Astronomers have discovered an unusual exoplanet, GJ 3090 b, that orbits its star in the opposite direction to the star's rotation. This kind of motion, called retrograde, is rare and challenges our current understanding of how planetary systems form. Typically, planets and their stars form from a rotating cloud of gas and dust, which collapses to create a star and a surrounding disk. Within this disk, planets form and usually orbit in the same direction as the star's rotation and in the same plane as the star's equator. These are called "aligned" systems. In our solar system, the planets orbit in the same direction as the sun, but their orbital planes are slightly tilted relative to the sun's equator. This small tilt, known as the angle Psi, varies between 3 to 7 degrees for different planets. Yann Carteret, a doctoral student in astronomy at the University of Geneva, led a study that measured the angle Psi for GJ 3090 b and found it to be approximately 136 degrees — far more misaligned than any other known exoplanet. Not only is the planet’s orbit highly misaligned, but it also orbits in the opposite direction to its star's rotation, a phenomenon known as retrograde motion. This discovery, published in the journal Astronomy & Astrophysics, suggests that the formation of the GJ 3090 planetary system may have followed a different process than the typical aligned systems. GJ 3090 is a red dwarf star, smaller and cooler than the sun. The planet GJ 3090 b, which orbits it, was first detected by NASA’s TESS space telescope, which identifies planets by observing periodic dips in a star’s brightness caused by a planet passing in front of it. GJ 3090 b has a radius 2.2 times that of Earth and a mass 4.5 times that of Earth, making it a sub-Neptune — a common type of exoplanet in the Milky Way. Additional observations using the NIRPS spectrograph, a Swiss-built instrument at the La Silla Observatory in Chile, confirmed the planet’s existence, measured its mass, and determined its unusual orbital configuration. The extreme misalignment of GJ 3090 b’s orbit is significant because no massive object, such as another star or planet, has been detected in the system that could explain the retrograde motion. This absence has led researchers to consider alternative explanations, such as the possibility that the star itself formed from a misaligned or retrograde disk. "The star could have accreted a misaligned, retrograde secondary disk in which the planets in the system then formed," suggested Vincent Bourrier, a researcher at the University of Geneva. This finding adds to the growing list of planetary systems that challenge our understanding of planet formation and evolution.