Scientists Say This Rogue Planet Contradicts Existing Models of Planetary Formation

HD 106906's debris disk (black circle is the star, removed from image)

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.

The findings are reported in The Astrophysical Journal Letters