Astronomers' hunt for elusive "Planet Nine" takes surprising twist

It's an evocative idea that has long bedeviled scientists: a huge and mysterious planet is lurking in the darkness at the edge of our solar system, evading all our efforts to spot it. Some astronomers say the strange, clustered orbits of icy rocks beyond Neptune indicate that something big is out there, which they have dubbed "Planet Nine."

Now, a U.S.-based trio hunting the elusive world has instead stumbled on what appears to be a new dwarf planet in the solar system's outer reaches. And the existence of the new kid on the block could challenge the Planet Nine theory, the researchers have calculated.

Named 2017 OF201, the new object is roughly 430 miles across, according to a preprint study, which has not been peer-reviewed and was published online last week. That makes it three times smaller than Pluto.

But that is still big enough to be considered a dwarf planet, lead study author Sihao Cheng of New Jersey's Institute for Advanced Study told AFP.

"Lucky" discovery

The object is currently three times farther away from Earth than Neptune. Its extremely elongated orbit swings out more than 1,600 times the distance between the Earth and the Sun, taking it into the ring of icy rocks around the solar system called the Oort cloud. It goes so far out, it could have passed by stars other than our sun in the past, Cheng said.

During its 25,000-year orbit, the object is only close enough to Earth to be observed around 0.5 percent of the time, which is roughly a century.

"It's already getting fainter and fainter," Cheng said.

The discovery suggests "there are many hundreds of similar things on similar orbits" in the Kuiper Belt beyond Neptune, Cheng said.

After taking a risk spending more than half a year sorting through a difficult dataset in search of Planet Nine, Cheng said he was "lucky" to have found anything at all.

The researchers are requesting time to point the James Webb, Hubble and ALMA telescopes at their discovery.

But Sam Deen, a 23-year-old amateur astronomer from California, has already been able to track the dwarf planet candidate through old datasets.

"OF201 is, in my opinion, probably one of the most interesting discoveries in the outer solar system in the last decade," Deen told AFP.

"See almost to the edge of the universe"

The icy rocks discovered in the Kuiper belt tend to have a clustered orbit going in a particular direction. Two decades ago, astronomers proposed this was due to the gravitational pull of a world up to 10 times larger than Earth, naming it Planet Nine and kicking off a debate that has rumbled since.

It is also sometimes called Planet X, a name proposed for a hypothetical world beyond Neptune more than a century ago.

Back in 1930, astronomers were searching for Planet X when they discovered Pluto, which became our solar system's ninth planet. But Pluto turned out to be too tiny — it is smaller than the moon —  and was demoted to dwarf planet status in 2006.

There are now four other officially recognized dwarf planets, and Cheng believes 2017 OF201 could join their ranks.

When the researchers modeled its orbit, they found it did not follow the clustered trend of similar objects. This could pose a problem for the Planet Nine theory, but Cheng emphasized more data is needed.

Samantha Lawler of Canada's University of Regina told AFP that this "great discovery" and others like it mean that "the original argument for Planet Nine is getting weaker and weaker."

The Vera Rubin Observatory, which is scheduled to go online in Chile this year, is expected to shed light on this mystery, one way or another.

Deen said it was discouraging that no sign of Planet Nine has been found so far, but with Vera Rubin "on the horizon I don't think we'll have to wonder about its existence for much longer."

For Cheng, he still hopes that this huge planet is out there somewhere.

"We're in an era when big telescopes can see almost to the edge of the universe," he said.

But what is in our "backyard" still largely remains unknown, he added.

Cheng made the discovery alongside colleagues Jiaxuan Li and Eritas Yang from Princeton University, and Li said the newly found object demonstrates the power of open science.

"All the data we used to identify and characterize this object are archival data that are available to anyone, not only professional astronomers," Li said in a statement. "This means that groundbreaking discoveries aren't limited to those with access to the world's largest telescopes. Any researcher, student, or even citizen scientist with the right tools and knowledge could have made this discovery, highlighting the value of sharing scientific resources."

The new discovery was officially announced by the International Astronomical Union's Minor Planet Center on May 21.

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