Up to now it was expected that exoplanets would all orbit in more or less the same plane, and that they would move along their orbits in the same direction as the star’s rotation — as they do in our Solar System. However, new results unexpectedly show that many exoplanets actually orbit at a large angle to their star’s spin axis. In the case shown here (WASP 8b) the orbit is completely reversed, or retrograde.
Image Credit: ESO/L. Calçada
Scientific Frontline: Extended "At a Glance" Summary: Retrograde Exoplanet GJ 3090 b
The Core Concept: The sub-Neptune exoplanet GJ 3090 b possesses a highly misaligned and retrograde orbit, revolving in the opposite direction of its host red dwarf star's rotation.
Key Distinction/Mechanism: Unlike typical planetary systems where planets align with the star's equatorial plane and rotational direction, GJ 3090 b exhibits an orbital angle of approximately 136 degrees without the gravitational influence of a massive companion object to explain the deviation.
Major Frameworks/Components:
- Planetary formation theory concerning circumstellar gas and dust collapse.
- Orbital dynamics and the measurement of the angle Psi to map alignment between a planet's orbital plane and a star's equatorial plane.
- The theoretical accretion of a secondary, retrograde protoplanetary disk to account for unexplained orbital misalignment.
Branch of Science: Astronomy, Astrophysics, and Exoplanetology.
Future Application: This system provides a new baseline for testing advanced planetary formation models, potentially prompting astronomers to search for signatures of secondary disk accretion in other misaligned stellar systems.
Why It Matters: The discovery challenges standard, idealized models of star and planet formation, demonstrating that severe orbital misalignment can occur and persist in sub-Neptune systems devoid of massive perturbing neighbors.
Using the NIRPS infrared spectrograph, an international team led by UNIGE identified the unique orbit of the exoplanet GJ 3090 b.
An international team led by the Department of Astronomy at the University of Geneva (UNIGE) has determined the orbital configuration of the exoplanet GJ 3090 b using the NIRPS spectrograph, which was partly built in Geneva. The planet is highly misaligned and orbits in the opposite direction of its star's rotation. This puzzle is especially challenging for astronomers because there is no massive companion in this planetary system capable of influencing the planet's orbit and thus explaining this configuration. The results are published in the journal Astronomy & Astrophysics.
A star and its planets form when a cloud of gas and dust collapses on itself in space. The cloud forms a disk around the nascent star, within which the planets form. The protostar and its disk rotate in the same direction.
In this ideal scenario, the planets formed in the disk rotate in the same direction as their star's rotation and in the same plane as the stellar equator, perpendicular to the axis of rotation. Astronomers refer to these as aligned systems.
In the solar system, the eight planets orbit in the same direction as the Sun, but there is a small angle between their orbital planes and the Sun's equatorial plane. This angle, often called Psi, varies from approximately 3 to 7 degrees, depending on the planet. For Earth, for example, it is just over 7 degrees. This small deviation from the ideal scenario may reflect perturbations that occurred during the formation of our solar system, such as an encounter with another star or an asymmetric collapse of the initial cloud.
“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,” notes Yann Carteret, a doctoral student in the Department of Astronomy at UNIGE’s Faculty of Science and first author of the study on the planet GJ 3090 b. “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.”
The GJ 3090 Planetary System
GJ 3090 is a red dwarf, a star that is cooler and smaller than our Sun. The planet GJ 3090 b, which orbits it, was first detected by the TESS space telescope, which scans the sky for transits (small, periodic variations in a star’s brightness caused by a planet passing in front of it). The planet has a radius 2.2 times that of Earth and a mass 4.5 times that of Earth, making it a sub-Neptune, the most abundant class of exoplanets in our Galaxy.
The Swiss-built NIRPS instrument, a spectrograph installed at the La Silla Observatory, was used to detect two other planets in the system, confirm the presence of GJ 3090 b, measure its mass, and, above all, determine its unusual orbital configuration. “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,” says Léna Parc, a doctoral student in the Department of Astronomy at UNIGE’s Faculty of Science and co-author of the study.
A Misaligned and Retrograde Orbit
The angle Psi for GJ 3090 b is approximately 136 degrees. Only five other multiplanetary systems are known to host a misaligned planet with an angle greater than 70 degrees. Unlike those other systems, however, there is no evidence of a massive object (another star or planet) orbiting GJ 3090 that could be responsible for this configuration.
“The absence of a massive companion to explain this unusual orbit will lead us to explore other hypotheses,” explains Vincent Bourrier, a lecturer and researcher in the Department of Astronomy at UNIGE’s Faculty of Science and co-author of the study. “In the case of GJ 3090, the star could have accreted a misaligned, retrograde secondary disk in which the planets in the system then formed,” he concludes.
Published in journal: Astronomy & Astrophysics
Title: Upside down: GJ 3090 b the first retrograde exoplanet around an M dwarf detected with NIRPS
Authors: Yann Carteret, Vincent Bourrier, Léna Parc, Andrew Winter, Romain Allart, Izan de Castro Leão, François Bouchy, Vera M. Passegger, Charles Cadieux, Pierrot Lamontagne, Étienne Artigau, Frédérique Baron, Susana C. C. Barros, Björn Benneke, Xavier Bonfils, Marta Bryan, Bruno L. Canto Martins, Ryan Cloutier, Eduardo Cristo, Jonathan Gagné, Neil J. Cook, Nicolas B. Cowan, Jose Renan De Medeiros, Xavier Delfosse, René Doyon, Xavier Dumusque, David Ehrenreich, Jonay I. González Hernández, David Lafrenière, Christophe Lovis, Lison Malo, Claudio Melo, Lucile Mignon, Christoph Mordasini, Francesco Pepe, Rafael Rebolo, Jason Rowe, Nuno C. Santos, Damien Ségransan, Alejandro Suárez Mascareño, Stéphane Udry, Diana Valencia, Gregg Wade, Khaled Al Moulla, Rillck Guilherme de Souza Barros de Amorim, Lisa Dang, Emily Deibert, Dasaev O. Fontinele, Thierry Forveille, Yolanda G. C. Frensch, Roseane de Lima Gomes, Dany Mounzer, Stefan Pelletier, Riley Rosener, Bennett Neil Skinner, Avidaan Srivastava, Atanas K. Stefanov, Valentina Vaulato, Joost P. Wardenier, and Drew Weisserman
Source/Credit: Université de Genève
Edited by: Scientific Frontline
Reference Number: astr092126_01
