A backward-spinning star reveals its likely origin

In our solar system, all the planets orbit in the same direction and in nearly the same plane, which also coincides with the Sun’s equatorial plane. But not all planetary systems are so well aligned: Observations of exoplanets over the past decade have turned up a multitude of planets that orbit in a different, or even opposite, direction to their host stars’ spins.

Many theories have been proposed to explain those orbital misalignments, but testing them has been a challenge. Now, through their observations of the exoplanet system K2-290, Simon Albrecht of Aarhus University in Denmark, his recent PhD student Maria Hjorth, and an international team of collaborators have found a neat confirmation of one mechanism that was proposed in 2012: Disrupted by the gravitational influence of an orbiting companion star, the protoplanetary disk is pulled out of alignment before the planets even form.

Exoplanets are too faint to image directly, so it might seem impossible to say anything at all about the directions of their orbits. But when Earthbound astronomers are lucky enough to see an exoplanet pass directly in front of its star (which is how most exoplanets are discovered ), spectroscopic analysis of that transit produces a wealth of information. A spinning star appears partly redshifted and partly blueshifted; a planet that transits the star in the same direction as the spin blocks the blueshifted part first and the redshifted part second. But if the planet orbits in the opposite direction, the reverse is true.

In the K2-290 system, not one but two planets regularly transit the primary star. That means, almost certainly, that the planets’ orbital planes coincide. (If they didn’t, the line of sight from Earth would have to lie on the intersection of the two planes, which is unlikely.) The researchers’ spectroscopic measurements showed that both planets orbit backward with respect to the star’s spin, as illustrated in the figure.

The coplanar orbits rule out most proposed mechanisms, such as planet–planet scattering, that could have misaligned the system after the planets formed, because those processes would act differently on the two planets and give them different orbital obliquities. But the primordial misalignment scenario is a good match: The system’s primary star is also orbited by a faint dwarf star, which the researchers’ simulations showed could have misaligned the protoplanetary disk over a period of a few million years.

It’s far too early to tell whether the primordial mechanism is responsible for most planetary misalignments, or even how common misaligned systems are. Exoplanet studies are still overwhelmingly dominated by the planets that are easiest to observe—giant, fast-orbiting planets whose formation and dynamics may not be representative of planetary systems in general. That situation is improving, however, as new spectrographs come online that can make detailed measurements of smaller planets. One of the K2-290 planets, for example, is just three times the diameter of Earth. (M. Hjorth et al., Proc. Natl. Acad. Sci. USA 118, e2017418118, 2021 .)