Evidence for 'Planet Nine' lurking on the fringes of the Solar System is building. So why can't astronomers spot it?

Scientists are still searching for Planet Nine. (Science Photo Library: FreelanceImages)

A huge unknown lurks in the far reaches of our Solar System — something massive enough to pull distant space rocks into extraordinarily long, thin loops around the Sun.

At least, this is what US astronomer Michael Brown believes.

In 2016, he and a colleague at the California Institute of Technology (Caltech) proposed something almost unfathomable: a huge planet, up to 10 times heftier than Earth, way out on the edge of our Solar System.

They called it "Planet Nine".

Now, they've published a study, yet to be peer-reviewed, that simulated the movements of objects on the Solar System's fringes, and found that the chance of a Planet-Nine-type object not existing was just one in a million.

"I don't see how we can have a Solar System without Planet Nine," Professor Brown says.

"It just has to be out there."

The only problem? Scientists still can't find it.

How to find a planet

It's been almost 200 years since astronomers last discovered a planet in our Solar System.

The honour of predicting it goes to mid-19th-century astronomers Urbain Le Verrier and John Couch Adams.

They noticed that Uranus (which was only discovered about 60 years earlier) had irregularities in its orbit that could only be explained by the presence of another, more distant planet.

Le Verrier in Paris and Adams in Cambridge calculated coordinates for this hypothetical planet, and when German astronomer Johann Gottfried Galle and his student pointed a telescope to that part of the sky in 1846, there it was: Neptune, the Solar System's eighth planet.

"They found it in the first night they were looking," Professor Brown says.

"[The observation] didn't take eight years, it took one night! I'm very jealous."

While Planet Nine might be found the same way, the most tantalising hints it exists come not from ice giants like Uranus, but the motion of dwarf planets and asteroids that typically orbit much further out in our Solar System.

They're known as trans-Neptunian objects or TNOs.

More than 3,000 of these objects — including dwarf planet Pluto — have been found so far, although most haven't yet been named or thoroughly investigated.

We do know that none have the right characteristics to be that mysterious ninth planet — not even Eris, the largest known trans-Neptunian object.

What some trans-Neptunian objects do have, though, are extreme orbits. Instead of trundling around the Sun in something resembling a circle, they're slingshotted out towards interstellar space before making the journey back in towards Neptune.

In 2003, researchers including Professor Brown discovered 90377 Sedna, a trans-Neptunian dwarf planet which had an incredibly long, thin orbit.

Other trans-Neptunian objects began to be discovered by other teams. Sedna's odd orbit was not a one-off.

It's these extreme trans-Neptunian journeys that have astronomers stumped. Based on what scientists currently understand about the Solar System and the laws of physics, there's no reasonable explanation yet for why they're out there.

And it was the 2016 study by Professor Brown and his colleague Konstantin Batygin that rocketed a potential explanation — Planet Nine — into the mainstream.

The evidence for Planet Nine

At the core of the study were six weirdly orbiting extreme trans-Neptunian objects, including Sedna.

The pair calculated a 99.993 per cent probability that the objects' peculiar paths were not due to chance.

They suggested the reason might be a planet that is five to 10 times the mass of Earth that lies around 20 times the distance of Neptune to the Sun, and takes 10,000 years to complete a single orbit.

Some astronomers were immediately excited.

Brad Tucker, an astronomer at the Australian National University, quickly set up a citizen science project to try and capture the elusive planet using data from the SkyMapper telescope at the Siding Spring Observatory.

"We found lots of objects, but nothing conclusive. We kind of expected that, but we wanted to give it a good look," he says.

Other groups around the world have looked since.

"No-one has found really anything that could possibly be Planet Nine," Dr Tucker says.

With no Planet Nine forthcoming, the recent arXiv preprint study by Professor Brown, Dr Batygin and two other colleagues went down a different route.

They looked at 17 trans-Neptunian objects with less extreme orbits that didn't take them as far out in the Solar System.

"All the things in the outer part of the Solar System are also being pushed inward," Professor Brown says.

The team ran multiple simulations, reporting that one-in-a-million chance that something like Planet Nine is not out there.

But even Professor Brown is well aware this isn't enough.

"Have we found Planet Nine? No. If we had found Planet Nine it would not be on arXiv for people to randomly find. Everybody would know already," he says.

"Until somebody points a telescope at it and says 'It's that dot in the sky right there', you haven't found it."

While indirect evidence for Planet Nine has strengthened over the past eight years, there are also now fewer places it could be hiding.

Another study published earlier this year by Professor Brown and his colleagues noted that 78 per cent of the sky in which Planet Nine could potentially be has already been searched.

So, either Planet Nine is hiding in some hard-to-find location in that final 22 per cent of sky, or something else is causing these strange trans-Neptunian object orbital results.

Dr Tucker says many astronomers are sceptical Planet Nine exists.

"There's still plenty of people who doubt it," he says.

But eight years after Professor Brown and Dr Batygin's first Planet Nine study, "I think there is more acceptance that there's something going on".

Why not everyone agrees

For Katherine Brown, a theoretical physicist at Hamilton College in the US, extraordinary planets require extraordinary evidence.

"The idea that there could be, in this day and age, an undiscovered planet lurking in the outer reaches of the Solar System is fairly remarkable," she says.

"Remarkable ideas are bound to engender some level of controversy until they are proved or disproved."

Some researchers suggested that because the six trans-Neptunian objects in the 2016 study were picked from a variety of sky surveys, this may have biased the analysis. Even with more extreme trans-Neptunian objects discovered and reanalysed, some have remained unconvinced.

Other scientists have suggest that a small black hole might be behind the trans-Neptunian objects' orbits.

A study Hamilton College's Professor Brown published last year in The Astronomical Journal might contain the wildest theory of all — that instead of a planet out there, our entire understanding of gravity is wrong.

Orbits of objects on the edge of our Solar System aren't the only cosmic phenomenon scientists can't yet explain.

To keep from falling apart, galaxies require more mass than what scientists can see.

Most astrophysicists think that "dark matter" is the missing mass here, but a small minority, including Katherine Brown, believe that the answer could lie in an as-yet-undiscovered aspect of gravity.

According to her, the hypothesis, called "modified Newtonian dynamics" or MOND, can also explain the weird orbits of the six trans-Neptunian objects Michael Brown analysed in his 2016 Planet Nine study.

"We are at the exciting juncture where either a new planet might be discovered in the outer Solar System, or a new law of gravity," she says.

"Even if the trans-Neptunian object's orbital alignment turns out to be a spurious artefact, we will learn something about gravity."

New eyes on the sky

Caltech's Michael Brown appreciates why other scientists are trying to understand what could be behind the strange orbits.

"Until it's found, the majority will be sceptical I think," he says.

Those that are convinced Planet Nine is out there are waiting for the new Vera Rubin Observatory to come online in Chile early next year.

The telescope has an 8.4-metre mirror, which makes it the largest camera ever built for astronomy.

"It's going to be doing something called the Legacy Survey of Space and Time, which is a massive survey — taking images of the sky every single night," Swinburne University of Technology astrophysicist Sara Webb says.

"This type of survey is going to allow us to try and put a limit for how bright we think Planet Nine might be."

The survey will map the sky in the Southern Hemisphere for 10 years, and because it can map so much of the sky, it can track when objects change or move.

Being so much more powerful than those that came before it, it will be able to peer deeper into the edges of the Solar System, and pick up the tiny amount of sunlight reflected from Planet Nine's surface.

If that doesn't work, Professor Michael Brown suggests ditching optical telescopes all together.

"If Vera Rubin doesn't find it by reflected sunlight, the next best thing is to find it not as reflected sunlight, but by using radio telescopes," he says.

"They're not designed to look at little planets; they're designed to look at the whole sky at once. It'll take a while for the telescopes to be able to see that this planet has moved from one place to the other, so it'll be a couple of years of those surveys before we know it's there.

"But it's going to be very difficult to hide from those radio telescopes."

The Caltech astronomer wants nothing more than to be able to stop searching for this planet. Eight years is a long time to be looking for something that might not exist.

"It feels like an eternity," he says.

But he's not ready to give up yet. For him, finding the planet would allow the truly fun part to begin.

"We have spent centuries studying the giant planets that we have. Imagine we get a new one all of a sudden. All the things we've done for studying the giant planets, we get to do all over again for the first time," he says.

"We will study it with every telescope you can imagine — on the Earth and off the Earth. I am certain that we will very quickly work on trying to get a spacecraft out there.

"It's going to be extremely exciting — as soon as we find it."