Researchers discovered what should be an impossible planet.
The planet has a mass 11 times greater than Jupiter's, and orbits its host star at a distance of about 650 times the average Earth-Sun distance, a University of Arizona news release reported.
The planet, dubbed HD 106906 b, calls into question popular theories on how planets are formed.
"This system is especially fascinating because no model of either planet or star formation fully explains what we see," Vanessa Bailey, who led the research. Bailey is a fifth-year graduate student in the UA's Department of Astronomy, said.
Planets like Earth that orbit close to their star are believed to spawn from "asteroid-like bodies born in the primordial disk of dust and gas that surrounds a forming star," the news release reported. This process is far to slow moving to create a large planet far away from the star it orbits. A collapse in disk material is another theory on how planets are born, but these primordial disks should not have enough mass to form a planet that far out.
The researchers suggested the planet could have been created 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."
A hole in this idea is that the mass ratio between two stars in a binary system has never been observed to be more than 10-to-1.
"In our case, the mass ratio is more than 100-to-1," Bailey said. "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."
The researchers detected leftovers of the "debris disk" from the birth of the star and planet.
"Systems like this one, where we have additional information about the environment in which the planet resides, have the potential to help us disentangle the various formation models," Bailey said. "Future observations of the planet's orbital motion and the primary star's debris disk may help answer that question."
The planet is believed to be relatively young (only 13 million years old) because it still glows from the heat leftover from its birth.
The team confirmed the planet's orbit by looking at Hubble Space Telescope data from eight years ago.
"Every new directly detected planet pushes our understanding of how and where planets can form," co-investigator Tiffany Meshkat, a graduate student at Leiden Observatory in the Netherlands, said in the news release. "This planet discovery is particularly exciting because it is in orbit so far from its parent star. This leads to many intriguing questions about its formation history and composition. Discoveries like HD 106906 b provide us with a deeper understanding of the diversity of other planetary systems."
