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Planet 9: “You kill a planet, then you go out and find a new one”

An artist’s impression of an iceball planet. A study published in August suggests that an unknown planet several times the size of Earth could be hiding in our solar system

An artist’s impression of an iceball planet. A study published in August suggests that an unknown planet several times the size of Earth could be hiding in our solar system. Photo: NASA/JPL-Caltech

After more than two decades deciphering some of the theoretical mysteries of the solar system, Professor Mike Brown is finally about to experience something tangible that had eluded him to date: an actual rocket being fired into space. “It’s my first launch, which is hard to believe,” says Brown from the house he has rented for the occasion near the Kennedy Space Center in Florida. It is 15th October 2021 and in a few hours Nasa will launch Lucy, a “robotic archaeologist” atop an Atlas V rocket, to examine the Trojan asteroids that surround Jupiter. “My father worked on the Saturn V rocket. He went to launches,” says Brown. “My stepfather worked on the space shuttle. He went to launches. I’m so excited to see my first one.”

Lucy will travel four billion miles, on a 12-year journey, in the hope of learning more about a collection of asteroids about which we know little. It’s a subject that has long fascinated Brown, an astronomer from the California Institute of Technology, who has spent most of his academic career trying to work out what is flying around in the darkest and coldest corners of the solar system. He may now have uncovered something that will alter our perception of it forever.

On 26th August, Brown and his research partner Konstantin Batygin released a paper that identifies the area of the solar system in which they believe we will find a large, hidden planet, which they have nicknamed ‘Planet Nine’. If found, Planet Nine would be one of modern astronomy’s greatest discoveries. Only three planets have been identified in our solar system since antiquity. Brown and Batygin first floated their theory in 2016 but it’s only now that they have been able to pinpoint where the new addition might be. “We’ve made a kind of treasure map,” says Brown.

“We know that it is about six times the mass of Earth,” continues Brown. Despite this, nobody can find Planet Nine. It has remained elusive after a five-year search and a thousands-strong team of citizen astronomers hunting for it, leading many astronomers to doubt its existence. Yet Brown is convinced. “It’s not that we can’t observe it, it’s that we haven’t found it yet,” he says. “If I hand you a grain of sand, it’s small but you can see it. It’s a very distinct grain of sand. And then I go and throw that grain of sand on a beach. It’s not hard to see. It’s just hard to find.”

You may, of course, remember being taught at school that the solar system already had nine planets. That officially changed in 2006 when Brown played a leading role in the creation of a new definition for what is and isn’t a planet. Pluto was demoted. The man who may have helped find a massive, hidden ninth planet at the edge of our solar system is, ironically, also the man who killed a ninth planet in the first place.

In August 2006 the International Astronomical Union [IAU] gathered in Prague for a now-notorious two-week conference. Formed in 1919, the IAU is responsible, among other things, for naming celestial bodies and standardising astronomical terms. But there was no official definition for a planet, given that they were so rarely discovered: US astronomer Clyde Tombaugh found the solar system’s last planetary addition, Pluto, in 1930.

In the early 2000s, Brown had been tracking objects that moved unusually in the outer reaches of our solar system, far beyond Neptune. Among these ‘trans-Neptunian objects’ he found three worlds comparable in size to Pluto: Eris, Makemake and Haumea (although a rival team of astronomers also claims Haumea’s discovery). Brown’s finding meant the IAU would have to define what a planet was and whether these new discoveries qualified.

This put Brown in an extraordinary position. He and many others believed that Pluto wasn’t a planet at all but just one of thousands of icy rocks flying around in the Kuiper belt, a swathe of planetary debris ejected to the fringes of our solar system during its formation.  Yet if the IAU agreed on a definition that meant Pluto remained a planet, then Eris, and likely Haumea and Makemake were planets too. And that would make Professor Brown, on paper, the greatest discoverer of planets in history. His name would sit alongside the German-British astronomer William Herschel, who in 1781 spotted Uranus, the first planet discovered since ancient times. Or Urbain Le Verrier, who in 1846 correctly calculated, without observing it directly, both the existence and position of Neptune.

Such an accolade didn’t sit right with Brown. Since he believed Pluto wasn’t a planet, by extension none of his trans-Neptunian discoveries was either. “One of those two [Herschel or Le Verrier] gets the crown [of greatest planet discoverer]. I do not get that crown, that’s ridiculous,” he says. And so, on the brink of becoming a household name, Brown went to ground. “I decided I didn’t really want to go to this meeting because it’s just awkward. So I went and hid on an island, up in the north-western part of the United States, hoping that no one would find me.”

American astronomer Clyde Tombaugh discovered the solar system’s last planetary addition, Pluto, in 1930

IAU meetings were normally of interest only to a small group of specialists, but the prospect of Pluto’s planetary status being downgraded attracted the world’s media. On 24th August, after days of heated debate, 424 IAU astronomers agreed to a new three-part definition for planets in our solar system. First, a planet has to orbit the sun. Second, it must be round. And third, it must have “cleared its neighbourhood”. In other words, any prospective planet must become gravitationally dominant in its orbit. Pluto failed the third test and joined a newly created class of ‘dwarf planets’, alongside Eris, and later Haumea and Makemake. “Pluto is dead!” declared Brown after returning home, having come to terms with being the public face of Pluto’s demotion. There were now, officially, eight planets in the solar system, and Brown’s conscience was clear. “Pluto should never have been called a planet to begin with,” he recalls of that vote 15 years ago. “We finally admitted that we screwed up in 1930 and now we’ve set the solar system straight again.”

Clyde W. Tombaugh,. discoverer of the planet Pluto

Clyde W. Tombaugh discovered the planet Pluto. Photo: Patrick T Fallon for The Washington Post via Getty Images

There was an angry response from some scientists to the new rules. “This definition stinks,” said Alan Stern, the leader of Nasa’s New Horizons probe to Pluto, which passed by the now dwarf planet in 2015 with some of Clyde Tombaugh’s ashes onboard. Others complained that the resolution lacked legitimacy as less than five percent of the IAU’s members had voted. Pluto’s advocates continue to commiserate on ‘Pluto Demoted Day’ every 24th August. While Brown expected some resistance from the science community, he wasn’t prepared for the furious reaction from the public who had grown up with Pluto as the most mysterious and enigmatic planet in the solar system, as well as the name of Mickey Mouse’s dog. (Both were named in 1930 but Disney says it can find no official link between the two.)

As the key proponent of the move to reclassify Pluto, Brown was getting dozens of angry emails a day at one point, as well as irate phone calls at all hours. “I loved Pluto as a kid. It had this appeal as an underdog, the cold place at the very edge, the oddball that doesn’t quite fit in,” he says. “And the fact that it’s associated with this cartoon dog. People just kind of love it.” For a while Brown revelled in the infamy. His Twitter handle is @plutokiller. His memoir is called How I Killed Pluto and Why It Had It Coming. But the constant stream of negative comments eventually got to him. “I was a little bit down for a couple of years,” he admits. “Then, seven or eight years ago, my daughter said: ‘Dad, do you want to know how to get everyone to stop hating you?’ And I was like, first off, ‘I object to the premise of this question. Second, yes, please let me know!’ She said you should go find a new planet, you should name it Pluto, and then Pluto can be a planet again.”

Pluto captured by the Long Range Reconnaissance Imager aboard Nasa’s New Horizons spacecraft as it passed the dwarf planet on 13th July 2015

Pluto captured by the Long Range Reconnaissance Imager aboard Nasa’s New Horizons spacecraft as it passed the dwarf planet on 13th July 2015. Photo: NASA

By pure chance Brown had been working on a problem that one of his former PhD students had flagged up a few years earlier: trying to explain why a cluster of objects in the outer solar system orbit the sun at different angles to everything else. The particular focus was on a handful of rocky objects in the Kuiper belt that all have the same oblong orbits set along an incline in relation to the solar system’s orbital plane. (All of the planets of the solar system orbit the sun in nearly the same plane, since all were created from the same disc of dust that swirled around the sun 4.6 billion years ago). When Brown and Batygin looked deeper into the data they theorised a large planet’s gravitational pull must be responsible. Brown didn’t take the planetary explanation seriously at first, because it would have been too absurd given his history. “So many people have proposed planets beyond Neptune over the past 170 years and everybody’s been wrong, every single time,” says Brown.  “And usually the people who are wrong are both wrong and ridiculous.”

One of those people was American tycoon Percival Lowell. Although Urbain Le Verrier correctly identified where to find Neptune in 1846 there were still slight anomalies in the planet’s orbit, and that of Uranus. Speculation grew that another body the size of Neptune lurked beyond it, and in 1894 Lowell funded the construction of the Lowell Observatory in Arizona, which still stands today, and set out to find what he called ‘Planet X’. A cultured and intellectually hyperactive man, Lowell was used to going against the scientific grain: his observatory was partly built to help him prove that Mars was covered in a network of deep canals built by an advanced alien civilisation, a theory that inspired HG Wells’ War of the Worlds but was debunked by the astronomical elite. Lowell spent until his death in 1916 looking for life on Mars and Planet X – but found neither.

It was 14 years later that Tombaugh, a young farmhand turned astronomer working at the Lowell Observatory, built on his predecessor’s work and discovered Pluto. Over the following decades it became clear that Pluto couldn’t be Planet X as its mass was too small to account for the changes in the orbits of Neptune and Uranus. Planet X remained a theoretical possibility until Voyager 2’s flyby of Neptune in 1989, when an accurate reading of Neptune’s mass was taken for the first time. The new measurement showed that it was Neptune – rather than some new phantom planet – that accounted for all the orbital anomalies. Few dared float the idea of a gravity-altering celestial body again – until Brown.

 Professors Mike Brown and Konstantin Batygin, planetary astronomers at the California Institute of Technology, stand in front of an artistic impression of Planet Nine

Professors Mike Brown and Konstantin Batygin, planetary astronomers at the California Institute of Technology, stand in front of an artistic impression of Planet Nine. Photo: Patrick T Fallon for The Washington Post via Getty Images

Despite the obvious possibilities for ridicule, he and Batygin went ahead and resurrected the possibility with their 2016 paper in The Astronomical Journal, ‘Evidence for a distant giant planet in the solar system’. “[The orbital anomalies in the Kuiper belt are] a pretty classic signature of the gravitational pull of a giant planet,” says Brown. “[But] we anticipated that astronomers would come up with alternative explanations.”

Professor Avi Loeb knows better than most what it’s like to face down disbelieving colleagues. For nine years, until 2020, the Israeli-born theoretical astrophysicist was the chair of Harvard’s astronomy department and is one of the most distinguished figures in his field. Yet in 2017, when a flat, oblong, shiny object passed through the solar system – the first interstellar object ever recorded traversing the solar system – Professor Loeb went against the grain. He speculated that the object – named ʻOumuamua, meaning “messenger from afar” in Hawaiian – wasn’t an iceberg. Or an asteroid. Maybe, he said, it was alien technology? “I was very curious about its nature,” Loeb says of ʻOumuamua. “It looks different to all the rocks we have seen in the solar system, and I suggested that maybe it’s artificial in origin. And that, of course, raised a lot of pushback.”

Loeb does not believe a giant planet is responsible for the anomalies in the Kuiper belt. In July 2020, he and his student Amir Siraj built on a previous theory that suggests the gravitational deviations on which Brown and Batygin have based their Planet Nine prediction are not caused by a massive, unseen planet but by a primordial black hole.

vi Loeb, the chair of Harvard's University's Astronomy Department, poses for a portrait in Cambridge

Professor Avi Loeb, the chair of Harvard’s University’s Astronomy Department. Photo: Lane Turner/The Boston Globe via Getty Images

When we think of black holes, it’s usually the cataclysmic aberrations of popular science and Hollywood fiction that come to mind: massive collapsed stars whose gravitational pull would be so powerful that they’d pose an existential threat to everything in their cosmic neighbourhood. A primordial black hole, though, would be a different beast. These are hypothetical objects thought to have formed just after the Big Bang, from particularly dense patches of the early universe’s hot, soupy expanse. Primordial black holes could theoretically have masses that are orders of magnitude tinier than the ‘classic’ black holes created by giant imploding stars. In A Brief History of Time, Stephen Hawking suggested that primordial black holes, if they still existed, would have the equivalent mass of a mountain (a billion tonnes), packed into an area smaller than the nucleus of an atom. The attractive force of a primordial black hole wouldn’t be enough to move a planet but could, theoretically, be enough to alter the orbits of the trans-Neptunian objects that caught Brown’s attention.

If we don’t find Planet Nine beyond a certain time, then we should admit that it’s not out there

If a primordial black hole is responsible, says Loeb, we might soon be able to prove it. His plan is to observe the flares that would emanate as a primordial black hole devours the passing comets which populate that distant region of space. “The idea was: can we calculate the rate of flares and whether they will be detectable? The answer is yes,” he says. Those flares, he believes, should be discernible when the Vera C Rubin Observatory in Chile, which has the largest digital camera ever made and can capture over half the entire night sky, becomes operational in 2022. Loeb also points to the fact that despite many efforts, Planet Nine has not been found. “Maybe [what’s responsible is] dark… a dark object that you can’t really find with telescopes, because it’s a black hole,” says Loeb.

Mike Brown is not convinced by that argument. “It could be a black hole that’s six times the mass of the Earth. It could be a hamburger that’s six times the mass. It could be a cat. I have a cat that’s nearly that size,” he says, tongue firmly in cheek. “Planets are pretty normal things to have even at those distances and so until there’s some reason to think that it’s not a planet, black hole ideas are cute but unnecessary.”

While Loeb admits that finding a black hole within our own solar system would be an incredible discovery, a Nobel Prize-worthy discovery, he bristles at the suggestion that the idea should be dismissed, comparing it to the same establishment myopia that greeted his ideas on ‘Oumuamua. “If you ever mention the possibility that Planet Nine is a primordial black hole to Mike Brown, he will get really upset. Did you ask him about the black hole possibility?” he asks.

I did, I reply.

Did he get upset? asks Loeb.

He mentioned a six-Earth-mass hamburger, I answer.

Loeb slaps his hands against his thighs, as if he’s heard that line before. “Because his vocabulary is rocks!” he says. “A black hole is as exotic [to Brown] as a tooth fairy or something.”

Dr Sarah Casewell Astronomer, University of Leicester

Dr Sarah Casewell, astronomer at the University of Leicester. Photo: University of Leicester

There are other reasons that might explain why Planet Nine hasn’t been found and, indeed, might not exist. The most commonly cited, and the explanation Loeb believes is still the most likely, is a simple statistical anomaly – that a small sample of objects have been observed behaving the same way but that there is no common cause linking those observations.

“Some people say it’s just a statistical fluke,” admits Brown. “We can calculate that probability and it’s pretty low. Almost precisely [like] flipping a coin and getting heads nine times in a row.” For Loeb, the longer the hunt for Planet Nine goes on, the less likely it is to be found. “If we don’t find it beyond a certain time, then we should admit that it’s not out there,” he says.

“I hope that Mike Brown would be sufficiently open to other possibilities.”

There are a lot of people looking for Planet Nine, including Dr Sarah Casewell from the University of Leicester and an army of volunteer astronomers. Casewell helps run the Backyard Worlds: Planet Nine project, a vast Nasa-funded citizen astronomer project begun in 2017 that has seen thousands of amateurs analyse photographs of space taken by Nasa’s Wide-field Infrared Survey Explorer (WISE) space telescope in the hope of finding Planet Nine. “We give our users a series of WISE images, and we blink through them,” explains Dr Casewell. “We ask them to flag things that move in the images.” The volunteers are people who are interested in science but never pursued it as a career. “Their enthusiasm is incredibly infectious,” says Casewell.

WISE orbits the Earth and its infrared telescope cuts through the Milky Way starlight that has made it so hard to identify Planet Nine. Casewell’s own suspicion is that Planet Nine may be a brown dwarf, a failed companion star to our sun that didn’t generate enough hydrogen fusion to sustain itself. “I’d like it to be a cool brown dwarf companion that’s been hanging around out there,” she says.

I’d like Planet Nine to be a cool brown dwarf companion that’s been hanging around out there

The Backyard Worlds team now has a little more help. Brown and Batygin’s August 2021 study refined the calculations from their 2016 paper and narrowed down the area where Planet Nine might be found, their “treasure map”. Given the new data, Casewell and her citizen astronomers are now discussing which space images to take a second look at. Meanwhile, Professor Brown is regularly sent data from the Panoramic Survey Telescope and Rapid Response System, or Pan-STARRS, in Hawaii. He checks his computer first thing every morning, and every morning he’s disappointed. Still, he believes Planet Nine is just months away from being discovered.

Finding a new planet would also bring Brown a degree of closure. Pluto may never be reinstated as an official planet in our solar system, as his daughter suggested, but if he is proved right and Planet Nine becomes Pluto’s official replacement, it would be a redemption arc worthy of Hollywood. “I could not have come up with this even if I wanted to: you kill a planet, then you go out and find a new one,” says Brown. “If it works out, it’s like, ok, that was a cute little bow around this part of my life.”

A guide to the five dwarf planets in the solar system officially recognised by the International Astronomical Union

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