A giant rogue world once described as the "planet that shouldn't be there" looks like it actually formed out in deep space, far from its host star and the cosmic material that usually births planets, according to new research.
The anomaly, called HD 106906b, is a young planet located approximately 300 light-years from Earth in the Crux constellation. HD 106906 was discovered in 2013, and what makes so unique is how distantly it orbits its star – at 650 astronomical units (au), or 650 times the distance from Earth to the Sun.
That epic stretch gives HD 106906b the record for the largest orbit around a single, Sun-like star, which takes the planet about 1,500 years to complete one loop.
The most puzzling thing about HD 106906b's aloofness is that its distant orbit places it well beyond the disk of cosmic debris surrounding HD 106906 – the dust and gas from which planets usually form.
HD 106906 and its debris disk bottom-left, with HD 106906b top right. Credit: ESO and A.M. Lagrange/Université Grenoble Alpes
In this case, the debris disk is about 10 times closer to the star than HD 106906b is, begging the question of just where did this bizarre rogue planet come from?
"Our current planet formation theories do not account for a planet beyond its debris disk," says astrophysicist Smadar Naoz from the University of California, Los Angeles.
Naoz and her team have now developed a model that can track HD 106906b's orbital path.
Since HD 106906b was first discovered, scientists have been trying to explain how the planet could have ended up so far removed from HD 106906, since the vast majority of exoplanets are thought to be located inside debris disks.
And the same holds true closer to home in our own Solar System, with all of the planets orbiting the Sun falling inside the Kuiper belt – the circumstellar disk that extends beyond Neptune, encompassing dwarf planets and several other smaller remnants left over from the formation of our Solar System.
Previous research had suggested that HD 106906b might have formed inside the debris disk before gravitational interactions ejected the planet into its far-off exile – but Naoz's team don't think that's the case.
One of the researchers – Erika Nesvold from the Carnegie Institution for Science – created a computer model called Superparticle-Method Algorithm for Collisions in Kuiper belts and debris disks (SMACK), which suggests that the planet formed outside the debris disk.
SMACK took the known data about the HD 106906 system and calculated how an outside planet like HD 106906b would affect the structure of the star's debris disk.
It's not known if the HD 106906 system contains any other planets, but the model suggests that the shape of the elliptical debris disk as it currently exists is compatible with the lone orbit of HD 106906b.
"We were able to create the known shape of HD 106906's debris disk without adding another planet into the system, as some had suggested was necessary to achieve the observed architecture." Nesvold says.
The model also indicates that HD 106906b most likely formed outside of the disk – if it initially formed inside and then later moved outward, gravitational effects would mean that the disk would hold a different shape to the one it has now.
While this means we still can't exactly explain how HD 106906b took shape so far from the dust and gas that gives birth to most planets, at least we've narrowed down the planet's origin story a little.
And if we can find more rogue outliers like HD 106906b, the SMACK model could help us learn more about how these planets could be possible.
"Other debris disks that are shaped by the influence of distant giant planets are probably likely," Nesvold says.
"My modelling tool can help recreate and visualise how the various features of these disks came to be and improve our understanding of planetary system evolution overall."
The findings are reported in The Astrophysical Journal Letters.
Image credit: HD 106906's debris disk (black circle is the star, removed from image). Credit: Erika Nesvold/Carnegie Institution for Science