There’s no Planet Nine after all, but there is another ...


There’s no Planet Nine after all, but there is another explanation

Scientists have a much simpler hypothesis for the strange orbits of 13 objects in the outer solar system

Sarah Knapton

When the mysterious “Planet Nine” was first hypothesised in 2016 it was hailed as the solution to the strange orbits of far-flung icy objects and the unexplained tilt in the solar system.
The finding came 10 years after Pluto had been unceremoniously downgraded to “asteroid number 134340” and led to hopes that our star system would soon be restored to nine worlds.
Yet despite widespread searches by the most powerful telescopes, Planet Nine has never been spotted, and now astrophysicists have put forward a “less dramatic” explanation.
They think that the Kuiper Belt objects are being tugged by the combined gravitational force of a rotating ring of icy chunks orbiting the sun beyond Neptune.
When the heavy spinning disc was punched into computer models of the solar system it created the same gravitational pull as Planet Nine, and is a far more likely explanation for the anomaly.
The explanation has been dubbed “the shepherding disk hypothesis”.
“The Planet Nine hypothesis is a fascinating one, but if the hypothesised ninth planet exists, it has so far avoided detection,” said Cambridge University student Antranik Sefilian.
“We wanted to see whether there could be another, less dramatic and perhaps more natural cause for the unusual orbits we see.
“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 disk beyond the orbit of Neptune and see what it does for us?”
The Planet Nine hypothesis was first proposed by researchers at Caltech (the California Institute of Technology) who discovered that 13 objects in the Kuiper Belt – a doughnut-shaped area of icy bodies beyond Pluto – were all moving together as if being “lassoed” by the gravity of a huge object.
After running computer simulations they concluded that only a massive planet 10 times the size of Earth could have such a huge effect. It was so big that researchers said it would be “the most planety planet of the solar system”.
The Kuiper Belt sits at the edge of the solar system and it is all that remains of the original disc of material that created the planets and sun about 4.6 billion years ago.
Most of the belt’s objects – know as Trans-Neptunian Objects (TNOs) – orbit the sun in a circular path, but in 2003 astronomers noticed that some had strange elliptical orbits, as if something was interfering with their rotation.
Professor Jihad Touma, from the American University of Beirut, and his former student Sefilian modelled the movement of the TNOs with the combined action of the giant outer planets and a massive icy disk beyond Neptune.
Their calculations showed that the model fully explained the odd elongated orbits.
“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,” added Sefilian.
“While we don’t have direct observational evidence for the disk, neither do we have it for Planet Nine, which is why we’re investigating other possibilities.
“It’s also possible that both things could be true – there could be a massive disk and a ninth planet. With the discovery of each new TNO, we gather more evidence that might help explain their behaviour.”
The new hypothesis is backed up by observations that show there is far more leftover debris at the edge of star systems after planet building than previously thought.
Computer models of the birth of the solar system also suggest there should be a lot of building blocks left over.
Until now, it was believed that the total mass of all objects in the Kuiper Belt was about one 11th the mass of Earth, but the new calculations suggest it could be up to 10 times more than Earth.
On New Year’s Day Nasa’s New Horizons spacecraft became the first to fly past a Kuiper Belt object, named Ultima Thule. Pictures beamed back to Earth showed a dark-reddish object that looked like a snowman, about 33.5km long and 16km wide, that spins on its axis once every 15 hours.
The new research was published in the Astronomical Journal.
– © The Daily Telegraph

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