Strange things are happening at the outer edges of our solar system. An object up to ten times the mass of Earth is pulling others towards it. Is it a planet, or something else?

As we head towards the end of another extraordinary year, BBC Future is taking a look back at some of our favourite stories for our "Best of 2021" collection. Discover more of our picks here.

Percival Lowell was a man of many errors.

The 19th-Century travel writer and businessman – fabulously wealthy, perennially moustachioed, and often found in crisp three-piece suits – had read a book on Mars, and on this basis, decided to become an astronomer. Over the coming decades, he made a number of wild claims.

First up, he was convinced of the existence of Martians, and thought he had found them (he hadn't). Others had documented strange lines traversing the planet, and Lowell suggested that these were canals, built as the last attempt of a dying civilisation to tap water from the polar ice caps. He used his fortune to build an entire observatory, just to get a better look. It turned out they were an optical illusion, created by the mountains and craters on Mars when viewed through low quality telescopes.

Lowell also believed that the planet Venus had spokes – seen in his notes as spidery lines emanating from its centre (it doesn't). Though his assistants tried to find them, it seemed that only he could see this unexpected detail. It's now assumed that they were shadows cast from the irises in his own eyes, as he looked through his telescope.

But most of all, Lowell was determined to find the ninth planet in our solar system – a hypothetical "planet X", which at the time was thought to be responsible for the rogue orbits of the furthest-known planets from the Sun, the cool-blue ice giants Uranus and Neptune. Though he never set eyes on this phantom behemoth, the quest consumed the last decade of his life – and after several nervous breakdowns, he died at the age of 61.

Little did he know, the search would still be going – with a few tweaks – in 2021.

A false trail

Undeterred by his own mortality, Lowell left a million dollars to the cause of finding planet X in his will. So, after a brief interlude involving a legal battle with his widow, Constance Lowell, his observatory kept looking.

Just 14 years later, on 18 February 1930, a young astronomer was looking at two photos of star-studded skies, when he noticed a speck amongst them. It was a tiny world. He had found Pluto – for a while considered the elusive planet X.

Alas, it was not to be. Soon scientists realised that this could not be what Lowell was looking for – it was not nearly large enough to pull Neptune and Uranus away from their rightful positions. Pluto was just an accidental interloper, which happened to be in the area.

The final blow to planet X came in 1989, when the Voyager 2 spacecraft swept by Neptune and revealed that it's fractionally lighter than anyone had originally thought. With this in mind, eventually a Nasa scientist calculated that the orbits of the outer planets had made sense all along. Lowell had instigated a search that had had never been needed.

But just as the concept of a hidden planet was killed off, the groundwork was laid for its resurrection.

In 1992, two astronomers who had "doggedly scanned the heavens in search of dim objects beyond Neptune" for years, according to Nasa, discovered the Kuiper Belt. This cosmic donut of frozen objects, extending just beyond the orbit of Neptune, is one of the largest features in the solar system. It's so vast, it's thought to contain hundreds of thousands of objects larger than 100km (62 miles) across, as well as up to a trillion comets.

Soon scientists realised that Pluto was unlikely to be the only large object in the outer reaches of the solar system – and began to question whether it was actually a planet at all. Then they found "Sedna" (around 40% of the size of Pluto), "Quaoar" (around half the size of Pluto), and "Eris" (almost the same size as Pluto). It became clear that astronomers needed a new definition.

In 2006, the International Astronomical Union voted to demote Pluto's status to a "dwarf planet", along with the newcomers. Mike Brown, a professor of planetary astronomy at the California Institute of Technology – Caltech – who led the team that identified Eris, is self-styled as the "man who killed Pluto" to this day. The ninth planet was no more.

A ghostly signature

At the same time, the discovery of these objects uncovered a major new lead in the search for a hidden planet.

It turns out that Sedna is not moving in the way everyone expected – tracing elliptical rings around the Sun, from within the Kuiper Belt. Instead, this dwarf planet takes a bizarre and unexpected path, swinging from just 76 Earth-Sun distances (roughly 11 billion k/7 billion miles) from the centre of our solar system to more than 900 (roughly 135 billion km/84 billion miles). Its orbit is so meandering, it takes 11,000 years to complete – the last time Sedna was at its current position, humans had only just invented farming.

It's as though something is tugging at Sedna and dragging it away.

Enter a hypothetical new addition to our solar system – but not as it was thought of before. In 2016, the same Mike Brown who had slain Pluto, together with his colleague Konstantin Batygin – also a professor of planetary science at Caltech – co-authored a paper proposing a massive planet, between five and 10 times the size of Earth.

Their idea came from the observation that Sedna was not the only object out of place. It was joined by six others, and all of them are being pulled in the same direction. There are also other clues, such as the fact that each is tilted on its axis in exactly the same direction. The pair calculated that the probability of all six objects being pulled in the exact same direction, with the same tilt by chance was just 0.007%.

"We thought 'this is quite interesting – how can this be?'" says Batygin. "It was quite remarkable because such a clustering, if left alone for a sufficiently long period of time, would disperse, just due to interaction with the gravity of the planets."

Instead, they proposed that Planet Nine had left its ghostly imprint in the outer reaches of our solar system, distorting the orbits of the objects around it with its gravitational pull. Several years on, and the number of objects that fit the eccentric orbital pattern and tilt has continued to increase, "We now have around 19 overall," says Batygin.

Though no one has yet seen the hypothetical planet, it's possible to infer a surprising amount about it. As with the other objects beyond the Kuiper Belt, the orbit of the new Planet Nine would be so distorted that its farthest reach is expected to be twice as far away as its nearest – around 600 times the distance from the Sun to Earth (90 billion km/56 billion miles), vs 300 (45 billion km/28 billion miles). Scientists have also hazarded a guess at its aesthetic – icy, with a solid core, like Uranus or Neptune.

Then there's the slippery question of where Planet Nine might have come from in the first place. So far, there are three main ideas. One is that it formed where it currently hides, which Batygin dismisses as relatively unlikely because this would require the early solar system to have stretched out as far as its distant refuge.

There's also the intriguing suggestion that the ninth planet is actually an alien imposter, an object that was stolen from another star long ago when the Sun was still in the stellar cluster in which it was born. "The problem with such a story is that you're just as likely to then lose the planet upon the next encounter," says Batygin. "So, statistically, that model runs into trouble."

Then there's Batygin's personal favourite, which he admits is also "not a complete slam dunk". In this scenario, the planet formed much closer to the Sun, at a time when the solar system was in its early stages and the planets were just beginning to coalesce out of the surrounding gas and dust. "It kind of hung around the giant planet formation region, before being scattered out by Jupiter or Saturn, and subsequently had its orbit modified by passing stars," he says.

An obscure hiding place

Of course, all this begs an obvious question – if Planet Nine is really there, why has no one seen it?

"I didn't have a particularly strong appreciation for just how difficult would be to find Planet Nine until I started looking together with Mike using telescopes," says Batygin. "The reason it's such a tough search is because most astronomical surveys are not looking for a single thing."

For example, astronomers would normally be looking for a class of objects, such as a particular kind of planet. Even if they're rare, if you survey a wide enough expanse of space, you're likely to find something. But hunting down a specific object such as Planet Nine is a whole different exercise. "There's only one tiny portion of the sky that has it," says Batygin, who explains that another factor is the slightly more prosaic challenge of booking time slots to use the right kind of telescope.

"Really, at the moment the only game in town for finding Planet Nine is the Subaru Telescope," says Batygin. This 8.2m behemoth – located at the summit of a dormant volcano, Maunakea, in Hawaii – is capable of capturing even the weak light of distant celestial objects. This is ideal, because the shadowy planet would be so far away, it's unlikely to be reflecting much light from the Sun.

"We have only one machine that we can use, and we get maybe three nights on it a year," says Batygin, who was fresh from a three-night run on the telescope the previous week. "The good news is that the Vera Rubin telescope is coming online within the next couple of years, and they are going to probably find it." This next-generation telescope, currently under construction in Chile, will be scanning the sky systematically – photographing the entire available view – every few nights, to survey its contents.

An intriguing alternative

However, there is one almost outrageously peculiar scenario in which the planet will never be found this way – it might not be a planet after all, but a black hole.

"All of the evidence for there being an object is gravitational," says James Unwin, professor of physics at the University of Illinois, Chicago, who first suggested the idea, along with Jakub Scholtz, a postdoctoral researcher from the University of Turin. While we're most familiar with the idea that planets exert a powerful gravitational pull, "there are other things that can generate it, which are more exotic", says Unwin.

Some plausible replacements for planet nine include a small ball of ultra-concentrated dark matter, or a primordial black hole. As black holes are among the most dense objects in the Universe, Unwin explains that it's entirely possible the latter could be warping the orbits of distant objects in the outer solar system.

The black holes we’re most familiar with tend to include “stellar” black holes, which have a mass that’s at least three times that of our own Sun, and “supermassive” black holes, which are millions or billions of times our Sun’s mass, While the former are born out of dying stars collapsing in on themselves, the latter are more mysterious – possibly beginning as colossal stars which implode, then gradually accumulate more and more mass by devouring everything in their surroundings, including other black holes.

Primordial black holes are different. They have never been observed, but are thought to originate in the hot energy-and-matter haze that formed in the first second of the Big Bang. In this uneven environment, some parts of the Universe may have become so dense, they were compressed into tiny pockets with the mass of planets.

Unwin points out that there is zero probability of the black hole being formed from a star, since they keep their potent gravitational pull – it’s just concentrated. Even the smallest stellar blackholes have masses three times that of our Sun, so it would be like having at least three extra Suns pulling at the planets in our solar system. In short, we would definitely have noticed.

However, Unwin and Scholtz say it could be a primordial black hole, since these are thought to be substantially smaller. "Because these things are born during the early stages of the Universe, the dense regions they formed from could have been particularly small," says Scholtz. "As a result, the mass contained in this black hole that eventually is formed out of it can be much, much less than a star – they even can be just a couple of pounds, like a chunk of rock." This is more in line with the predicted mass of Planet Nine, which is thought to be equivalent to up to ten Earths.

What would it look like? Should we be worried? And could this be even more exciting than discovering a planet?

First, even primordial black holes are dense enough that no light can escape. They are the truest form of black. This means that this one would not show up on any kind of telescope that currently exists. If you were to look straight at it, the only clue to its presence would be a blank void – a tiny gap in the blanket of stars in the night sky.

Which brings us to the real snag. While the mass of this black hole would be the same as that of the proposed Planet Nine – up to 10 times Earth's – it would be condensed into a volume roughly the size of an orange. To find it would require some ingenuity.

So far, suggestions include looking for the gamma rays that are emitted by objects as they fall into black holes, or releasing a constellation of hundreds of tiny spacecraft, which might – if we're lucky – pass close enough so that they'd be pulled towards it ever-so-fractionally, and accelerate by a detectable amount.

Since the mysterious gravitational pull is emanating from the farthest reaches of our solar system, the probes would have to be sent via an Earthbound laser array, which could propel them to 20% of the speed of light. If they travelled any slower, they might take hundreds of years to arrive – an experiment that would, naturally, stretch well beyond a human lifetime.

As it happens, these futuristic spacecraft are already being developed for another ambitious mission, the Breakthrough Starshot project, which aims to send them to the Alpha Centauri star system, 4.37 light-years away.

If we were to discover a lurking black hole, rather than a frigid planet, Unwin says there would be no need to panic. "There's a supermassive black hole in the centre of our galaxy," he says. "But we don't worry about our solar system falling into it, because we're in a stable orbit around it." So, while a primitive black hole will suck up anything within its path, this would not include the Earth, which – like the other inner planets – doesn't ever come close.

"It's not like a vacuum cleaner," says Unwin. He explains that from the perspective of anyone on Earth, having an undiscovered black hole in the solar system is not that different to having a concealed planet there.

But while stellar and primordial black holes are essentially the same, the latter have never been found or studied – and difference in scale is expected to lead to some bizarre phenomena. “I would say that the things that happen with small black holes are more interesting than what happens with large black holes," says Scholtz.

One example is the aptly-named process of “spaghettification”, which is often illustrated by the fable of an astronaut who ventured too near a black hole’s event horizon – the point beyond which no light can escape – and fell in headfirst. Though her head and feet were just metres from each other, the difference in the gravitational forces acting on them would be so great, she would be stretched like spaghetti.

Intriguingly, the effect should be even more dramatic, the smaller the black hole is. Sholtz explains that it’s all about relative distances – if you’re two metres tall, and you’re falling through an event horizon that’s one metre from a primordial black hole’s centre, the discrepancy between the location of your head and feet is larger, compared to the size of the black hole. This means you’ll be stretched far more than if you fell into a stellar one that’s a million miles across.

"And so, peculiarly enough, they're more interesting," says Scholtz. Spaghettification has already been seen via a telescope, when a star got too close to a stellar black hole 215 million light years from Earth, and was ripped apart (no astronauts were harmed). But if there is a primordial black hole in our own solar system, it would provide astrophysicists with the opportunity to study this behaviour – and many others – up close.

So what does Batygin make of the possibility that the long-sought ninth planet could actually be a black hole instead? "It's a creative idea, and we cannot constrain what its composition is even in the least bit," he says. "I think maybe it's just my own bias, being a planetary science professor, but planets are a little bit more common…"

While Unwin and Scholtz are rooting for a primeval black hole to experiment with, Batygin is just as keen for a giant planet – citing the fact that the most common type throughout the galaxy are those which have around the same mass as Planet Nine.

"Meanwhile most exoplanets that orbit Sun-like stars, are in this weird range of being bigger than the Earth and considerably smaller than Neptune and Uranus," he says. If scientists do find the missing planet, it will be the closest they can get to a window into those elsewhere in the galaxy.

Only time will tell if the latest quest will be more successful than Lowell's. But Batygin is confident that their missions are totally different. "All of the proposals are quite distinct in both the data they seem they seek to explain, as well as the mechanisms they use to explain it," he says.

Either way, the search for the legendary ninth planet has already helped to transform our understanding of the solar system. Who knows what else we'll find before the hunt comes to an end.

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