The thermal infrared image that led to HD 106806 b’s discovery, and shows just how unfathomably far it is from its star. Image: Vanessa Bailey

It seems like every time astronomers find a new exoplanet, we’re forced to rethink everything we know about planets and how they’re made. The latest to challenge our perceptions of planetary formation is HD 106906 b, which orbits its star further than any other planet we’ve found.

HD 106906 b, discovered by a team of astronomers at the University of Arizona, is 11 times as massive as Jupiter, but its size isn’t the interesting part. What’s fascinating—and surprising—for astronomers is that this massive planet orbits 650 astronomical units (AU) from its Sun-like star. That’s an orbit 650 times the distance from the Earth to the Sun, or roughly 60,450,000,000 miles.

This isn’t something astronomers have seen before. The research describing the find is available on Arxiv, and will soon be published in the Astrophysical Journal Letters. As Vanessa Bailey, the fifth-year graduate student who led the University of Arizona team, said, “no model of either planet or star formation fully explains what we see.”

The leading theories of planetary formation tend to focus on and explain systems like our own solar system. Planets are thought to form from the primordial disk of gas and dust that surrounds a young, still forming star. From that disk, Earth-like planets close to their stars are thought to gradually coalesce from small asteroid-like bodies over time. But this happens far too slowly to explain giant planets forming so far from their parent stars. It’s possible that these distant giants form from a fast, direct collapse of the primordial disk.

These images show the newly discovered planet in the top right and its sun at bottom left, via the arXiv paper

There’s a problem with this model when it comes to explaining HD 106906 b: primordial disks rarely have enough material on their outer edges to collapse into a planet this large. 

There are other theories that might explain this odd planet. HD 106906 b might be the failed star of a would-be binary system. Binary systems can form when two adjacent clumps of gas collapse into stars independently of one another but close enough to become and stay gravitationally bound to one another. So HD 106906 b might be a star that just didn’t have quite enough material to ignite and start burning.

But even this failed binary explanation doesn’t totally work. The mass ratio between two stars in a binary system is typically 10-to-1. In the case of HD 106906 b, the mass ratio between the planet and its star is more than 100 to 1.

“This extreme mass ratio is not predicted from binary star formation theories,” Bailey explained, drawing the parallel between this unpredicted relationship and planetary formation’s inability to predict this planet’s existence at all.

Obligatory artist’s rendering showing a ringed planet far, far away from its star. Via NASA/JPL-Caltech

There’s still a chance astronomers will solve the mystery. HD 106906 b’s system still has observable traces of the debris disk that formed it, and watching this disk as it develops might help astronomers explain the currently unexplainable planet. It’s only 13 million years old and about 2,700 degrees Fahrenheit, which means it’s still glowing with the residual heat from its formation, and emits energy in the form of infrared light.

Imaging this planet then demanded the team use special infrared cameras, in this case the Magellan Adaptive Optics (MagAO) system and Clio2 thermal infrared camera. Both technologies were developed at the University of Arizona and both are mounted on the 21.3-foot diameter Giant Magellan telescope in the Atacama Desert in Chile. Using the telescope’s adaptive secondary mirror, which has 585 actuators moving 1,000 times a second, the team was able to remove the blurring effects of the atmosphere to get a clean shot of the planet.

The same technology, and other instruments used in tandem, will help the team gather more information on the system and unlock the many remaining mysteries. HD 106906 b will certainly be an object of study for astronomers in the coming months and years, and what we learn in unraveling this mysterious planet’s formation will help shine light on just how planet systems form.