Beyond Pluto: The hunt for our solar system’s new ninth planet

“It’s more likely than unlikely to exist”

You’d think that if you found the first evidence that a planet larger than the Earth was lurking unseen in the furthest reaches of our solar system, it would be a big moment. It would make you one of only a small handful of people in all of history to have discovered such a thing.

But for astronomer Scott Sheppard of the Carnegie Institution for Science in Washington DC, it was a much quieter affair. “It wasn’t like there was a eureka moment,” he says. “The evidence just built up slowly.”

He’s a master of understatement. Ever since he and his collaborator Chad Trujillo of Northern Arizona University, first published their suspicions about the unseen planet in 2014, the evidence has only continued to grow. Yet when asked how convinced he is that the new world, which he calls Planet X (though many other astronomers call it Planet 9), is really out there, Sheppard will only say: “I think it’s more likely than unlikely to exist.”

As for the rest of the astronomical community, in most quarters there is a palpable excitement about finding this world. Much of this excitement centers on the opening of a giant new survey telescope named after Vera C Rubin, the astronomer who, in the 1970s, discovered some of the first evidence for dark matter.

Scheduled to begin its full survey of the sky in 2022, the Rubin observatory could find the planet outright or provide the clinching circumstantial evidence that it’s there.

Discovery of the planet would be a triumph, but also a disaster for existing theory about how the solar system was created.

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“It would change everything we thought we knew about planet formation,” says Sheppard, in another characteristic understatement. In truth, no one has a clue how such a large planet could form that far from the sun.

The distant solar system is a place of darkness and mystery. It encompasses an enormous volume of space that begins at the orbit of Neptune, some 30 times further from the sun than Earth, or 30 astronomical units (AU)the , and extends to about 100,000AU. That’s almost one-third of the distance from the sun to the next nearest star.

It was in the inner regions of this volume that American astronomer Clyde Tombaugh discovered Pluto in 1930. Although Pluto possessed just two-thirds of the diameter of our moon, it was originally classed as a planet.

Read more: The Guardian