Among some astronomers, there is a growing suspicion that our solar system's distant reaches conceal a large, ninth planet that we have not yet seen. New findings about a small ice world far beyond Pluto buttress this idea.
Last week, astronomers led by Dr Scott Sheppard of the Carnegie Institution for Science in Washington revealed the orbital details of the world, which they have nicknamed the Goblin. Mr Sheppard and his colleagues first spotted the world, which for now carries the official designation of 2015 TG387 as part of a systematic search three years ago for new worlds in the outer system, including the hypothesised Planet Nine. But only with additional observations did they realise how far out TG387 really is.
"It took us three years to figure out that it has an interesting orbit," Dr Sheppard said.
The astronomers have submitted a paper describing the discovery to The Astronomical Journal.
The world, estimated to have a width of a couple of hundred kilometres, is currently about 11.9 billion km from the sun, or about 2.5 times farther away than Pluto. But that is near to the closest it ever gets to the Sun.
At the other, most distant end of its elliptical, 40,000-year orbit, TG387 is nearly 70 times farther from the sun than Pluto - more than 321 billion km.
At TG387's most distant spot in its orbit, travelling there would be equivalent to circling the Earth seven times, or travelling three-quarters of the way to the moon.
The world, estimated to have a width of a couple of hundred kilometres, is currently about 11.9 billion km from the sun, or about 2.5 times farther away than Pluto. But that is near to the closest it ever gets to the sun. At the other, most distant end of its elliptical, 40,000-year orbit, TG387 is nearly 70 times farther from the sun than Pluto - more than 321 billion km.
Because TG387 remains far beyond the pull of the gravitational heavyweights of the solar system - Jupiter, Saturn, Uranus and Neptune - that raises the question of how it got thrown into its current orbit.
Astronomers - including Dr Sheppard's team in 2014 - have discovered several bodies with such distant, stretched-out orbits. The orbits of these objects seem to not be entirely random.
In 2016, Professor Michael Brown and Assistant Professor Konstantin Batygin of the California Institute of Technology published a detailed prediction of what they called an unseen planet, bigger than Earth yet smaller than Neptune, that was shepherding the movement of these distant worlds and could explain the odd journeys around the Sun of these faraway worlds.
They named it Planet Nine.
Assistant Professor Ann-Marie Madigan, an astronomer at the University of Colorado, has suggested that gravity from a massive ring of small worlds early in the solar system's history could explain the distant orbits. Prof Madigan's ideas could help explain how the ice worlds were cast out there, but not any clustering in their orbits.
"This new object does look like it's quite good for the Planet Nine theory," she said.
Prof Brown, who is known best for his role in Pluto's demotion to the status of a dwarf planet and is leading his own search for Planet Nine, agreed. "Mostly it's just another piece that fits in the puzzle very nicely," he said.
For almost all of its orbit, TG387 would be too dim to be seen by any telescope on Earth. Thus, either the astronomers were extremely lucky to locate an oddball at the rare moment it was visible - or actually it is just one of a multitude of similar worlds, with a few happening to be close enough to be seen.
"We think there are thousands of these, and most of them are too distant to detect," Dr Sheppard said.
He said that there might even be a million bodies 48km wide and larger, with a cumulative mass equal to one-tenth of Earth's.
Still, the existence of Planet Nine remains unproven. "I would say it's more likely than not," Dr Sheppard said. "It's not a slam dunk."