A ‘rebellious’ exoplanet orbits in the opposite direction of its star

A star and its planets form when a cloud of gas and dust collapses in on itself somewhere in space. The cloud then forms a disk around the nascent star, within which the planets will form. This protostar and its disk then rotate in the same direction.

In this ideal scenario, the planets formed in the disk will then rotate in the same direction as their star and in the same plane as the stellar equator, perpendicular to the axis of rotation. Astronomers refer to these as aligned systems.

For 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,” said Yann Carteret, a doctoral student in the Department of Astronomy at the 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 research is published in the journal Astronomy & Astrophysics.

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 of 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.

NIRPS, a Swiss-built 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,” said Léna Parc, a doctoral student in the Department of Astronomy at the UNIGE’s Faculty of Science and coauthor of the study.

A misaligned and retrograde orbit

The angle Psi for GJ 3090 b is approximately 136 degrees. Only five other multiplanet 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,” said Vincent Bourrier, a lecturer and researcher in the Department of Astronomy at the UNIGE’s Faculty of Science and coauthor 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 concluded.

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Swati Mestri

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Robert Egan

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