The Planet Nine hypothesis was introduced in 2016 in a bid to explain the “eccentric” orbits exhibited by some objects on the far fringes of the solar system. This unexplored region of space known as the Kuiper belt is home to a vast asteroid field and a number of dwarf planets, including the dethroned Pluto. The Kuiper belt sits approximately 50 astronomical units or 4.6 billion miles (7.5 billion km) from the Sun, past the orbit of Neptune. And it is in this part of the solar system that some trans-Neptunian Objects (TNOs) appear to cluster together and orbit the Sun in an unusual elliptical manner.
If a planet between five to 10 times the size of Earth exists somewhere beyond Neptune, it would explain what is happening in the Kuiper belt.
As of 2019, no observations of Planet Nine have been made although scientists are hopeful this will soon change.
Some astronomers, however, are now questioning the Planet Nine model of the solar system and are suggesting other, equally plausible explanations.
Two researchers from the University of Cambridge and the American University of Beirut have proposed the bizarre Kuiper orbits can be explained by not one but many smaller objects.
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Professor Jihad Touma from Beirut and former student Antranik Sefilian from Cambridge believe a disc of small and icy objects in the Kuiper belt is to blame.
All of these object grouped together would have a mass approximately 10 times that of the Earth – enough for the combined gravitational forces to affect other objects around them.
The theory was presented in the peer-reviewed Astronomical Journal.
Mr Sefilian, who is currently a PhD student at Cambridge, said: “The Planet Nine hypothesis is a fascinating one, but if the hypothesised ninth planet exists, it has so far avoided detection.
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“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?”
The Kuiper belt is an ancient remnant from the earliest days of the solar system with rocks and asteroids present during the system’s formation.
Since 2003, astronomers have spotted at least 30 TNOs within the Kuiper region, which follow elliptical orbits unlike the other objects in the belt.
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Michael Brown, a professor of astronomy at the California Institute of Technology (Caltech), suggested in 2016 a giant planet beyond the direct reach of our telescopes was affecting their orbits.
However, Mr Sefilian said: “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.”
But the astronomer also conceded the possibility of both models being correct, that is, Planet Nine and the doc of icy objects both exist far out in space.
The scientist said each new discovery of a TNO provides new clues to unravel the mysteries of the solar system.
