Astronomers discover planet that shouldn’t be there

The discovery of a giant planet orbiting its star at 650 times the average Earth-sun distance has astronomers puzzled over how such a strange system came to be.

An international team of astronomers, led by a University of Arizona graduate student, has discovered the most distantly orbiting planet found to date around a single, sun-like star. It is the first exoplanet – a planet outside of our solar system – discovered at the UA.

Artist’s conception of a young planet in a distant orbit around its host star. The star still harbors a debris disk, remnant material from star and planet formation, interior to the planet’s orbit (similar to the HD106906 system). Image credit: NASA/JPL-Caltech

Weighing in at 11 times Jupiter’s mass and orbiting its star at 650 times the average Earth-Sun distance, planet HD 106906 b is unlike anything in our own Solar System and throws a wrench in planet formation theories.

“This system is especially fascinating because no model of either planet or star formation fully explains what we see,” said Vanessa Bailey, who led the research. Bailey is a fifth-year graduate student in the UA’s Department of Astronomy.

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 106906 b 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,” Bailey explained. “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.”

According to Bailey, 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,” she 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.”

See full size | This is a discovery image of HD 106906 b in the thermal infrared (4µm wavelength) from MagAO/Clio2, processed to remove the bright light from its host star, HD 106906 A. The planet is than 20 times farther away from HD 106906 A than Neptune is from our Sun. AU stands for Astronomical Unit, the average distance of the Earth and the Sun. Image: Vanessa Bailey

This system 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,” Bailey added. “Future observations of the planet’s orbital