Astronomers discover sub-Neptune exoplanet orbiting retrograde around red dwarf

Astronomers have discovered a sub-Neptune exoplanet, GJ 3090 b, that orbits in the opposite direction of its parent red dwarf star. Located approximately 72 to 73 light-years away in the constellation Phoenix, the planet is roughly 2.2 times the width of Earth and 4.5 times its mass, completing an orbit every 2.85 to 2.9 days.

Using the Near-InfraRed Planet Searcher (NIRPS) on the European Southern Observatory’s 3.6-meter telescope in Chile, researchers identified the retrograde motion by observing the Rossiter-McLaughlin effect. This phenomenon occurs during a transit when the planet blocks specific parts of the star's rotating surface, causing detectable shifts in light. In this case, the planet crossed the star's red hemisphere before the blue hemisphere, indicating an orbit contrary to the star's rotation.

The discovery challenges standard planet formation theories, which suggest planets typically orbit in the same direction as their host star's rotation. Notably, researchers have been unable to identify a massive companion star or planet that might have gravitationally pushed GJ 3090 b into such a highly misaligned, retrograde path.