Massive Newfound Planet Defies Traditional Planetary Formation Theories

Astronomers form the University of Arizona have discovered a distant exoplanet with a set of characteristics so bizarre that they say it should not even exist.

With a mass about 11 times that of Jupiter and an orbit about 650 times greater than the average Earth-Sun distance, newfound planet HD 106906 b, the UA astronomers say, is throwing a wrench into existing planet-formation theories.

"This system is especially fascinating because no model of either planet or star formation fully explains what we see," said research leader Vanessa Bailey, who is a fifth-year graduate student at UA's Department of Astronomy.

Bailey and her colleagues' research on planet HD 106906 b has been accepted for publication in The Astrophysical Journal. An advance copy of the paper can be read here.

The existence of planet HD 106906 b does not coalesce with leading planet formation theories. The planet is too far from its star to have formed from colliding asteroid bodies as the star was first coming to life, and the planet is too massive to have formed from gasses in the primordial disk of its forming star - typically the primordial disk, at such a distance from the star itself, does not have enough material to support the formation of such a large planet.

Several alternative formation theories have been put forward, including that the planet formed in a mini binary star system.

"A binary star system can be formed when two adjacent clumps of gas collapse more or less independently to form stars, and these stars are close enough to each other to exert a mutual gravitation attraction and bind them together in an orbit," Bailey said. "It is possible that in the case of the HD 106906 system the star and planet collapsed independently from clumps of gas, but for some reason the planet's progenitor clump was starved for material and never grew large enough to ignite and become a star."

However, there is a hole in the mini binary system theory. The mass ratio of the two stars in a binary system is typically no more than 10-to-1.

"In our case, the mass ratio is more than 100-to-1," Bailey explained. "This extreme mass ratio is not predicted from binary star formation theories - just like planet formation theory predicts that we cannot form planets so far from the host star."

At 13 million years old, planet HD 106906 b is relatively young. It still glows from the residual heat of its formation. For comparison, Earth is about 4.5 billion years old, roughly 350 times older than HD 106906 b.

Because the region of space where the newfound planet is in orbit has been observed before for another project, there is a wealth of data on the region.

"Such detailed information is rarely available for directly imaged exoplanets, making HD 106906 b a valuable target for future study," said Megan Reiter, a study co-author and also a UA astronomy graduate student.