GJ 3090 b runs backward around its M dwarf at 136 degrees to the star's equator

An obliquity above 90 degrees means the planet travels against its star's spin, and this one sits 44 degrees short of exactly anti-aligned ^[1]. The quantity being measured is the angle between the planet's orbital plane and the star's equatorial plane, which is how NASA's Jet Propulsion Laboratory defines obliquity ^[10]. For the eight planets of the Solar System, that angle runs from around 3 to 7 degrees depending on the planet, according to a statement on the study from the Institute of Astrophysics of the Canary Islands ^[11]. GJ 3090 b's is roughly 20 to 45 times as large ^[2]. Its mass and radius imply a mean density about 0.42 times Earth's ^[3].

In the standard account, a cloud of gas and dust collapses to form a star, the leftover material flattens into a protoplanetary disk, and planets are born inside it ^[16]. Star and planets end up turning the same way because they came from the same material ^[16]. Andrew Winter, an astrophysicist at Queen Mary University and the study's lead author, said the planet is "not simply tilted relative to its star" but "orbiting in the opposite direction" ^[4]. "That immediately raises the question of how such an unusual orbit could have formed," Winter said ^[4].

A retrograde orbit can also be produced long after the planets form. The team first considered a gravitational interaction with a companion star or with a nearby planet massive enough to disturb the orbit and tilt it over time ^[12]. Neither turned up ^[13]. The published account calls the threshold a "sufficiently massive" outer planet and gives no mass or separation limits for that search ^[13].

The alternative the team proposes puts the misalignment at the beginning. The star may have acquired a second disk of gas and dust from its surrounding environment, and planets formed from that material would take on an unusual orientation ^[14]. "It suggests that the environment around a young star can play a much bigger role in determining the architecture of its planets than we might have expected," said Vincent Bourrier, an astronomer at the University of Geneva and a study author ^[5].

One system does not set a rate. How often planets around M dwarfs end up misaligned depends on the denominator, the number of such planets with a measured spin-orbit angle, and this is the first retrograde case among them ^[2]. The researchers say more observations are needed to verify the formation scenario and to determine whether similar orbital configurations exist in other planetary systems ^[15].