As astronomers are looking for a hypothetical “Planet Nine” in our solar system, an exoplanet 336 light years from Earth is looking more like the Planet Nine of its star system.
Planet Nine, potentially 10 times the size of Earth and orbiting far beyond Neptune in a highly eccentric orbit around the sun, was proposed in 2012 to explain perturbations in the orbits of dwarf planets beyond Neptune’s orbit, so-called detached Kuiper Belt objects. However, it has yet to be found, if ever it exists.
A similarly weird extrasolar planet was discovered far from the star HD 106906 in 2013 was much heavier than the predicted mass of Planet Nine at probably 11 times the mass of Jupiter, or 3,500 times the mass of Earth. And it was located in an unexpected location, far above the dust plane of the planetary system and tilted at an angle of about 21 degrees.
It is not known whether the planet, HD 106906 b, is in an orbit perpetually bound to the binary star — which is 15 million years old compared to the 4.5 billion-year age of our sun or on its way out of the planetary system, never to return.
In a paper published on Dec. 10, 2020, in the Astronomical Journal, astronomers answer that question. By precisely tracking the planet’s position over 14 years, they determined that it is likely bound to the star in a 15,000-year, highly eccentric orbit, making it a distant cousin of Planet Nine.
If it is in a highly eccentric orbit around the binary, “This raises the question of how did these planets get out there to such large separations,” said Meiji Nguyen, a recent UC Berkeley graduate and first author of the paper. “Were they scattered from the inner solar system? Or, did they form out there?”
According to senior author Paul Kalas, University of California, Berkeley, the resemblance to the orbit of the proposed Planet Nine shows that such distant planets can really exist and that they may form within the first tens of millions of years of a star’s life. “Something happens very early that starts kicking planets and comets outward, and then you have passing stars that stabilize their orbits,” he said.
What makes HD 106906 unique is that it is the only exoplanet that we know that is directly imaged, surrounded by a debris disk, misaligned, and widely separated, Nguyen said. “This is what makes it the sole candidate we have found thus far whose orbit is analogous to the hypothetical Planet Nine.”