Astronomers have discovered an unusual exoplanet whose orbit is almost completely misaligned with the rotation of its star. GJ 3090 b is located about 73 light-years from Earth and orbits a red dwarf in the direction opposite to the star’s rotation. What makes this especially puzzling is that there is no massive nearby object that could explain such a configuration, Space.com reports.

An orbit that should not exist here

Planets form from a disk of gas and dust surrounding a young star. The material in this disk rotates in a particular direction, and the planets that form from it usually inherit that motion. As a result, their orbits generally lie in roughly the same plane as the star’s equator and are oriented in the same direction.

The angle between the plane of a planet’s orbit and the star’s equatorial plane is called Psi. It can change under the influence of gravitational perturbations during the formation and subsequent evolution of a planetary system. For Earth, this angle is about 7 degrees.

For GJ 3090 b, however, the situation is completely different: researchers determined a Psi value of 136 degrees. At such an inclination, the planet is effectively moving in a retrograde orbit — in the direction opposite to the rotation of its star.

“To our great surprise, the planet GJ 3090 b is not only on a highly tilted orbit, but it also moves in a retrograde direction, opposite to the rotation of its star,” said study participant Yann Cretier of the University of Geneva.

A small planet with an unusual system

GJ 3090 b was discovered in 2022. In size, it is about 2.2 times larger than Earth, and its mass is approximately 4.5 times that of Earth. The planet is classified as a sub-Neptune — a class of worlds that fall in size between Earth and Neptune and are considered among the most common types of planets in the Milky Way.

What makes it unusual is not so much its size as the geometry of its orbit. Systems with a Psi greater than 70 degrees are extremely rare: GJ 3090 b has become only the fifth known planet with such a strong mismatch between its orbit and the rotation of its star.

Moreover, the other known systems all share one important feature — a massive neighbor. This may be a gas giant, a brown dwarf, or another star. Its gravity can significantly alter a planet’s orbit and tilt it relative to its star.

No such object has yet been detected in the GJ 3090 b system.

What could have flipped the orbit

It is precisely the absence of a massive companion that is forcing scientists to look for another explanation for the origin of this unusual orbit.

One proposed hypothesis is connected to the protoplanetary disk itself. According to the researchers, the star may have absorbed a second disk of gas and dust that was originally highly inclined relative to its rotation and was moving in the opposite direction. The system’s planets may then have formed from this material instead.

In this scenario, the unusual orbit of GJ 3090 b would not be the result of later gravitational influence from a giant neighbor, but rather a feature of the material from which the planetary system itself formed.

For now, this is only a hypothesis. But the observation of GJ 3090 b shows that the architecture of planetary systems may turn out to be far more diverse than standard formation scenarios suggest.

This unusually large angle was measured thanks to the NIRPS infrared spectrograph installed at the La Silla Observatory. Its precision proved sufficient to determine the mutual orientation of the orbit relative to the star’s equatorial plane even for a comparatively small planet.

The study was published on September 21 in the journal Astronomy & Astrophysics.