Could our solar system contain one more planet beyond the orbit of Neptune that has yet to be discovered? So-called "Planet Nine" is more than just fanciful speculation; it's a theory with compelling circumstantial evidence behind it.
For instance, since 2003 astronomers have discovered a suspicious number of trans-Neptunian Objects (TNOs) — bodies found in the far reaches of our solar system, in a region known as the Kuiper Belt — that have similar spatial orientation and which are on highly elliptical orbits. This type of clustering and orbital behavior can't be explained by our existing eight-planet solar system architecture, and it's too uncanny to be a coincidence.
One thing that would explain it? The existence of a Planet Nine, with a mass of about 10 Earths, creeping around in the darkest parts of the solar system, dragging around these TNOs in its gravitational wake. Even more compelling: scientists haven't been able to come up with a theory that explains this TNO behavior better than theories which postulate Planet Nine.
Or at least, that used to be the case. Researchers at the University of Cambridge and the American University of Beirut have successfully modeled a new theory that does away with Planet Nine entirely. Rather than posit a whole new planet, they instead proposed the existence of a disc populated with a collection of small icy bodies that cumulatively possess a mass of about ten Earths, reports Phys.org.
Looking at the problem a different way
"The Planet Nine hypothesis is a fascinating one, but if the hypothesised ninth planet exists, it has so far avoided detection," explained co-author Antranik Sefilian. "We wanted to see whether there could be another, less dramatic and perhaps more natural, cause for the unusual orbits we see in some TNOs. We thought, rather than allowing for a ninth planet, and then worry about its formation and unusual orbit, why not simply account for the gravity of small objects constituting a disc beyond the orbit of Neptune and see what it does for us?"
This isn't the first theory to propose that the gravitational forces of a massive disc made of small objects could avoid the need for a ninth planet, but its the most comprehensive such theory, and the first to account for all of the major gravitational variables in the solar system.
Researchers were able to identify ranges in the disc's mass, its "roundness" (or eccentricity), and forced gradual shifts in its orientations (or precession rate), which faithfully reproduced the outlier TNO orbits. It's a remarkably detailed account that might just be the death knell for Planet Nine truthers.
"If you remove Planet Nine from the model and instead allow for lots of small objects scattered across a wide area, collective attractions between those objects could just as easily account for the eccentric orbits we see in some TNOs," added Sefilian.
Of course, scientists won't really know for sure whether the Kuiper Belt contains a Planet Nine or a massive disc of small bodies, not until we go out and actually look for these objects. But we haven't yet glimpsed any massive planets lurking about, and small objects are notoriously difficult to detect. It will take thorough exploration before either theory can be definitely ruled out.
"It's also possible that both things could be true — there could be a massive disc and a ninth planet. With the discovery of each new TNO, we gather more evidence that might help explain their behavior," said Sefilian.