There’s something going on in our outer solar system. But it couldn’t be Planet Nine. Something wonky going on somewhere in the outer reaches of the Solar System, beyond Neptune’s orbit. Some objects orbit differently from all else, and we don’t know why.
A popular hypothesis is that these orbits could be messed with an unseen object called Planet Nine. Astronomers are searching avidly for this planet. But physicists came up with an alternative explanation earlier this year that they think is more plausible.
The orbital Wobblies could be caused instead of one big object by the combined gravitational force of a number of smaller Kuiper Belt or trans-Neptunian objects (TNOs). That’s according to astrophysicists Antranik Sefilian from the University of Cambridge in the United Kingdom and Jihad Touma from the American University of Beirut in Lebanon.
If it sounds familiar, that’s because Sefilian and Touma aren’t the first to think about this idea. But their calculations are the first to explain important features of these objects strange orbits while taking into account the other eight planets in the Solar System.
A hypothesis was first announced in a 2016 study for Planet Nine. Astronomers studying a dwarf planet in the Kuiper Belt noticed that several TNOs were “detached” from the powerful gravitational influence of the gas giants of the Solar System and had strange looping orbits different from the rest of the Kuiper Belt.
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But these six objects ‘ orbits were also clustered together in a way that did not seem to be random. Something seemed to tug them into that position. A giant, previously unseen planet could do so, according to modeling.
This planet has remained elusive so far-not necessarily odd, as there are considerable technical challenges to seeing such a distant dark object, especially when we don’t know where it is. But its evasiveness leads scientists to search for alternative explanations.
“The hypothesis of the Planet Nine is fascinating, but if the hypothesized ninth planet exists, detection has so far been avoided,” Sefilian said back in January when his study was released. Adding that the team wanted to see if there was a less dramatic explanation of the strange TNO orbits.
“Instead of allowing a ninth planet and then worrying about its formation and unusual orbit, we thought why not just take into account the gravity of small objects that constitute a disc beyond Neptune’s orbit and see what it does for us?”
When researchers first detected there’s something going on in our outer Solar System they created a computer model of the detached TNOs, as well as the Solar System planets (and their gravity), and a huge debris disc past the orbit of Neptune. They were able to recreate the clustered looping orbits of the detached TNOs by applying tweaks to elements such as mass, eccentricity, and disc orientation.
“If you remove Planet Nine from the model and instead allow lots of small objects scattered across a wide area, collective attractions between those objects could account for the eccentric orbits we see in some TNOs just as easily,” Sefilian said.
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This solves a problem faced by the University of Colorado Boulder scientists when they first floated last year’s collective gravity hypothesis. While their calculations could account for the gravitational effect on the detached TNOs, they were unable to explain why their orbits were all tilting the same way.
And there is yet another problem with both models. The Kuiper Belt needs a collective gravity of at least a few Earth masses in order to produce the observed effect. However, current estimates put the Kuiper Belt mass at only 4-10% of the Earth’s mass.
But, according to the Solar System formation models, it should be much higher. And, Sefilian notes, when you’re inside it, it’s hard to see the whole of a debris disc around a star, so it’s possible that the Kuiper Belt is much more than we can see.
“While we don’t have direct observational evidence for the disc and we don’t have it for Planet Nine, so we’re investigating other possibilities,” said Sefilian.
“It is also possible that both things could be true there could be a massive disk and a ninth planet. We are gathering more evidence with the discovery of each new TNO that could help explain their behaviour.”