Somewhere in the outer reaches of the Solar System, beyond the orbit of Neptune, something wonky is happening. A few objects are orbiting differently from everything else, and we do not know why
A popular hypothesis is that an unseen object called Planet Nine could be messing with these orbits; astronomers are avidly searching for this planet. But now physicists have come up with an alternative explanation that they think is more plausible
Instead of one big object, the orbital wobbles could be caused by the combined gravitational force of a number of smaller Kuiper Belt or trans-Neptunian objects (TNOs ). That's according to astrophysicists Anthony Sefilian of the University of Cambridge in the UK and Jihad Touma of the American University of Beirut in Lebanon
If it sounds familiar, that's because Sefilian and Touma are not the first to think of this idea ̵
A hypothesis for Planet Nine was first announced in a 2016 study. Astronomers studying a dwarf planet in the Kuiper Belt noticed that several TNOs were "detached" from the strong gravitational influence of the Solar System's gas giants, and had weird looping orbits that were different from the rest of the Kuiper Belt
. orbits of these six objects were also clustered together in a way that did not appear random; something seemed to have tugged them into that position.
So far, this planet has remained elusive – not necessarily odd, since there are considerable technical challenges to seeing a dark object that far away, especially when we do not know where it is.
"The Planet Nine hypothesis is a fascinating one, but if the hypothesized ninth planet exists, it has so far avoided detection," Sefilian said, adding that the team wanted to see if there was a less dramatic explanation of the weird TNO orbits
"We thought, rather than allowing for a ninth planet, and then worry about its formation and unusual orbit, why not just account for the gravity of small objects constituting a disk beyond the orbit of Neptune and see what it does for us ?
The researchers created a computer model of the detached TNOs, as well as the planets of the Solar System (and their gravity), and a huge disk of debris past the Neptune's orbit
By applying tweaks to elements such as the mass, excentricity and orientation of the disk, the researchers were able to recreate the clustered looping orbits of the detached TNOs
"If you remove the Planet Nine from the model, and instead allow for lots of small objects scattered across "
This solves a problem that scientists from the University of Colorado Boulder had when they first floated d the collective gravity hypothesis last year. Although their calculations were able to account for the gravitational effect on the detached TNOs, they could not explain why their orbits were all tilting the same way
And there is still another problem with both models: in order to produce the observed effect, the Kuiper Belt needs a collective gravity of at least a few Earth masses
Current estimates, however, put the mass of the Kuiper Belt at just 4 to 10 percent of Earth's mass.
But, according to Solar System formation models, it should be much higher; and, Sefilian notes, it's hard to see the whole of a debris disc around a star when you're inside it, so it's possible that there's a lot more to the Kuiper Belt than we can see
we do not have direct observational evidence for the disc, nor do we have it for Planet Nine, which is why we're investigating other possibilities, "said Sefilian.
" It's also possible that both things could be true – there "
The team's research is due to appear in the Astronomical Journal and you can find the pre-print on arXiv