same direction.

Within the solar system, the largest angle of misalignment between a planet’s orbit and the Sun’s equator—which defines the plane of the Sun’s spin—is Earth’s at just over 7°. Exoplanet scientists have been able to make similar measurements of the spin-orbit alignment within other planetary systems. “Is 7° a small value or a large value?” asked Simon Albrecht, an astronomer at Aarhus University in Denmark and lead author on the recent study. “The jury on that is still out.”

“That alignment in our solar system is part of what led us to believe that planets form out of a disk that’s around the star,” added astrophysicist and coauthor Rebekah Dawson of Pennsylvania State University in University Park. The prevailing theory of planet formation posits that a large cloud of dust and gas collapses under its own gravity to create a star in the center. The leftover material flattens out into a disk that coalesces into one or more planets (see video at right). In that simplified model, all of the star- and planet-forming material swirls in the same direction, which should make the resulting star and planets all spin in a common direction.

However, “we have known for over a decade that there are planets that are not orbiting in the same plane as their star,” Dawson explained. Although most exoplanets orbit in the same direction as the star’s spin (prograde) and with a very small angle between spin and orbit (0°), there are plenty whose orbits don’t follow suit, including some that orbit opposite to the direction of the star’s spin (retrograde) and others that travel completely backward (180°). “The angle between the planet’s orbit and the star’s spin was some of the first three-dimensional information that we started to get about other planetary systems.…We have to imagine something that’s different or more complicated than the history that we’ve naively invoked for our solar system.”

Astronomers can calculate the angle of inclination between the exoplanet’s orbit and the star’s spin by measuring the transit of the planet in different wavelengths and comparing the different transit profiles, a method called the Rossiter-McLaughlin effect (Figure 1).

Fig. 1. If a star’s spin axis is not pointed toward Earth, some of the light from the star will appear to be moving toward observers (blueshifted), and some of the light will appear to move away from observers (redshifted). Here this apparent movement is represented by the stars (large circles) colored blue and red as they spin from left to right (dashed arrow). Exoplanets (black circle with white halo) will block varying amounts of blueshifted and redshifted light as they transit the star (solid arrow). The pattern of how much of the bluer or redder light is blocked over time, known as the Rossiter-McLaughlin effect, can reveal the direction of the planet’s orbit relative to the star’s spin. Credit: Kimberly M. S. Cartier

Usually, however, astronomers can measure only one dimension of a star’s 3D spin—the component of the spin that’s pointed at Earth. “That can tell you that something is misaligned but not by how much,” Dawson said. How much of the star’s total spin we can see and measure depends on the geometry of our vantage point: If a star’s spin axis points directly at Earth, we would measure no spin at all and see no planetary misalignment. To understand the physical reasons why planetary systems are misaligned, it’s not the perceived angle of misalignment that matters, but the true one.

A recent mathematical advancement helped Albrecht and his team calculate our viewing angle for 57 stars that host misaligned planets. With that additional information, the researchers determined that the planets’ misalignments weren’t as random as previously thought. In fact, they found that a significant number of the true misalignment angles were close to 90°, meaning that the planets orbit their stars from pole to pole rather than across the star’s equator.

More Questions Than Answers

For now, the data on perpendicular planets are outpacing the theories that explain them. There’s no obvious commonality that groups these stars and planets together that might explain why misaligned planets end up on polar orbits: The stars range from hot to cold, the planets range from Neptune mass to more massive than Jupiter, and the planetary orbits range from very close in to quite far away.

“The biggest thing these planets have in common is that we can measure this [viewing angle] for them,” Albrecht said. There are no models of planetary dynamics that predict a preference for perpendicular planets, he explained, because, quite simply, no one knew that their models needed to explain it.

No one theory can yet explain all of the perpendicular planetary systems.

Regardless, Albrecht and his team offered a few potential ideas to start with, although they acknowledged that no one theory can yet explain all of the perpendicular planetary systems they analyzed. Three of the proposed explanations rely on the gravity of another object—the star, an unseen planet, or the planet-forming disk—tugging a planet’s orbit into a 90° misalignment; the fourth theory invokes a magnetic interaction during planet formation.

J. J. Zanazzi, a postdoctoral researcher at the Canadian Institute for Theoretical Astrophysics in Toronto, said that the team “did a great job summarizing the primary theories which can lead to their very exciting result that spin-orbit misalignments come in two flavors,” well aligned or perpendicular. “All the mechanisms have different strengths and weaknesses, and each mechanism fails to explain some part of [the] observation.” Zanazzi was not involved with this research.

The good news, Zanazzi said, is that “all of the astrophysical mechanisms which have been proposed make specific predictions when the mechanism does not work.…For me, a big thing observers can do in the near future is look for companion planets or stars which can cause the required tilts.” If they fail to find any that fit the bill, such a pattern would narrow down the potential explanations.

Moreover, Albrecht said, as theorists begin to refine their models to explain a cluster of polar orbits, those models can help guide the observers toward the right planetary systems to take a closer look at. Will polar orbits be more prevalent around cool stars or hot stars? Will perpendicular planets be found mostly in multiplanet systems or as loners? More observations, new theories, and time will tell.

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer


Cartier, K. M. S. (2021), Peculiar planets prefer perpendicular paths, Eos, 102, https://doi.org/10.1029/2021EO161264. Published on 29 July 2021.

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