An international team of astronomers announced the discovery of a giant extrasolar planet orbiting its star at more than 20 times the average Neptune-Sun distance.
The planet, named HD 106906b, weighs in at 11 times Jupiter’s mass. It orbits a white main-sequence star, HD 106906A, located in the constellation Crux around 300 light-years away from Earth.
HD 106906b is unlike anything in our own Solar System and throws a wrench in planet formation theories.
At only 13 million years old, the planet still glows from the residual heat of its formation.
Because at 1,500 degrees Celsius the planet is much cooler than its host star, it emits most of its energy as infrared rather than visible light. Earth, by comparison, formed 4.5 billion years ago and is thus about 350 times older than HD 106906b.
“This system is especially fascinating because no model of either planet or star formation fully explains what we see,” said Vanessa Bailey of the University of Arizona, who is the lead author of a paper accepted for publication in the Astrophysical Journal Letters (arXiv.org).
It is thought that planets close to their stars, like Earth, coalesce from small asteroid-like bodies born in the primordial disk of dust and gas that surrounds a forming star. However, this process acts too slowly to grow giant planets far from their star. Another proposed mechanism is that giant planets can form from a fast, direct collapse of disk material. However, primordial disks rarely contain enough mass in their outer reaches to allow a planet like HD 106906b to form. Several alternative hypotheses have been put forward, including formation like 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. 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.”
“One problem with this scenario is that 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. 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,” Vanessa Bailey said.
HD 106906b is also of particular interest because researchers can still detect the remnant debris disk of material left over from planet and star formation.
“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.”
“Future observations of the planet’s orbital motion and the primary star’s debris disk may help answer that question,” Vanessa Bailey concluded.
Bailey V et al. 2013. HD 106906 b: A Planetary-mass Companion Outside a Massive Debris Disk. ApJL, accepted for publication; arXiv:1312.1265