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University of Cambridge researchers propose alternative to Planet Nine theory

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How many planets are there in our solar system?

The Kuiper Belt is made of remnants from the formation of the solar system (6682994)
The Kuiper Belt is made of remnants from the formation of the solar system (6682994)

Since Pluto was downgraded from planet to ‘dwarf planet’ - because it has not cleared the neighbourhood around its orbit - the accepted answer is eight.

But some astronomers have hypothesised that an as-yet unseen ninth planet exists in the farthest reaches of our solar system, and is responsible for the strange clustering of objects that orbit the Sun well beyond Neptune.

Now researchers at the University of Cambridge and the American University of Beirut have put forward an alternative to the Planet Nine hypothesis.

In a paper published in the Astronomical Journal, they suggest the unusual orbital architecture could be explained by the combined gravitational force of a disc of small icy bodies with a combined mass as much as 10 times that of Earth.

The new theory is not the first to propose a massive disc of small objects but is the first to explain the significant features of the observed orbits, while accounting for the mass and gravity of the eight planets we have seen in our solar system.

Co-author Antranik Sefilian, a PhD student in Cambridge’s Department of Applied Mathematics and Theoretical Physics, said: “The Planet Nine hypothesis is a fascinating one, but if the hypothesised ninth planet exists, it has so far avoided detection.”

The area of interest lies beyond the orbit of Neptune in the Kuiper Belt, which is comprised of small bodies left over from the formation of our solar system - including Ultima Thule, recently photographed by NASA’s New Horizons spacecraft.

It is known that Neptune and the other giant planets have a gravitational influence on objects in this belt and beyond, known as trans-Neptunian Objects (TNOs), which follow near circular paths around the Sun.

But since 2003, about 30 TNOs on highly elliptical orbits have been spotted and share similar spatial orientation. The clustering of these ‘extreme TNOs’ can be explained by our eight-planet solar system architecture, prompting the suggestion that a planet 10 times the size of Earth is shepherding them in the same direction.

“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,” says Antranik. “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?”

Professor Jihad Touma, from the American University of Beirut, and his former student Sefilian, a Gates Cambridge Scholar, calculated the impact of the combined action of giant outer planets and a massive extended disc of small objects beyond Neptune.

The model can explain the curious spatially clustered orbits of the extreme TNOs.

Sefilian, a member of Darwin College, says: “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.”

The researchers were able to identify the ranges in the disc’s mass, its roundness or eccentricity, and forced gradual shifts in its orientations.

But if they are right, the combined mass of the Kuiper Belt must be a few to 10 times the mass of the Earth - earlier estimates have only added up to about one-tenth.

“When observing other systems, we often study the disc surrounding the host star to infer the properties of any planets in orbit around it,” adds 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. Nevertheless, it is interesting to note that observations of Kuiper belt analogues around other stars, as well as planet formation models, reveal massive remnant populations of debris.

“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 behaviour.”

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