Scientists have found that a distant world has a real rebellious streak, outwardly defying its red dwarf host star.
The extrasolar planet, or exoplanet, which is designated GJ 3090 b and is located about 73 light-years away, has a highly misaligned orbit. In fact, it's rotating in the opposite direction to the rotation of its parent star.
This is something scientists could explain if GJ 3090 b had a massive companion, be it a gas giant planet, a brown dwarf, or another star. But it doesn't. No, this world, discovered in 2022, seems to be a singleton.
"To our great surprise, not only is the planet GJ 3090 b on a highly misaligned orbit, but it also orbits retrogradely, in the opposite direction to the rotation of its star," team member Yann Carteret, of the Université de Genève (UNIGE) Department of Astronomy, said in a statement.
Big Psi for rebel exoplanet
Planets form from disks of gas and dust that surround infant stars. This matter rotates in the same direction as the central young star, meaning planets that may emerge shuld also rotate in the same direction.
Likewise, the planets should also share a similar orbital plane to their parent star. The difference in orientation of a star's angular plane and each of its planets is called Psi, and it emerges as a result of perturbations during the planet formation phase. As an example, Earth's Psi is 7 degrees.
"Measuring the angle Psi provides clues about the formation and evolution of planetary systems. It is therefore natural for us to try to measure it for other planetary systems," Carteret said.
GJ 3090 b has a radius 2.2 times that of Earth and a mass 4.5 times that of our planet, making it a "sub-Neptune." Sub-Neptunes are one of the most common planets in the Milky Way. Yet, when the team studied the world with La Silla Observatory's NIRPS instrument, this exoplanet appeared to possess something far from common: a Psi of 136 degrees.
"It is the performance of NIRPS in the infrared that made it possible to achieve the precision needed to measure the angle between the planet's orbital plane and the star’s equatorial plane for such a small planet," Léna Parc of UNIGE said.
This planetary system one of only five ever seen with a Psi greater than 70 degrees — but what is really different here is those other systems possess a massive object that clearly seems to have caused the extremely misaligned orbits.
"The absence of a massive companion to explain this unusual orbit will lead us to explore other hypotheses," Vincent Bourrier of UNIGE said, also proposing an explanation for this massive Psi. "In the case of GJ 3090, the star could have accreted a misaligned, retrograde secondary disc in which the planets in the system then formed."
The team's research was published on Monday (Sept. 21) in the journal Astronomy & Astrophysics.
