In the nearly 12 months since scientists first proposed the theoretical existence of a “Planet 9,” we’ve seen several rounds of information and fact-checking. Last year, scientists reported that unusual orbital patterns in multiple Kuiper Belt Objects (KBOs) might be explained by the influence of a single, larger planet far beyond the orbit of Neptune. Later refinements to this theory have set some size limits and theoretical orbital patterns for any hypothetical planet. If it coalesced around Sol, Planet 9 is almost certainly an ice giant, smaller than Neptune, and so far from our sun that it could take 10,000 – 20,000 years to complete a single orbit. Neptune and Pluto are both speed demons by comparison, completing one orbit every 164.8 and 248 years, respectively.
According to scientists, there’s another interesting possibility: Planet 9 might not be from our solar system at all. Instead, it might represent what is sometimes called a “rogue” planet, or a planet wandering the cosmos rather than being in stable orbit around a companion star. Despite the term, rogue planets are thought to be relatively common, possibly outnumbering planets orbiting stars. Thus far, only a handful of good rogue planet candidates have been identified, but we have located at least two objects that could fit the definition — CFBDSIR 2149-0403, an object containing no more than 13 Jupiter masses (making it a Jupiter-sized planet or lower-end brown dwarf candidate) and PSO J318.5-22, with estimated cloud temperatures exceeding 800C despite the complete absence of a host star.
We bring up these two candidates, because available data on them suggest two very different planets — and illustrate that whatever Planet 9 might be, it could be fascinatingly strange. If you’re wondering how a rogue planet can remain hot without a star, yet simultaneously fail to show up on infrared, scientists have done some research on that topic. Under the right conditions, a planet that forms with a sufficiently think atmosphere could retain its internal heat, even in the absence of sunlight. The Earth, after all, remains geothermally active despite the fact that no sunlight striking the surface of the planet is transferred as heat to the planetary core. If a rogue planet had a geodynamo and associated magnetosphere, it would help protect the upper atmosphere from being stripped away while the planet exited its original solar system or from being slowly eroded over hundreds of millions of years. The same insulative tendencies that could trap heat at the surface of these worlds would also make them unlikely to show up via infrared telescopy.
Research into whether Sol could’ve captured a rogue planet of Planet 9’s theorized size and structure indicates it could, according to Space.com. In most cases, a rogue planet entering our system would spin out again, possibly dragging one of our own native planets along with it. In roughly 40% of cases, however, the rogue could be captured, either without perturbing the orbits of nearer planets or by booting one of our own planets out in the process. Exactly which of these scenarios is likely depends on the parameters of the experiment, and it’s also possible that Planet 9 either doesn’t exist or exists but formed in the orbit of our sun like the other known planets. Since any rogue planet larger than Neptune would likely have perturbed the orbit of the inner planets, the simulations suggest an upper boundary on Planet 9’s theoretical size that agrees with other predictions. Planet 9’s hypothesized orbit is consistent with a rogue planet that Sol captured, but the most likely orbits don’t require a rogue — they just allow for one.
Finding a new planet in the outermost reaches of our solar system would, of course, be a momentous occasion. Finding a planet in our solar system that formed around another star would be one of the most important scientific discoveries in history. Our ability to explore other worlds via probe or manned mission is currently limited to our own solar system. A rogue planet in our system would represent a phenomenal research opportunity to explore how planetary formation takes place in other star systems, and careful examination of the rogue might even give us clues about the star system it could have come from.
Of course, all of this depends on Planet 9 existing in the first place, and we’ve got to find the thing before we can seriously speculate on its origin. Scientists are expected to ramp up the search in 2017.