"Planet Nine" may be a distant, dusty disc of rock and ice
Considering we've now detected almost 4,000 planets orbiting distant stars, it's easy to assume we already have an accurate head count in our own neighborhood – but that might not be the case. In recent years astronomers have found evidence that there might be a huge "Planet Nine" lurking on the fringes of the solar system. A new study has now poured some cold water on the idea, instead pitching the possibility that a dusty disc is causing all the weirdness out there.
The Planet Nine hypothesis took off in 2016, when a pair of Caltech astronomers noticed that several objects in the Kuiper Belt had strange orbits. These trans-Neptunian objects (TNOs) were all tilted 30 degrees off-kilter from everything else, in a way that didn't make sense based on the known arrangement of eight planets.
Using mathematical models, the team proposed that a huge ninth planet, around 10 times more massive than Earth, could be tugging on them from somewhere out there and causing the unusual orbits. Further evidence of its influence was found in other TNOs and even the wobble of the Sun itself.
It's not as crazy as it sounds, either. Lately, new dwarf planets have been turning up on the edges of the solar system with surprising regularity, like the Goblin and Farout. It's plausible that a mysterious Super-Earth could be hiding in the shadows – or maybe the observed phenomena can be explained some other way.
"The Planet Nine hypothesis is a fascinating one, but if the hypothesized ninth planet exists, it has so far avoided detection," says Antranik Sefilian, co-author of the new study. "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?"
To investigate, the team ran models of the full spatial dynamics of TNOs, along with the influence of the giant planets and, crucially, a disc of smaller objects extending out beyond Neptune. Intriguingly, the researchers found that if there's enough mass out there – say, between a few times and 10 times the mass of Earth – the outlier TNO orbits became possible.
"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," says Sefilian.
The new study isn't the only one to suggest other explanations. Last year, researchers at the University of Colorado Boulder ran similar simulations and found that distant dwarf planets like Sedna and other TNOs could be jostling each other like "gravitational bumper cars," which may produce similar orbits to those observed. Whatever the answer, the hunt will no doubt continue.
"It's also possible that both things could be true – there could be a massive disc and a ninth planet," says Sefilian. "With the discovery of each new TNO, we gather more evidence that might help explain their behavior."
The research was published in the Astronomical Journal.
Source:University of Cambridge