Planet around a red dwarf moves against the star’s rotation for the first time

Astronomers have confirmed that the Neptune-sized planet GJ 3090 b orbits its red dwarf in the direction opposite to the star's rotation. This is the first reliably confirmed case of a retrograde orbit for a planet around an M dwarf, the most common type of star in the Galaxy. The work was carried out by an international team including researchers from Queen Mary University of London, and the paper was published in Astronomy & Astrophysics Letters.

Usually a star and its planets are born from a single rotating disk of gas and dust, so the planets move in the same direction as the star. For GJ 3090 b, the orbital tilt was measured at about 136 degrees. This is the angle between the star's rotation axis and the planet's orbital axis, and at such a value the planet orbits the star almost "upside down".

How the orbit was measured

The three-dimensional orientation of the orbit was reconstructed from observations with the NIRPS spectrograph, which splits the star's near-infrared light by wavelength. One of the lead authors, Andrew Winter of Queen Mary University of London, noted that the planet is not merely tilted relative to the star but moves in the opposite direction, which immediately raises the question of how such an orbit originated.

The authors consider the possibility that the star captured from its surroundings a second disk of gas and dust, tilted relative to the original one, and the planets formed in it. Vincent Bourrier of the University of Geneva calls this idea especially interesting: it suggests that the environment of a young star has a stronger influence on the structure of its planetary system than expected.

What could have flipped the planet

An alternative explanation involves gravity: a sufficiently massive external neighbor, such as a large planet or a second star, could flip the orbit. Yan Carteret, a PhD student at the University of Geneva, said that the researchers searched for such a companion but found no confirmation of its existence. So another explanation must be sought.

GJ 3090 b has also become the smallest planet around an M dwarf for which the orbital tilt has been measured in three dimensions, demonstrating the capabilities of near-infrared observations. The authors emphasize that further measurements are needed to test the second-disk scenario and to understand whether such extreme orbits occur in other systems.

Image: Rijksmuseum CC0 · License

Translation editor: Ловушкин А.