Exoplanet around distant star resembles our reputed ‘Planet Nine’
Credit: ESA/Hubble, M. Kornmesser

Astronomers are still searching for a hypothetical “Planet Nine” in the distant reaches of our solar system, but an exoplanet 336 light years from Earth is looking more and 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 about the sun, was proposed in 2012 to explain perturbations in the orbits of dwarf planets just beyond Neptune’s orbit, so-called detached Kuiper Belt objects. It has yet to be found, if it exists.

A similarly weird extrasolar planet was discovered far from the star HD 106906 in 2013, the only such wide-separation planet known. While much heavier than the predicted mass of Planet Nine — perhaps 11 times the mass of Jupiter, or 3,500 times the mass of Earth — it, too, was sitting in a very unexpected location, far above the dust plane of the planetary system and tilted at an angle of about 21 degrees.

The big question, until now, has been whether the planet, called HD 106906 b, is in an orbit perpetually bound to the binary star — which is a mere 15 million years old compared to the 4.5 billion-year age of our sun — or whether it’s on its way out of the planetary system, never to return.

In a paper appearing Dec. 10 in The Astronomical Journal, astronomers finally 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, adjunct professor of astronomy, 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. And based on the team’s other recent discoveries about HD 106906, the planet seems to favor a scenario where passing stars also play a role.

“Something happens very early that starts kicking planets and comets outward, and then you have passing stars that stabilize their orbits,” he said. “We are slowly accumulating the evidence needed to understand the diversity of extrasolar planets and how that relates to the puzzling aspects of our own solar system.”

Source/Further reading: Berkeley University of California