GJ 3090 b Orbits Backward Around Its Star

GJ 3090 b is a transiting sub-Neptune that completes an orbit every 2.85 days. High-resolution observations with NIRPS, the Near-Infrared Planet Searcher on ESO’s 3.6-metre telescope at La Silla Observatory in Chile, allowed astronomers to measure the orbit’s three-dimensional geometry. They found an obliquity of roughly 136° — well above 90° — meaning the planet travels around its star in the direction opposite to the star’s rotation. 810

What NIRPS measured

During a transit, a planet crosses the face of its rotating star. Precision spectroscopy can detect the subtle changes this produces in the star’s spectral features, enabling researchers to infer how the planet’s orbital plane is oriented relative to the stellar equator.

For GJ 3090 b, NIRPS’s near-infrared observations delivered the precision needed to determine that angle. A value of Ψ ≈ 136° is not a slight mismatch: it confirms a strongly misaligned, genuinely retrograde orbit. 48

The same observing programme also refined the planet’s mass to 4.52 ± 0.47 Earth masses. Together with a radius of 2.18 ± 0.06 Earth radii, that places GJ 3090 b in the sub-Neptune category. 3

Why this planet stands out

GJ 3090 is a small, cool M-type red dwarf. GJ 3090 b is the first known planet on a retrograde orbit around a star of this kind. 815

Extreme orbital tilts have more often been measured around hotter, larger stars, where observations can be more straightforward. NIRPS’s infrared capability was crucial here because it made it possible to probe the orbital geometry of a relatively small planet around a cool star. 410

No obvious culprit

A common explanation for highly tilted or retrograde orbits is later gravitational disruption. A distant giant planet or companion star can, in some systems, gradually alter the orbital plane of an inner planet.

But observations of GJ 3090 found no evidence for either a wide stellar companion or an outer planet massive enough to provide that explanation. 8 That does not rule out every possible dynamical history, but it means the simplest version of the “hidden giant perturber” scenario is not supported by the available data.

The system’s confirmed second planet

The best-established additional detection is GJ 3090 c, a non-transiting sub-Neptune with a 15.9-day orbit, a minimum mass of 10.0 ± 1.3 Earth masses and an orbital distance of about 0.10 AU. 324

Some announcements say NIRPS detected “two other planets” in the system. 45 However, the supplied sources consistently name and confirm only GJ 3090 c. Without secure details such as a name, period or mass for another body, it should not be presented as an equally confirmed planet.

Could a second tilted disk explain it?

One alternative shifts the explanation to the system’s earliest history. The young star may have later accreted gas and dust whose angular momentum was sharply tilted — or even opposed — to that of its original disk. Planets that formed within this second disk would naturally inherit its different orbital direction. 6

This remains a hypothesis rather than a demonstrated history of GJ 3090. Still, it offers a way to explain a backward orbit without requiring a massive companion that is still present today: the unusual geometry may be primordial, preserving evidence of how the protoplanetary disk was fed rather than the aftermath of a violent later upheaval. 68

Finding more systems like this would strengthen the case that planets can form in successive disks with different orientations — and that a planet’s orbit does not always have to line up with the final rotation of its star.