Researchers from Queen Mary University of London, along with an international team of scientists, have determined the retrograde orbit of GJ 3090 b , a planet roughly the size of Neptune.
This is the first known instance of a planet orbiting in retrograde motion around an M dwarf star, a small, cool group of stars that makes up the majority of the Milky Way galaxy.
The discovery raises a big question: Why does GJ 3090 b have such a strange orbit?
GJ 3090 b is moving in the opposite direction to its host star.
When a star and its surrounding planets form from the same rotating disk of gas and dust, the planets are generally expected to orbit the star in the same direction as the star's rotation.
This is a natural consequence of the formation of planetary systems from a disk of matter with shared angular momentum.
However, GJ 3090 b does not follow that rule.
Its host star is an M-dwarf, a type of star with a surface temperature lower than the Sun and common in our galaxy. GJ 3090 b orbits this star in the opposite direction, also known as retrograde orbit.
What's remarkable isn't just that the planet is tilted sharply relative to its star.
Measurements show that its orbital plane is actually oriented almost opposite to the rotation direction of its host star.
Dr. Andrew Winter, the lead author of the study from Queen Mary University of London, said this is a special planetary system because the planet is not simply tilted relative to the star, but is also moving in the opposite direction.
This immediately raises questions about the mechanism by which such an unusual trajectory is formed.
To accurately determine the direction of movement of GJ 3090 b, the research team used high-resolution observations from NIRPS, a near-infrared spectrometer capable of separating light into its component wavelengths.
This data allows astronomers to study the planet's motion in three dimensions, rather than simply observing its orbit from a limited perspective.
The results showed that the orbital inclination, the angle between the star's axis of rotation and the planet's orbital axis, was approximately 136 degrees. An orbit with an inclination exceeding 90 degrees is considered retrograde.
Therefore, the 136-degree figure is clear evidence that GJ 3090 b is orbiting the star in the opposite direction to the host star's rotation.
The research, published in the journal Astronomy & Astrophysics Letters, opens a new approach to understanding the origins of planetary systems with unusual architectures.
Was GJ 3090 b "born" in a retrograde orbit?
One of the hypotheses considered by the research team is that GJ 3090 b may have formed in this particular orbit from the beginning, rather than being pushed into a retrograde orbit by a violent event that occurred later.
According to this scenario, the star GJ 3090 may have absorbed additional gas and dust from its surroundings during the early stages of its star system formation.
This new material could form a second disk, but it would be tilted in a different direction than the original disk.
If planets form from the second disk, they may inherit the direction of motion of that disk itself.
In the case of GJ 3090 b, this could explain why the planet has an extremely retrograde orbit.
Associate Professor Vincent Bourrier of the University of Geneva noted that the idea that a planetary system could be reconstructed from a second disk with a significantly different orientation is particularly noteworthy.
This suggests that the environment surrounding a young star may play a much larger role than previously thought in determining the structure of planetary systems.
If this hypothesis is correct, the orbit of GJ 3090 b could be a "fossil" recording the history of the star system's formation. In other words, the planet's retrograde orbit could preserve traces of the material that created it.
However, there is another mechanism that can create a retrograde orbit: gravitational interaction.
A giant planet located at a great distance, or a sufficiently large companion star, can exert a gravitational influence on the planet for an extended period.
Such interactions have the potential to drastically alter the direction of the orbit, even tilting the orbital plane to the point where the planet moves into retrograde motion.
This is one of the mechanisms commonly used to explain the existence of planets with extreme orbits.
Therefore, the research team searched the GJ 3090 system for a large companion star or an exoplanet with sufficient mass to cause an unusual tilt of GJ 3090 b.
But the results did not find convincing evidence for such an object.
This suggests that scientists may need to consider a different approach to the formation of the unique architecture of the GJ 3090 system.
M-dwarf stars may hold even more secrets.
GJ 3090 b also holds another notable record. It is the smallest planet orbiting an M-dwarf star whose orbital inclination in all three spatial dimensions has been measured by astronomers.
This result also demonstrates the value of near-infrared observations in studying planetary systems orbiting M dwarf stars.
Because M dwarf stars are small, cold, and very common, understanding how planets form around them is crucial for studying the diversity of planetary systems in galaxies.
Further observations will be crucial in verifying the hypothesis about the "second disk" being deflected.
Scientists need to determine whether the GJ 3090 system may have actually formed from a second disk of matter, and also search for other planetary systems with similar extreme orbital structures.
If more instances like GJ 3090 b are discovered, they could force astronomers to adjust their understanding of planetary system formation.
Instead of always forming in a stable and coplanar structure, some systems can undergo far more complex histories, in which matter from the surrounding environment contributes to shaping the orbits of worlds from the very first stages.
(According to SciTechDaily, Space, Astronomy)