Image Courtesy: Scitech Daily
Astronomers have discovered a Neptune-sized exoplanet orbiting its star in the opposite direction from the star’s rotation, an unusual configuration that could provide new clues about how planetary systems form and evolve.
The planet, GJ 3090 b, orbits an M dwarf, a class of small, cool stars that accounts for most stars in the Milky Way. It is the first known planet around an M dwarf to have a confirmed retrograde orbit, raising questions about how it ended up following such an extreme path.
Researchers from Queen Mary University of London and an international team used high-resolution observations from the NIRPS near-infrared spectrograph to measure the planet’s orbital orientation in three dimensions. They determined an orbital obliquity of about 136 degrees, confirming that GJ 3090 b travels around its star in a backward direction. The findings were published in Astronomy & Astrophysics.
Normally, planets are expected to orbit in the same general direction as their host star because both form from the same rotating disk of gas and dust. A retrograde orbit therefore suggests that GJ 3090 b may have experienced an unusual formation history.
One possibility is that the star acquired a second disk of gas and dust from its surrounding environment. If that material was oriented differently from the original disk, planets forming from it could inherit a dramatically different orbital direction.
“The idea that a planetary system could be rebuilt from a second, differently oriented disk is particularly exciting,” said Vincent Bourrier of the University of Geneva. Such a scenario could explain the planet’s orbit without requiring a later gravitational encounter to flip it.
The researchers also searched for another explanation. A massive outer planet or companion star could potentially disturb a planet’s orbit and push it into an extreme configuration. However, the team found no evidence of a sufficiently massive companion in the GJ 3090 system.
“This is a remarkable planetary system because the planet is not simply tilted relative to its star, it is orbiting in the opposite direction,” said Andrew Winter of Queen Mary University of London.
GJ 3090 b is also the smallest planet around an M dwarf for which astronomers have measured a three-dimensional orbital obliquity. Further observations will be needed to determine whether its backward orbit resulted from a second, misaligned disk and whether similarly unusual planetary systems exist elsewhere.
