Over the last two decades, astronomers have spotted several distant objects beyond the orbit of Neptune that exhibit incredibly strange orbits. According to a landmark study published in 2016, they indicate the presence of a large, hypothetical world, which has come to be known as “Planet Nine.”
Since scientists predicted its existence, “Planet Nine” has never been directly observed. While some studies have strengthened the hypothesis, others—like a paper recently published in the Astronomical Journal—have proposed alternative explanations.
According to the authors of the Astronomical Journal study—from the University of Cambridge and the American University of Beirut—the combined gravitational force of a massive disc consisting of icy bodies, which has a mass up to ten times higher than that of the Earth, could account for the strange orbits.
To date, other groups of scientists have proposed similar alternative hypotheses centered around the gravitational influence of such a disc. However, the new explanation, according to the researchers, is the first to take into account important characteristics of the strange orbits, as well as the mass and gravity of the eight known planets in the Solar System.
Planet Nine’s existence was predicted because of the highly elliptical orbits of around 30 so-called Trans-Neptunian objects (TNOs)—which are clustered together in a way that it is extremely unlikely to have occurred by chance. Furthermore, the architecture of the Solar System as we know it cannot explain them. The hypothesis, thus, describes a planet about ten times more massive than the Earth, which exerts an influence on these TNOs, accounting for their peculiar clustering of orbits.
“The Planet Nine hypothesis is a fascinating one, but if the hypothesized ninth planet exists, it has so far avoided detection,” co-author of the study, Antranik Sefilian, from Cambridge’s Department of Applied Mathematics and Theoretical Physics, said in a statement. “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?” he said.
The team came to their conclusions after modeling the dynamics of TNOs, the large, known outer planets and a massive, extended disc beyond Neptune. They say that this set up can explain the bizarre clustering of orbits.
“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,” Sefilian said.
There is currently no direct observational for the disk of debris proposed in the study, because our vantage point from inside the Solar System makes it difficult to study.
“When observing other systems, we often study the disc surrounding the host star to infer the properties of any planets in orbit around it,” said Sefilian. “The problem is when you’re observing the disc from inside the system, it’s almost impossible to see the whole thing at once. While we don’t have direct observational evidence for the disc, neither do we have it for Planet Nine, which is why we’re investigating other possibilities.”
“It’s also possible that both things could be true—there could be a massive disc and a ninth planet,” he said. “With the discovery of each new TNO, we gather more evidence that might help explain their behavior.”