Researchers have discovered a dwarf planet far beyond the orbit of Pluto they believe strengthens the case for a large planet at the outer reaches of our solar system.

The newly found object is called 2015 TG387 or "The Goblin," because of its letters and the fact it was first seen near Halloween.

It was discovered about 80 astronomical units (AU) from the sun — one AU is equivalent to the distance from the sun to Earth or about 150 million km. But that's almost as close as it gets: its orbit can take the dwarf planet as far away as 2,300 AU, making it the most distant object in our solar system.

In a paper published in The Astronomical Journal on Tuesday, researchers suggest that its strange orbit, along with that of two other distant objects — Sedna and 2012 VP113 — supports the idea of a planet about seven times bigger than Earth that has yet to be detected at the outer edge of our solar system.

2015 TG387 is seen here, centre, in this animation. The images were taken with the Subaru 8-metre telescope in Hawaii, about three hours apart. (Scott Sheppard)

The existence of a Planet X — more recently referred to as Planet 9 — has been theorized since the early 1800s. First, it was used to explain the perturbations in Uranus's orbit, which ended up being Neptune. Later, it was proposed to explain Neptune's orbit. Though Pluto was discovered in 1930, it wasn't large enough to account for any gravitational influences.

In the 1990s, more precise measurements of the outer planets were taken by the Voyager spacecraft and the existence of Planet X was put to rest.

But not quite.

Fast forward to 2003, when CalTech astronomer Mike Brown and colleagues discovered Sedna, a small object beyond Pluto (which eventually led to the "demotion" of Pluto from planet to dwarf planet). Sedna is a planet smaller than Mercury.

Questions were raised about the nature of its orbit that didn't seem to align with what astronomers observed with other planets.

Fortuitous discovery

2015 TG387 was first discovered in October, 2015, using the Subaru Telescope in Hawaii. The closest it gets to Earth is about 65 AU. While 2012 VP113 and Sedna come closer, they don't trek as far out.

The dwarf planet — only 300 kilometres in diameter — is believed to have an orbital period of 40,000 years. The orbit takes it beyond the Kuiper Belt, an area past Neptune where small icy worlds are found.

The fact that its orbit is so long means its discovery was somewhat fortuitous: with current instruments it would be detectable only one per cent of the time.

A comparison of 2015 TG387 at 65 AU with the solar system’s known planets. Saturn can be seen at 10 AU. Earth sits at 1 AU, as the measurement is defined as the distance between the sun and our home planet. (Roberto Molar Candanosa and Scott Sheppard, courtesy of Carnegie Institution for Science)

So why can astronomers find dwarf planets and not something that could be seven times larger than Earth?

"We think the big object is … between 500 to 1,000 astronomical units away. And we think it's at its most distant point on its elongated orbit, so it's very far away," said Scott Sheppard, co-author of the paper in The Astronomical Journal and astronomer at the Carnegie Institution for Science.

"Whereas these smaller objects have elongated orbits as well, but we're finding them at their closest approach to the sun, so they'd be brighter and be seen by reflected sunlight."

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The findings support a theory for a large planet whose orbit could be between 50,000 and 100,000 years.

"It's basically exactly what we would predict," said Brown, who was not involved in this paper. However, in 2016, he encouraged other astronomers to hunt for Planet 9.

"At this point, the Planet 9 hypothesis explains the alignment of these distant objects, these weird 90-degree twisted objects, a strange alignment of close objects," Brown said. "I must say it all fits together really well."

There are two leading theories for how this planet could have ended up out there: either it was captured by gravity as it passed our solar system or it formed in the inner solar system and was flung out when it came too close to Jupiter or Saturn. The latter theory draws more backing from astronomers.

"This is another piece of the puzzle," said study co-author Chad Trujillo. "It's not like we know exactly where this planet is. But the more objects we find that point to the planet, the more data we have, the more likely we are to find it."

And because it could be so distant, astronomers would have to know exactly where in the sky to search for it.

Confidence in Planet 9

The jury's still out on declaring with 100 per cent certainty that Planet 9 is lurking out there.

Brown, however, is confident that it exists and its discovery is on our doorstep.

"I am eternally optimistic," Brown said. "I would not be surprised if we find it this year."

This image compares the orbits of the new extreme dwarf planet 2015 TG387 with the orbits of other planets in the solar system. (Roberto Molar Candanosa and Scott Sheppard, courtesy of Carnegie Institution for Science)

Trujillo is a little more cautious, even when it comes to the chance it exists at all.

He believes that there's a 25 per cent chance that a giant planet can explain it all. He uses the example of famed French astronomical mathematician Urbain Le Verrier who, after the discovery of Uranus in 1781, theorized a giant planet could explain Uranus's perturbations. He was right (it was Neptune). But later, he theorized there was a planet within the orbit of Mercury (given the nickname Vulcan). He was wrong.

"He was right once and he was wrong once, and this was one of the greatest mathematicians of all time for astronomy, and he was only at 50 per cent," Trujillo said. "If we can do 25 per cent, that's good."

Sheppard seems to be in the middle of these dwarf planet hunters.

"I would say we're at the 80 to 90 per cent level; it's not a guaranteed thing," Sheppard said. "But there are a lot of things pointing to it existing out there.

"The hunt continues."