EXOPLANET GJ 3090 B FOUND ORBITING ITS STAR BACKWARDS

Astronomers have discovered an exoplanet, designated GJ 3090 b, that orbits its star backwards, according to a study published on 21 September in the journal Astronomy and Astrophysics. Researchers from the University of Geneva, including co-author Yann Carteret, said the direction of the orbit, known as retrograde, is unusual because stars and planets normally form from the same rotating disc of gas and dust, called a protoplanetary disc, and newly formed objects typically keep rotating in the same direction as that disc. Reversing that direction requires a large amount of energy applied in a specific way, the researchers said. Astronomers have identified a small number of exoplanets with retrograde orbits previously, usually caused by the gravitational pull of other massive objects in the same star system disrupting the planet's path.

To make the discovery, the team analysed 23 hours of observations gathered by two exoplanet-hunting instruments on a 3.6-metre telescope at the La Silla Observatory in Chile. They recorded the planet passing in front of its star ten times, with each transit dimming the starlight reaching Earth; measuring that dimming revealed the planet's size and orbital details. The star is an M dwarf, a type that is small, cool and dim enough that even the nearest examples cannot be seen with the naked eye, though the researchers noted such stars make up around three-quarters of those in the Milky Way. The system contains two, or possibly three, planets smaller than Neptune, each completing an orbit in under 16 days.

Carteret said the team searched for a massive companion object that could have forced the planet into its retrograde path but found no evidence of one in this system. "We may need to think differently about how this system acquired its unusual architecture," Carteret said. The researchers said the finding could help explain how exoplanets form around M dwarfs, the most common type of star in the galaxy.