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Bizarre alien world may be analog of our solar system's putative Planet Nine

Artist’s illustration of the exoplanet HD 106906 b located a great distance away from its central binary star and the disk of dusty material that surrounds it.
Artist’s illustration of the exoplanet HD 106906 b located a great distance away from its central binary star and the disk of dusty material that surrounds it.
(Image: © Image courtesy of ESA/Hubble, M. Kornmesser)

Planet Nine may not be such an oddball after all, if it does indeed exist.

The exotic exoplanet HD 106906 b is an analog of sorts of Planet Nine, the Neptune-size world hypothesized to lurk in the outer reaches of our own solar system, a new study suggests.

HD 106906 b was discovered in 2013. It's about 11 times more massive than Jupiter and lies 336 light-years from Earth, near the double star HD 106906.

Related: When will we find Planet Nine?

The HD 106906 stellar duo is very young — just 15 million years old — and is still surrounded by a dusty debris disk. HD 106906 b apparently emerged from that disk but is now quite removed from it, zooming along high above the disk's plane. In addition, the giant exoplanet currently lies 737 astronomical units (AU) from the double star, about 25 times more distant from the pair than Neptune is from our sun. (One AU is the average Earth-sun distance, which is about 93 million miles, or 150 million kilometers.)

So HD 106906 b is a pretty curious character. Indeed, scientists have been debating whether the alien planet is still a bona fide member of the HD 106906 system or if it's now speeding away from the two stars, having been booted out by a gravitational interaction. 

There's evidence that such a booting did take place: photos captured a few years ago by the Gemini South Telescope in Chile show that HD 106906's outer dust disk and inner comet disk are lopsided. HD 106906 b could have done this sculpting if the planet formed close to the system's center and was then kicked outward after a close encounter with the double stars, previous modeling work has indicated.

Such a disturbance would ordinarily send a planet out of its home system entirely, turning it into a "rogue" that wanders the galaxy alone, free of any host star — unless its departure was arrested by another gravitational interaction, this time with a star that wandered close to the HD 106906 system.

Last year, a group that included two of the three authors on the new study identified several stars that could have provided such a nudge about 3 million years ago. And the new paper suggests that one of those interloping stars may indeed have kept HD 106906 b from becoming a runaway. 

In the new work, researchers led by Meiji Nguyen, a recent graduate of the University of California, Berkeley, analyzed observations of the HD 106906 system made between 2004 and 2018 by NASA's Hubble Space Telescope. The astronomers also compared the Hubble images with data gathered by Europe's Gaia spacecraft, which is precisely mapping the positions and movements of billions of stars in our Milky Way galaxy.

Related: The evidence for 'Planet Nine' in our solar system (gallery)

These combined data sets allowed the three scientists to nail down the positions of the HD 106906 binary and the giant planet. They found that HD 106906 b is likely in an elliptical and very long — but stable — orbit around its two parent stars, completing one lap every 15,000 years.

"Though it’s only been 14 years of observations, we were still able to, surprisingly, get a constraint on the orbit for the first time, confirming our suspicion that it was very misaligned and also that the planet is on an approximately 15,000-year orbit," Nguyen said in a statement.

"The fact that our results are consistent with predictions is, I think, a strong piece of evidence that this planet is, indeed, bound. In the future, a radial-velocity measurement is needed to confirm our findings," Nguyen added. (Radial-velocity measurements quantify the gravitational tug exerted by planets upon their host stars.)

What does this have to do with Planet Nine? The inferred history of HD 106906 b is similar to that proposed to explain how Planet Nine — or Planet Next, Planet X or Giant Planet Five, for those who will always regard Pluto as our solar system's ninth planet — could have gotten into, and remained in, its putative orbit, a highly elliptical path that keeps it hundreds of AU from the sun.

"What I really think makes HD 106906 b unique is that it is the only exoplanet that we know that is directly imaged, surrounded by a debris disk, misaligned relative to its system and is 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."

The new study was published online this week in The Astronomical Journal. You can read a preprint of it for free at arXiv.org.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook. 

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