Is there no Planet Nine after all?

A 9th planet could be shepherding objects in the outer Solar System into strange orbits, or it could be the gravity of objects beyond Neptune.

The existance of Planet Nine was first suggested by Konstantin Batygin and Mike Brown in 2016. Several surveys have since launched to search for the planet, but it has not been found. Image Credit: Caltech/R Hurt (IPAC)
Published: January 21, 2019 at 12:00 pm
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In 2016, the world was excited to learn a Planet Nine could be hiding in the outskirts of our Solar System. Now a new study suggests that it might not exist after all.


The idea of a ninth planet was first proposed to explain the strange orbits of several trans Neptunian Objects (TNOs) – the icy bodies that orbit the Sun in a disc beyond Neptune.

Around 30 TNOs are at an angle to the rest of the disc, and current ideas of the Solar System’s layout fail to explain how they arrived at such a strange configuration.

At first it was suggested that a ninth planet with a mass around ten times that of Earth’s could be responsible for ‘shepherding’ the bodies into their orbits.

But after two years of searching, there’s been no sign of Planet Nine, nor has there been a definitive explanation of how a planet that size could come to orbit so far out.

“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 Sefilian from the University of Cambridge’s Department of Applied Mathematics and Theoretical Physics who co-wrote the study.

Sefilian, along with colleague Jihad Touma from the American University of Beirut, created a computer model of the disc of TNOs to see the effects of their cumulative gravity.

“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.

However, this new theory is not without flaws either.

To create the orbits observed, the disc would need to be as much as ten Earth masses in size.

Current estimates of the mass are around 0.1 Earth masses, though these are not thought to be very accurate.

“The problem is when you’re observing a disc from inside the system, it’s almost impossible to see the whole thing at once,” says Sefilian.

“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.”


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.


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