Ideas about a possible ninth planet have been kicking around since shortly after we discovered the eighth in 1846. But so far, all that we've come up with is Pluto and a handful of other objects orbiting out in the Kuiper Belt. And these dwarf planets simply don't have the mass to have a significant gravitational influence on our Solar System.
But our inability to find anything big beyond the known planets may just have been because we weren't thinking radically enough. One of the people responsible for the discovery of a number of Kuiper Belt Objects noticed an odd alignment in their orbits. When running models of how that oddity could be produced, he and his team found that a large planet with an extreme orbit would work.
Calling it Planet 9, they suggested it could be over 10 times Earth's mass and so far out it takes 20,000 years to complete one orbit. Planet 9, they speculated, has a lopsided orbit that's tilted relative to the other planets and much closer to the Sun on one side.
Unfortunately, there's no observational evidence of its existence yet. But the people behind the first announcement found another way to provide evidence that Planet 9 might be out there: they tested whether it could explain an odd tilt in the Sun's axis of rotation.
The Sun and its planets formed through a single process. As gravity drew in the large blob of gas and dust they formed from, it naturally started spinning. This spin, in turn, flattened the blob out to a disk with the Sun at the center. Most of the matter in the disk fell into the Sun, but some of the remainder condensed into planets further out in the disk.
The rotating disk should thus end up imparting its rotation to the Sun, and all the planets should form in the same plane as the disk. Thus, barring outside influences, the Sun's axis of rotation should be perpendicular to the plane where the planets orbit.
Except it's not. The Sun's axis of rotation has been off by about six degrees, and scientists have been pondering why for at least 50 years. And they've come up with a variety of things that could have thrown things off early in the Solar System's history. These include magnetic interactions between the Sun and the disk, asymmetries in the disk itself, and another star having passed sufficiently close.
Looking at exosolar systems we've now observed, issues like this appear to be common. Some systems have tilts that are up to several times our Sun's; others have planets that rotate in the opposite direction from their host star.
So, we know the Sun's tilt is nothing unusual, and we have several plausible ways of explaining it. Isn't that enough? In a paper describing the new work, the authors argue that the situation is inherently dissatisfying: all the explanations happened early in the Solar System's history and are thus inaccessible to observation. We can never test the ideas.
Planet 9 is different. Given its weird, extreme orbit, it would produce an asymmetrical tug on the Sun that, over time, could produce a tilt in its axis.
But its orbit would have to have specific qualities in order to do so. And the same thing is true if Planet 9 is altering the orbits of Kuiper Belt objects. So, the new analysis mostly involves determining whether these two sets of properties overlap.
This, once again, involves numerous simulations that run the full course of the Solar System's 4.5 billion years of existence, including the gravitational influences of both Planet 9 and all the known giant planets. Of course, there are indications that the giant planets didn't start out in their current positions. Since we don't know where they were instead, this has to be viewed as a potential source of error in the analysis.
The overall conclusion is that, if Planet 9 is out there, it would necessarily influence the Sun's axis of rotation. And, in some orbital configurations, it can account for the entire six degrees of tilt. But those configurations don't overlap with the high-probability orbits identified in the analysis of the Kupier Belt Objects.
That leaves things ambiguous. That ambiguity is certainly still consistent with Planet 9 existing, in that none of the likely orbits produced a larger tilt than we see. Maybe Planet 9 is only one of a number of influences on the tilt, allowing it to occupy one of the high-probability orbits. Or, it could be in a lower-probability orbit and produce all the tilt. Astronomers are no closer to knowing where to look for the planet.
But this still represents progress. In addition to direct observation evidence, science tests ideas by seeing if they're consistent with available evidence and whether they have the power to explain known phenomena. Making sure the Planet 9 proposal clears those two hurdles is part of the process.
The arXiv. Abstract number: http://arxiv.org/abs/1607.03963v2 (About the arXiv). To be published in Astrophysical Journal.
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