A small planet has just revealed an orbit almost opposite to what its system should normally have produced.
GJ 3090 b orbits a red dwarf, a star smaller and cooler than the Sun. This planet has a radius about 2.2 times greater than Earth's and a mass about 4.5 times greater. It completes one full orbit in just 2.9 days.
Artist's impression of an exoplanet following a retrograde orbit around its star.
Credit: ESO/L. Calçada.
When a star and its planets form in the same disk of gas and dust, they normally rotate in the same direction. GJ 3090 b does almost the opposite. Its orbit is strongly tilted relative to the rotation of its star and proceeds in the opposite direction: it is described as retrograde.
To measure this orientation, the researchers observed several passages of the planet in front of its star. They mainly used NIRPS, an instrument installed on the 3.6 m telescope at the La Silla Observatory in Chile. Six transits were monitored, several of which were also observed with the HARPS instrument.
The measurement of the star's rotation is based on the shift in its emitted light. Part of its surface is moving toward us, slightly shifting the received light toward blue, while the other side is moving away, shifting it toward red. When the planet passes in front, it successively blocks small areas whose light carries this information. The order of these occultations makes it possible to reconstruct the direction of its movement.
The researchers obtain an angle of about 136° between the planet's orbit and the star's rotation axis. Beyond 90°, an orbit is retrograde. GJ 3090 b thus becomes the first known planet following such a trajectory around a red dwarf.
A usual explanation would be the gravitational influence of a large companion. Another massive planet or a second star can gradually tilt an orbit. Yet searches conducted using several methods have revealed no object massive enough and distant enough to easily explain the observed configuration.
Another possibility therefore goes back to the system's birth. The star may have received gas late in its formation, arriving with a different orientation. A second disk of material could then have formed at an angle, or even opposite to the star's rotation. The planets would subsequently have formed in this disk before migrating inward.
The system contains at least one other confirmed planet, which will make it possible to test these hypotheses. By measuring the orientation of the other orbits, the researchers will be able to determine whether the entire system shares the same tilt or whether GJ 3090 b underwent a particular evolutionary history.