In our Solar System, how many planets are there? You might think that the answer you learned in kindergarten is obvious, but the number has fluctuated between 8 and 9, depending on whether you consider Pluto to be big enough for a planet. But there is another enigmatic planet, the so-called "Planet Nine" that could lurk at the edge of the Solar System. Everything we know about it is by inference, reached through judging the impact on its surroundings that this future world might have. Now for the first time in a separate solar system, scientists have measured the motion of a large exoplanet that could show activity close to that of the hypothesized Planet-Nine.
336 light-years away from Earth are the exoplanet HD106906 b. It orbits a double star system with a mass of as many as 11 Jupiters. Scientists first discovered it in 2013, but now, thanks to NASA's Hubble Space Telescope, they have been able to get much clearer knowledge about its orbit. In reality, Hubble has collected 14 years of accurate exoplanet data, enabling a wealth of new insight.
The exoplanet is located 730 times as far away from its host stars as it is from Earth to the Sun. While in just 14 years it's slow 15,000-year-long orbit does not usually allow for very definitive observation, the Hubble team discovered that its orbit is extreme, both very inclined and elongated, remaining on the outside of the twin stars forming a debris disk.
The University of California, Berkeley's Meiji Nguyen, who led the research, commented on how strange the orbit of the exoplanet is:
"To highlight why this is weird, we can just look at our own Solar System and see that all of the planets lie roughly in the same plane," shared Nguyen. "It would be bizarre if, say, Jupiter just happened to be inclined 30 degrees relative to the plane that every other planet orbits in. This raises all sorts of questions about how HD 106906 b ended up so far out on such an inclined orbit."
How has such an orbit formed on this planet? The scientists think it likely developed far closer to the host stars, but the drag from the system's gas disk may have influenced its orbit. It may have brought it closer to the twin stars, the gravitational force of which then hurled it into an eccentric orbit, almost all the way into interstellar space. As the press release of the Hubble team states, a moving star then theoretically corrected the orbit of the exoplanet and prevented it from leaving the home system altogether.
This theory is close to what scientists predict would have moved the prospective Planet Nine beyond the Kuiper belt to the edges of our Solar System. It was likely that Jupiter had affected its orbit, kicking it out of the inner solar system. Planet Nine might have continued going past Pluto, but by altering its orbit, a passing star would have prevented it.
A member of the Hubble team, Paul Kalas of the University of California, Berkeley, commented that studying what happened to exoplanet HD106906 b is like exploring our own history.
"It's as if we have a time machine for our own Solar System going back 4.6 billion years to see what may have happened when our young Solar System was dynamically active and everything was being jostled around and rearranged," elaborated Kalas.
None of this means of course, that we have actually discovered Planet Nine, whether it still exists. The unusual behavior of small space bodies past Neptune, whose unexplained orbits may be the result of being influenced by the gravitational force of an unknown planet, is evidence for its corporeality.
The James Webb Space Telescope will carry out further investigations into the formation and debris mechanism of the exoplanet HD106906 b, scheduled for launch in October 2021. This promises to shed more on the peculiar system of the exoplanet and by implication, on our own mystery, Planet Nine.