Sharon Forbes spends her evenings on the sofa, searching for another world. At her house in Wiltshire, with the television on in the background, the 63-year-old joins thousands of citizen-scientists in an astronomical treasure hunt that stretches back more than a century.
Every night, she scours images of the sky on her iPad, searching for a giant planet hidden at the edge of our own solar system. It’s a dark and icy world - ten times heavier than our own – with an iron core and an atmosphere of hydrogen and helium.
This impossibly distant wanderer could solve long-standing mysteries of our solar system, and explain strange anomalies in the orbits of objects circling far beyond Neptune. Or it could be an illusion – mere fuel for the fires of conspiracy theorists, who claim there’s been a government cover-up and the planet is on a collision course with Earth.
Astronomers are deeply divided, but intent on finding the truth. They’re using the world’s largest telescopes and most powerful supercomputers, and enlisting the help of thousands of amateurs like Forbes, who plays her part in this epic, astronomical search in between episodes of Love Island. Together, they will either pinpoint the location of this mysterious world and give the solar system a ninth planet, or rule out its existence once and for all.
The hunt can quickly spiral into obsession. Forbes first got involved about 18 months ago, after seeing a television documentary inviting volunteers to join the Backyard Worlds citizen science project. Soon, she was spending six or seven hours a day flicking through faint pictures of the night sky, scanning each one for about thirty seconds before moving on to the next. “It’s addictive,” she says. “I just find it so fascinating.”
She’s not the first to feel the pull of the unknown. In the early twentieth century, Percival Lowell – a wealthy businessman, writer and astronomer who built his own observatory in Flagstaff, Arizona – set out to solve the mystery of why Uranus and Neptune did not seem to be following their predicted orbits.
For Lowell, it was a chance to regain some scientific credibility. He had spent much of the previous two decades claiming to have seen a complex network of canals on the surface of Mars, which he thought had been dug by an advanced civilisation. It turned out to be an optical illusion.
By 1906, Lowell was convinced that the errant paths of the two ice giants could be explained by the gravitational pull of another large planet – seven times bigger than Earth - hiding somewhere beyond Neptune. He called it Planet X, and it consumed the last decade of his life.
Finding a planet is a search for movement. The ancient Greeks called them ‘wandering stars’, and the basic premise of discovering distant worlds hasn’t changed much since. Astronomers find planets by looking for points of light in the sky that don’t stay in the same place from night to night. Today they use computer software to automatically compare images taken hours, days, or weeks apart, but in the past it was a much more laborious process.
In 1929, Clyde Tombaugh – a 22-year-old who built homemade telescopes at his parents’ Kansas farm – arrived at the Lowell Observatory in Arizona and was introduced to the blink comparator. This bulky contraption allowed the user to click back and forth between two images of the night sky taken at different times. Distant stars and galaxies stay static, but anything that has moved appears to jump back and forth.
Tombaugh took up the search for Planet X, which had eluded Lowell until the end of his life (he died of a stroke in 1916, and a friend said the failure to find the planet had “virtually killed him”). He would sit in a darkened room at the observatory, staring into the eyepiece for hours at a time, clicking back and forth between images.
On February 18, 1930 – after about 7,000 hours of this over the course of 10 months – that rhythmic clicking suddenly stopped. Tombaugh had found something in images taken a few weeks earlier – a faint moving speck in a patch of sky close to Lowell’s predicted location for Planet X.
Fourteen years after his death, it seemed as if Lowell had been vindicated – his Planet X had been found. It was christened Pluto – at least partly because the first two letters matched Lowell’s initials. But there was a problem.
It slowly became apparent that Pluto could not be the planet that Lowell had envisaged. It is 500 times smaller than Earth – nowhere near big enough to pull Uranus and Neptune out of line. The killer blow arrived in 1989, when the Voyager II probe flew past Neptune and discovered that the eighth planet was one per cent lighter than previously thought.
When astronomers took the new figure into account, the unexplained oddities in the orbit of Uranus simply disappeared. Lowell’s theory was based on a measurement error. Planet X was dead. But then, in 2003, a trio of American astronomers made a discovery that changed everything.
The discovery of Pluto has to go down as one of science’s great flukes. Today, we know the distant clump of rock is nothing special. It’s just one of thousands of trans-Neptunian objects, or TNOs, orbiting our Sun in the vast space beyond Neptune, mostly in a band of icy shrapnel called the Kuiper Belt.
It was a massive coincidence that there happened to be a visible object in the area Lowell had earmarked for Planet X, and that Tombaugh was dedicated and diligent enough to spot it. This is neatly illustrated by the fact that it took until 1992 for the second TNO to be spotted. Now there are more than 2,000 that we know about, and the list is getting longer all the time.
These objects are thought to be debris leftover after the formation of the planets. The solar system started life about 4.7 billion years ago as a spinning disk of gas and dust, whirling around the young Sun. Over time, it coagulated into planets, still on the same plane as that original disk.
But TNOs like Pluto aren’t on that plane – they tend to orbit the Sun at inclined angles relative to the major planets. To explain this, astronomers developed the Nice model, which argues that the solar system used to be much more compact, and that the giant planets migrated outwards over time, dragging debris with them. “As the planets moved out to their current positions, they scattered objects outwards,” explains Susanne Pfalzner of the Max Planck Institute for Radio Astronomy in Bonn, Germany. “These are the Kuiper Belt objects like Pluto and the others.”
Then, in November 2003, a telescope at the Palomar Observatory near San Diego spotted an object that didn’t fit into the Nice model. Sedna, a reddish dwarf planet about 1,000 kilometres wide, is on an orbit so far removed from the Sun that it couldn’t have been scattered by the migration of the giant planets.
In space, distances are measured using astronomical units. One AU is equal to the distance between Earth and the Sun – about 93 million miles. Neptune orbits at about 30 AU, the furthest point in Pluto’s orbit is 49 AU. Then there’s Sedna.. Its closest approach to the Sun it is twice as far away as Neptune, and at its furthest point it is a staggering 936 AU – 84 billion miles - away. It takes 11,400 years to complete one circuit of the Sun.
Over the following decade, as astronomers found more Sedna-like objects with extremely distant orbits, they noticed something strange. In 2014, Chad Trujillo and Scott Sheppard pointed out that twelve TNOs on the most far-flung orbits seemed to be clustered together. Not only were they angled together relative to the solar plane, they were also all grouped on one side of the Sun.
Two astronomers at Caltech, Michael Brown and Konstantin Batygin, decided to investigate. Brown had been part of the team that discovered Sedna (with Trujillo and David Rabinowitz) and had gained some infamy beyond planetary circles too. It was his discovery of the dwarf planet Eris, the biggest TNO identified to date, which served as the final nail in the coffin for Pluto’s full planetary status. Over a few days of tense argument in 2006, Brown went from being the discoverer of a potential tenth planet to the destroyer of the ninth, something reflected by his Twitter handle (@PlutoKiller), and his memoir, How I Killed Pluto – And Why It Had it Coming.
Batygin is in his early thirties. He was born into science-minded family in the last days of the Soviet Union – his father was a physicist who worked on particle accelerators – and attended primary school in Japan before moving to California at the age of 13. He chose to attend university in nearby Santa Cruz so he could keep singing and playing guitar in his rock band, The Seventh Season, and fell into the field almost by chance when he went to register for an engineering major and a stranger told him astrophysics would be “dope”.
Today, solar system research is as likely to be conducted on a supercomputer as a telescope, and Batygin and Brown spent two years scribbling equations on chalkboards and trying different models to explain the weird clustering of these extreme TNOs. Eventually, they settled on an unexpected answer – something seven to ten times bigger than Earth, circling the Sun on a vast 15,000-year orbit that takes it 40 times further than Neptune. Its name? Planet Nine.
In 2016, Brown and Batygin published a paper confirming Trujillo and Sheppard’s observations, and identifying six extreme TNOs whose orbits could be explained by the existence of a giant planet in the outer solar system. The idea to introduce a ninth planet into their simulations was, Batygin admits, something of a last resort. They had exhausted all the other options. “The first year that we worked on this, the notion that there was a planet seemed laughable to us,” he says. But when they introduced Planet Nine to their model, everything clicked.
In the outer solar system, things get weird. There are TNOs circling the Sun at a right angle to the rest of the Kuiper Belt, and even objects that seem to be orbiting in the opposite direction to everything else. “They’re driving the wrong way on a one-way street,” says Batygin. It’s hard to account for these anomalies, but “Planet Nine explains them beautifully,” he says. Because Planet Nine is so far away, it pulls these objects into an extremely elliptical orbit – an oval that is so stretched out that it starts to look more like a straight line.
When this happens, it’s like pulling a ball on the end of a rubber band and then releasing it. “The orbits naturally forget about the fact that they were in some plane,” says Batygin. When their orbit brings them back towards the Sun, they effectively return in a random direction, like that rubber ball snapping back towards the centre.
Batygin and Brown suggest that Planet Nine coalesced from the same spinning disc as the rest of the solar system, but was flung into a distant, tilted orbit soon afterwards by a close encounter with Jupiter or Saturn. This could also explain one of the solar system’s enduring mysteries – why the orbits of the eight major planets are tilted six degrees off centre relative to the Sun. The gravitational pull of Planet Nine is making their orbits wobble, says Batygin, like a finger nudging a spinning top.
Another quirk of our solar system is that it has a relatively equal split of relatively small planets – Mercury, Venus, Earth and Mars – and relatively big planets – Jupiter, Saturn, Uranus and Neptune, but nothing in the middle. Planets about five to ten times the mass of Earth are “exceedingly common around Sun-like stars,” says Batygin. “It’s weird that our solar system doesn’t have one.”
The publication of Brown and Batygin’s 2016 paper sounded the horn on the hunt for our solar system’s missing super-Earth. Scores of scientists from disparate fields have joined the search, using advanced computing techniques and the world’s most powerful telescopes. At the same time, theoretical work has been trying to narrow down its location. “Things are progressing very well,” says Batygin, who has a new Planet Nine paper coming out soon based on seven months of computer simulations. “On the theoretical side, the model is holding up beautifully and we’re slowly chipping away at the observational side.”
You might ask how, given that we regularly find evidence of planets orbiting other stars in our galaxy, we haven’t been able to confirm the location of one in our own backyard. But, at the limits of our Sun’s influence, it’s almost impossibly dark, rendering Planet Nine almost invisible.
“It’s right on the edge of what’s possible with today’s telescopes,” says Batygin. “You have to have the stars align – in a metaphorical sense – to get the data. No turbulence in the upper atmosphere, really good weather, no Moon.”
In order for us to actually see a planet in our solar system, light from the Sun has to reach it, bounce back, and then find its way to a telescope on Earth. As a result, a planet twice as far away is sixteen times fainter. Planet Nine – depending on where it is in its orbit – is between two and twelve times more distant than the farthest known object in the solar system.
“It’s a huge, huge leap in distance beyond where almost everything in the solar system is,” says David Gerdes, an astrophysicist at the University of Michigan, who’s amongst the clutch of scientists around the world hunting for Planet Nine. His work focuses on images captured by the Dark Energy Survey at the Cerro Tololo observatory in northern Chile. The project covers an eighth of the night sky, and has been running for five years – it started a final six-month stint earlier this month. The survey’s intended purpose is to explore the expansion rate of the universe, but Gerdes has been co-opting the data to study the solar system as well.
Within minutes of being taken in Chile, the raw image data is sent to the National Centre for Supercomputing Applications at the University of Illinois for some intense processing. “Raw telescope data is crap,” says Gerdes, bluntly. “It’s full of detector effects and artefacts. A lot of processing needs to be done before the data is useful for anything.”
Once the data is processed, Gerdes and his colleagues follow a similar process to most astronomical surveys. Like Tombaugh, they’re looking for objects that appear in a patch of sky on one night, but not the next. Software speeds up the process, automatically comparing any objects identified with a catalogue of known planets and stars, and helping to make sense of the rest by trying to fit them to a potential orbit. They could be planets, comets, or stars exploding in a distant galaxy.
Surhud More is using a slightly different technique. Instead of comparing two images, he uses software to subtract one from the other in a technique known as difference imaging. Once you’ve controlled for variations in the images caused by the weather and other variables – not an easy task – anything you’ve got left should be something that only appears in one of the images. “If you want to find a needle in a haystack, it’s better to have less hay,” says More, who has been working with images taken by the Japanese-funded Subaru telescope in Hawaii, and recently moved to the Inter-University Centre for Astronomy and Astrophysics in Pune, India.
Difference imaging could prove crucial if Planet Nine ends up being parked in an area of the sky like the galactic plane or the Milky Way, where there are a lot of background objects. If Planet Nine is hiding in front of one of these objects it could be very difficult to spot with a traditional catalogue search. “Difference imaging will subtract all these galaxies and you can still see this object,” says More.
But computers aren’t perfect and algorithms are easily fooled by random noise and data defects. That’s one of the reasons Brad Tucker and the Backyard Worlds project turned to citizen scientists. “Humans are very good at spotting things which can move,” says Tucker, an astrophysicist at the Australian National University who spearheaded one of the searches that Sharon Forbes took part in. Each image is tinted a different colour, and then stacked on top of each other. “If there’s something that moves you will see these coloured dots moving in a line that humans can pick out right away,” Tucker explains.
A clutch of different searches have used the power of the crowd to comb through old data in the hope of spotting Planet Nine. Last year, a one search identified four potential objects of interest that are being investigated further.
“This is an interesting way of making science accessible for everyone,” Tucker says. “Everyone on Earth has played some form of spot the difference.” Right now, almost 50,000 users are analysing 300,000 images from Nasa’s Wide-field Infrared Survey Explorer (WISE) telescope. “You don’t have to be a professor or a scientist,” says Forbes, who has reined in her addiction to about half an hour a night. “It could be someone like me sitting at home who sees something that leads to the discovery of the ninth planet.”
Gil Holder is a cosmologist at the University of Illinois who uses sensitive radio telescopes to glean clues about the origins and evolution of the universe from the cosmic microwave background. But when he heard about Planet Nine, he realised that if it was out there, the equipment he was using should be able to detect its faint heat signature. “I got out a piece of paper and sketched some numbers,” he says. “In terms of signal to noise, it’s so easy to see this thing. How have we not seen it yet?”
Potentially, it’s because radio telescopes have not generally been pointing in the right direction. Because they’re interested in the Big Bang, cosmologists tend to aim their equipment away from any distractions in the foreground – they only ever look at planets when they’re calibrating their equipment. Planet Nine is changing that – and it won’t be long before the entire sky has been covered by radio telescopes. “It’s hard for me to think of a place where it could hide,” says Holder. “It might not be easy, but we’ll find it eventually.”
Holder isn’t the only scientist from a different field who has been drawn into the search. Makan Mohageg, a physicist at Nasa’s Jet Propulsion Laboratory has suggested using super-cooled atoms to build a network of incredibly precise accelerometers to triangulate the position of Planet Nine. “All of the objects in space exert tidal forces upon each other – and so you would build an accelerometer that’s sensitive enough to perceive the tidal force of that object,” he says.
Gerdes, More and Tucker all started out studying other things before being drawn into the search for Planet Nine. It’s an inviting concept, and Brown and Batygin’s paper sparked a wave of interest that continues to grow. But it also exposed them to the full force of the online conspiracy machine.
The idea that there’s a hidden planet on a collision course with Earth has been folded into the doomsday literature since the mid-1990s. It’s remarkably persistent: Planet X has been due to destroy the world in 1997, 2003, 2012 and 2017. The theory – also known as the Nibiru Cataclysm – has a habit of attaching itself to any scientific work on large objects within our solar system, and so when Brown and Batygin published their evidence for Planet Nine, they inadvertently opened the floodgates.
“I’ve had far too many encounters with that group,” says Batygin, who still gets worried messages saying things like, “Please tell me if the Earth is going to explode in three days, because I saw it on the internet.”
When Brad Tucker started working on the Backyard Worlds: Planet 9 project, he got about an email an hour from people who had been seduced by the conspiracy theorists toolbox: news footage and interviews intercut with stock footage of explosions. “I replied to every single one,” he says. “I was explaining to them that one of the problems with the Nibiru and Planet X is that the theory would always change, and it would never be able to die.” There’s a reason why Batygin and Brown are very careful never to refer to their proposed object as Planet X.
The question of names is a heated one in planetary science. Even the working title Planet Nine is contentious – those who dispute Pluto’s planetary demotion insist on referring to this new potential object as Planet X.
In their informal discussions, Batygin and Brown call it ‘Fatty’. Most of the people interviewed for this article had thoughts, ranging from the light-hearted – Jeff, PlanetyMcPlanetFace – to the mythological. Mohageg, whose technique for finding Planet Nine would rely on precision timing, suggests a horological name: Chronos. If it’s found using images from the Siding Spring observatory in Australia that he works with, Tucker would name it after a local Aboriginal deity. Gerdes suggests something similar – he would call it Nanabozho, after a native American trickster god. “If Planet Nine is there, it’s hard to find, and you have to be clever and use lots of tricks to notice its fingerprint,” he explains.
More than a thousand people signed a petition asking Batygin and Brown to name it after David Bowie, who passed away shortly after their initial paper was published. “I’ve really kind of grown to like it,” says Batygin. “And if it has moons we can have Ziggy Stardust and Major Tom. It just makes sense.”
Of course, talk of names and moons is extremely premature, and the longer we go without actually seeing Planet Nine, the more the doubts begin to grow. “I am more sceptical than I was a year ago,” says Gerdes. “Certainly the fact that it hasn’t turned up yet isn’t for a lack of people trying.”
There are other ways of explaining the strange clustering of the orbits of extreme TNOs. As Michele Bannister, of Queen’s University Belfast puts it, something is “picking these things up by their bootstraps and yanking them outwards,” but it doesn’t necessarily have to be a planet.
About four billion years ago, during a period known as the Late Heavy Bombardment, something sent a huge number of comets crashing through the inner solar system. They collided with the young planets, and left our Moon pitted and scarred.
The driving force behind this assault is thought to have been a stellar fly-by – a close encounter between our Sun and another star. “Most stars don’t form in isolation, they form in groups,” explains Pfalzner, who has published work on this hypothesis. When the Sun was formed, the stars were much more densely packed, so close encounters with other stars were much more common than they are today.
The gravitational pull of such an encounter could have pulled some TNOs out of line with the rest of the solar system into the orbits we see today. Or, our Sun could have plucked them from the grasp of another star – there’s some evidence that objects in more inclined orbits are composed of slightly different materials to those closer to the solar plane, which would support that theory.
Bannister has an alternative explanation – the galactic tide. This is a constant outwards tug on everything in the solar system, from the combined mass of all the other stars in the galaxy. Close to the Sun, it’s too weak to have an effect, but as you get further out it could start to pull TNOs into more and more extreme orbits.
She suggests that the orbit of TNOs randomly walk in and out over time through a process called diffusion – if Neptune happens to be nearby when these objects make their closest approach to the Sun, its gravity can give them a little bump – just enough to hook them into the galactic tide and pull them away.
Ann-Marie Madigan, an Irish astronomer at the University of Colorado, doesn’t think you need any external explanation at all. She says existing research hasn’t paid enough attention to the collective gravity of TNOs, and how these small bodies interact with each other over millions of years. “The gravitational interactions they have with each other aren’t just random,” she says. “The fact that they repeat the same orbits over and over means that gravitational forces build up to become very strong over long timescales.”
Her models show that these self-gravitational forces alone can explain the existence of extreme TNOs. “They change the shape and orientation of each other’s orbits,” she says. “They make each other have more circular orbits, and they also incline each other's orbits off the plane.”
What Madigan and Bannister’s theories can’t explain, however, is why the orbits of extreme TNOs are so clustered together. But some are starting to have their doubts about that too.
“The sky is enormous, and resources on telescopes are limited,” says Cory Shankman, who was previously an astronomy PhD candidate at the University of Victoria in British Columbia, Canada. “There are lots of different biases that come into play here.” We don’t search the sky evenly, in other words, which means we’re more likely to spot some objects than others. For example, it’s much easier to see things when they’re near their perihelion, their closest approach to the Sun.
There’s also a seasonal pattern to when the weather is good enough to spot the faintest objects – long, clear nights happen more often at certain times of year. And then there are bright areas like the Milky Way that are typically avoided when looking for new TNOs, because it’s so hard to see anything. “That can create a strong bias for where we’re able to detect these objects on really large orbits,” says Shankman.
Bannister and Shankman are among those who worked on the Outer Solar Systems Origins Survey (OSSOS), which tried to measure and counteract these biases. Starting in 2013, it surveyed the sky while also keeping careful track of where it was looking, when it was looking, and how deep it was able to go at each point.
OSSOS identified nine objects with same kinds of orbits as the ones originally flagged up by Batygin and Brown. But when Shankman ran a simulation using this independent, controlled dataset, he found no evidence of any clustering. The distribution of these objects was entirely consistent with a randomly distributed solar system. Like Lowell’s Planet X, Planet Nine could be a statistical quirk – a solution to a puzzle that doesn’t actually exist.
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Hunting season for Planet Nine starts around late September each year, as the northern hemisphere nights grow long. There is a ten per cent chance of finding it each year, according to Batygin, which means we should have a definitive answer within the next decade.
A project under construction in Chile should help. When it opens in 2023, the Large Synoptic Survey Telescope will scan the sky with an 8.4 metre mirror, connected to a 3.2 gigapixel sensor – the largest digital camera ever built. “It will be a defining word in the search for Planet Nine,” says Batygin.
By adding to the number of known objects in the outer solar system, LSST will either help pinpoint the location of Planet Nine, or consign it to history. “This is not something that you can just speculate about forever,” says Batygin – although conspiracy theorists might disagree. “It’s either there or it’s not there, and time will simply tell.”
Whether Planet Nine exists or not, it’s an idea that has captured the imagination, just as Planet X did a century ago. We know more than ever about the farthest reaches of the universe, but a hidden planet in our solar system somehow feels more tangible – almost within reach. “For me, what’s alluring about is realising there are still unanswered questions and mysteries in our own backyard,” says Gerdes.
This captivating, elusive world has driven scientific progress for a century, and inspired thousands to turn their attentions towards the stars. “The solar system is our cosmic home, and understanding it is just an extension of humankind’s exploratory nature,” says Batygin. “When we were in caves we explored what is beyond the hill, and when we built ships we explored what is beyond the ocean. This continued sense of exploration is now pushing us to understand what is beyond our planet. It sparks that natural curiosity that we all have.”
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This article was originally published by WIRED UK
