In the wise words of Douglas Adams – DON’T PANIC. When I tell people my most fervent wish is for the Solar System to have a black hole of its own, they look at me with revulsion and horror. But as we’ve learnt previously, black holes are not hoovers – in the Solar System a black hole’s role would be more one of a gravitational shepherd. So having a black hole in the Solar System wouldn’t be a bad thing: it would be coooooool.

Unfortunately there’s been no confirmed reports (or ‘sightings’ – geddit?!) of a black hole in the Solar System yet. The closest known black hole to Earth is V616 Monocerotis, which may sound like some sort of disease, but is actually a black hole 6.6 times more massive than the Sun, squished into a space a touch smaller than the planet Neptune. It is fairly close to us at 3,000 light years away (about 28 million billion miles), but much further away than the nearest star to the Sun, which is only four light years away. So in the grand scheme of things, it’s astronomically close but still not exactly what you or I would class as popping to the shops.

Thankfully, V616 Monocerotis is happily sat orbiting another star, one fairly similar to our Sun, which the black hole is slowly dragging material from into an accretion disk that gives off an occasional flare of X-rays, just to let us know that it’s there. Apart from that, it doesn’t have anything particularly remarkable about it, except for its proximity to Earth. And we’ve already agreed that there’s nothing particularly special about our backwater region of the Milky Way.

The one thing that makes it truly special to the human race is not that we have detected light coming from the material spiralling around it, but that we have also sent a light signal towards it. On 15 June 2018, three months after the death of British astrophysicist Stephen Hawking, who devoted his life to understanding the mathematics of black holes, the European Space Agency sent a broadcast out in the direction of V616 Monocerotis in his honour. It’ll arrive in the year 5475, and will be the first ever human ‘communication’ with a black hole.

V616 Monocerotis is only the closest known black hole, though. What if it’s not truly the closest black hole? There could be another that’s closer, perhaps a pair of black holes orbiting each other, like the system LIGO detected gravitational waves from, that don’t have any material around them to heat up to flag to us with X-rays that they’re there. Or perhaps even one hiding in plain sight in our very own Solar System?

That’s not as mad as it first sounds, I promise. There’s good reason to think that there might just be a tennis-ball-sized black hole hanging around on the edge of the Solar System, way out beyond the orbit of Pluto, stirring sh*t up. At first, the reason was that astronomers thought Uranus and Neptune’s orbits were a little weird. So weird, that after Neptune was discovered in 1859 (after Le Verrier famously predicted where it would be), people immediately began searching for another planet (‘Planet 9’) beyond Neptune that could be disturbing both Uranus and Neptune’s orbits: pulling on them due to gravity and making their orbits a lot more elliptical than those of other planets in the Solar System.

This elusive ‘Planet 9’ was finally thought to have been found in 1930, when, aged just twenty-four, American astronomer Clyde Tombaugh discovered Pluto. Tombaugh had taken up the mantle of searching for Pluto from fellow American astronomer Percival Lowell. Lowell was born into the Boston elite, and so of course studied at Harvard University. After graduating he ran a cotton mill in the city for six years and then decided to travel far and wide across Asia for the next decade. When he finally returned to the USA at the end of the nineteenth century, he decided to take up a career in astronomy. He went about it not as you or I would go about it, by applying for a job, but instead used his inherited and earned wealth to found a brand new observatory: the Lowell Observatory, just outside Flagstaff, Arizona, USA. Lowell specifically chose the location due to its high altitude and distance from city lights – the best conditions possible for astronomy – marking the first time the location of an observatory had been chosen in this way (rather than through convenience of location). It is now how the locations of all professional observatories are chosen, with the common themes of distance from populated areas, and high and dry climate. Think Mauna Kea, Hawai‘i; the Atacama desert, Chile; or the Warrumbungle National Park, Australia.⁷³

It was at Flagstaff in 1906 that Lowell started a dedicated search for ‘Planet 9’ (or ‘Planet X’, as he referred to it). Like at Harvard College Observatory, where they were classifying stars, Lowell hired a team of women computers to do the tedious searching of photographic plates, which was headed up by Elizabeth Langdon Williams. Williams had just graduated with honours from MIT in 1903 with a degree in physics, becoming one of the first women ever to do so. She was initially hired by Lowell in 1905 to edit his scientific publications, before being asked to lead the team of computers at the observatory. Lowell had given Williams a rough idea of where he thought Pluto would be (orbiting in the same plane as Uranus, at about forty-seven times the Earth–Sun distance), and she was left to do the grunt work of calculating possible orbits for ‘Planet 9’ in order to recommend the regions of the sky that should be searched.

Lowell would then observe those areas of the sky with the telescope at the observatory frequently, comparing the most recent images with those taken previously to see if anything had moved in front of the background stars (again, by today’s definitions, Williams was doing the astrophysics and Lowell the astronomy). He continued searching right up until his death in 1916, but never found what he was looking for. Although, with hindsight, we now know that the Lowell Observatory captured two very faint images of Pluto in 1915, but they were missed in the search.⁷⁴

After Lowell’s death, the search halted for over a decade; in that time Williams married a British astronomer also based at the observatory, George Hall Hamilton, and was promptly dismissed from her position as lead computer because early twentieth-century views on women were ridiculous it apparently wasn’t appropriate to employ a married woman. So, when the search finally resumed in 1929, it was the newly employed Clyde Tombaugh who took up the mantle. Tombaugh had impressed the observatory director, Vesto Melvin Slipher,⁷⁵ with the scientific drawings of Mars and Jupiter he’d made using a telescope he’d built and tested himself on his family’s farm in Kansas.

Tombaugh was given the rather tedious job of searching for ‘Planet 9’ by blinking back and forth between pairs of photographs of regions of the night sky taken a week apart. After a year of searching, he finally found an unknown object that had moved in images taken a few weeks prior, in January 1930. A few more observations confirmed the object was real and continuing to move in the same direction, and the discovery was finally announced to the world in March 1930.

The discovery made headlines around the world, and the question on everyone’s lips was what to call the new planet in the Solar System. The Lowell Observatory had the right to name it, by virtue of discovering it, and they received over 1,000 suggestions through the post from eager astronomy lovers the world over. Constance Lowell, Percival’s widow, who had taken over managing the estate, suggested Zeus (after the Greek God of the Sky), and even her and her husband’s names: Percival and Constance. All of these were unsurprisingly dismissed by Slipher and Tombaugh (including Zeus, as all the other planets in the Solar System have Roman names, not Greek: Jupiter is Zeus’s Roman equivalent).

Pluto is the Roman God of the Underworld, and according to Clyde Tombaugh, the name was originally proposed by an eleven-year-old in Oxford: Venetia Burney. This wasn’t just any ordinary eleven-year-old though; this was the granddaughter of a retired librarian at the University of Oxford’s Bodleian Library, Falconer Madan. Madan had friends in high places who he could relay the suggestion to, specifically the Savilian Professor of Astronomy and director of the University of Oxford’s Radcliffe Observatory, Herbert Hall Turner (remember, the author of Modern Astronomy from the prologue). Turner then sent a telegram to his colleagues at the Lowell Observatory, who included it on a shortlist of potential names (including Minerva and Cronus). A vote was held by the observatory staff, which was unanimous, and Lowell’s ‘Planet X’ was officially named Pluto on 24 March 1930.⁷⁶

In the end, Pluto was found just six degrees away from where Lowell (with Williams doing the calculation) predicted it would be. So at first, physicists were confident that Pluto was responsible for the oddities of Uranus and Neptune’s orbits. Its mass was estimated based on how big it would need to be to affect them: seven times more massive than Earth. But due to how faint Pluto appeared (if it was bigger it would reflect more light and appear brighter) that mass was cast into doubt. By 1931, that estimate had been revised down to somewhere between 0.5–1.5 times the mass of the Earth, and it kept dropping through the twentieth century. Dutch astronomer Gerard Kuiper himself estimated it to be just 10 per cent of the Earth’s mass in 1948, but this was still a gross overestimate.

It was in 1978 that Pluto’s moon, Charon, was discovered by astronomers Robert Harrington and Jim Christy working at the United States Naval Observatory. From the orbit of Charon they were able to work out that the mass of Pluto was a measly 0.15 per cent of Earth’s (that actually short changes Pluto a bit; modern estimates put it around 0.22 per cent of Earth’s mass). This was far too small to account for the oddities of Uranus’s orbit and it once again spurred searches for a planet beyond Pluto. These searches were brought to a halt by the results from the Voyager 2 flyby of Uranus in 1986 and Neptune in 1989 (the only craft ever to have visited either planet), which gave astronomers a more accurate estimate for both their orbits and masses. Taking all of these new measurements into account, the supposed weirdness to both of their orbits disappeared, along with the need for Lowell’s supposed ‘Planet X’. The fact that Lowell’s predictions coincided with the area of sky where Tombaugh discovered Pluto is considered a happy coincidence.

What followed instead, throughout the rest of the twentieth century, was the discovery of many more small objects out beyond the orbit of Neptune in an area now known as the Kuiper Belt, after Gerard Kuiper. The Kuiper belt is an asteroid belt of sorts, but far larger (roughly twenty times wider) and more massive (up to 200 times more matter) than the asteroid belt between Mars and Jupiter. The search was kicked off by British-American astronomer David Jewitt and Vietnamese-American astronomer Jane Luu discovering the first two objects in the Kuiper Belt after Pluto in the early 1990s (1992 QB1 and 1993 FW). There are now over 2,000 known Kuiper Belt objects, but there are thought to be over a hundred thousand more small icy objects out there in the far reaches of the Solar System.

In 2005, three American astronomers working at the Palomar Observatory outside San Diego, California (Mike Brown, Chad Trujillo and David Rabinowitz) announced the discovery of a new object in the Kuiper Belt. At first this object was called 2003 UB313, but was eventually dubbed Eris (after the Greek goddess of strife and discord). Like Pluto, there are ‘pre-discovery’ images of Eris dating all the way back to 1954. Eris’s moon was discovered a few months later, allowing Brown to calculate that Eris is 27 per cent more massive than Pluto. This made it the most massive object discovered in the Solar System since Neptune’s moon Triton in 1846.

The world’s press then dubbed it the ‘tenth planet’, but in astronomy circles that name was incredibly controversial. There were some in the community that thought Eris’s discovery, along with other Kuiper Belt objects found at the same time, such as Makemake and Haumea, were the best argument for there only being eight planets in the Solar System – otherwise you would have more like fifty-three. Some astronomers started to argue that Pluto should be reclassified, but were wary of public reaction. When the Hayden Planetarium in New York displayed a model of the Solar System in 2000 with only eight planets, leaving Pluto off their model, it made headlines around the world because of the sheer volume of complaints that rolled in from visitors who were big Pluto fans.

Things finally came to a head in 2006, when at a meeting of the International Astronomical Union the official definition of a planet in the Solar System was decided upon by vote. A proposal for the definition had been put forward by committee and then members at the meeting were eligible to vote at a session chaired by none other than Jocelyn Bell Burnell (who discovered the first pulsar). The proposal passed the vote, and there are now three criteria needed to classify an object in the Solar System as a planet:

  (i)   It must be in orbit around the Sun

 (ii)   It must have achieved ‘hydrostatic equilibrium’ (i.e. it has enough mass that gravity has rounded it from a lumpy potato-shaped asteroid to something close to spherical)

(iii)  It must have cleared the neighbourhood around its orbit

It’s the third of those criteria that Pluto falls down on, along with all other objects of the Kuiper Belt, as they all inhabit the same neighbourhood of the Solar System.⁷⁷ Instead, they’re classed as ‘dwarf planets’, along with a few other objects, like Ceres in the asteroid belt. It’s safe to say the world did not react well to the decision. The American Dialect Society even chose ‘plutoed’ as its 2006 Word of the Year; to pluto something meant to demote or devalue it. I still don’t think the internet is quiet over this demotion – anytime I bring it up it is met with absolute outrage. Although I like to point out to all those Pluto fans that now you can at least consider it ‘King of the Dwarves’.

The study of these newly dubbed dwarf planets in the late 2000s unearthed yet more orbit peculiarities that couldn’t be explained. For example, the dwarf planet Sedna has what’s known as a ‘detached’ orbit. Unlike the other ‘Trans-Neptunian Objects’ of the Kuiper Belt (or TNOs), Sedna never crosses the orbit of Neptune; the orbits are elliptical, so you could say that its closest point to the Sun is still further away than Neptune’s furthest point from the Sun (unlike Pluto and Eris, which do get closer to the Sun than Neptune’s furthest point and were probably shepherded there by the gravity of Neptune during the Solar System’s formation). Sedna actually orbits three times further out than Neptune on a highly elliptical orbit which takes over 11,000 Earth years. How did Sedna get into such a strange and distant orbit? One option is that it could be an object that was wandering interstellar space and was captured by the Sun. Another option is that it could’ve got pulled out there if the Sun had an interaction with a passing star, or, the most exciting option, by another massive planet on the edge of the Solar System.

It’s this last idea that’s favoured by the person who discovered Sedna: American astronomer Mike Brown (who also discovered Eris, leading to the demotion of Pluto that has earned Brown the nickname ‘Pluto killer’). After the discovery of six more objects found with detached Sedna-like orbits at huge distances through the early 2010s, Brown and his Caltech colleague Konstantin Batygin (a Russian-American astronomer) investigated further. They found that not only did these objects have similar distances from the Sun, but they all orbited in the same plane, as if they had been shepherded there by an object on the far reaches of the Solar System. Brown and Batygin worked out that the most likely explanation was a planet somewhere between five to fifteen times more massive than Earth orbiting at the far edge of the Solar System.

Overnight, Brown and Batygin single-handedly sparked the search for yet another ‘Planet 9’ in the Solar System; but in the words of Carl Sagan, ‘extraordinary claims require extraordinary evidence’. Planet 9 still remains a hypothetical planet, and despite many searches nothing has been found. One such search was done by volunteers on the Zooniverse citizen science online platform.⁷⁸ Similar to how Tombaugh found Pluto, volunteers were shown two infrared images from NASA’s Wide-field Infrared Survey Explorer (WISE) mission which blinked back and forth so that they could spot if anything had moved. While the project didn’t find ‘Planet 9’, it did find 131 new brown dwarf stars beyond the Solar System and ruled out a huge area of sky for future Planet 9 searches.

What makes the search for Planet 9 so difficult is that, if it exists, it is estimated to orbit at a distance of over 500 times the Earth–Sun distance. That means it will take an incredibly long time for it to complete one orbit of the Sun, and so it’s not expected to move that much on the sky on human timeframes. And so ‘Planet 9’ remains both hypothetical and elusive, with the orbits of the Sedna-like objects unexplained.

However, in 2020 a paper was published by Jakub Scholtz and James Unwin linking not only this unexplained phenomenon but another that, at first, appears completely disconnected. The Optical Gravitational Lensing Experiment (OGLE) run by the University of Warsaw uses a telescope in the Atacama Desert in Chile to spot if anything changes brightness in the sky. That can be anything from pulsing stars, to supernovae, or something called a microlensing event. This is when a compact object, like a neutron star or black hole, passes in front of a background star. The light of the background star gets bent as it travels along the curved space around the compact object, which acts like a lens to briefly brighten the background star. From how much the background star changes in brightness, and how long the change lasts, you can work out, once again using Einstein’s general relativity equations, how massive the compact object doing the lensing is.

The OGLE survey has been running since 1992 and in that time has spotted many a gravitational lens caused by black holes in the Milky Way, all formed when a star went supernova, giving a black hole anywhere above the Tolman–Oppenheimer–Volkoff limit of around three times the mass of the Sun. But the OGLE team also reported that they’d observed six ultra-short micro-lensing events in the direction of the centre of the Milky Way (which also crosses the plane of the Solar System) that had to be caused by objects just 0.5–20 times the mass of the Earth. Such a low mass meant that either this had to be a population of rogue, free-floating planets that had been ejected from whichever star system they formed in, or a population of primordial black holes. A primordial black hole is a hypothetical type of black hole that formed in the very early Universe when the Universe was much denser; if real, they’d be the oldest black holes in existence. Theoretically, if enough matter happened to clump together randomly during that time, a tiny black hole could form, an idea that was developed by Stephen Hawking in the 1970s.

What Scholtz and Unwin pointed out in their paper, entitled ‘What if Planet 9 is a Primordial Black Hole?’,⁷⁹ was that the two mass ranges, as predicted by Brown and Batygin for Planet 9 (5–15 times Earth mass) and seen by the OGLE team (0.5–20 times Earth mass) were remarkably similar, and perhaps one could help explain the other. Perhaps Planet 9 was once part of this population of objects causing the microlensing events seen by OGLE; either a captured free-floating planet or a captured primordial black hole.

Capture of Planet 9 is just one possible explanation for how the hypothetically rather large Planet 9 might have formed on the edge of the Solar System. Other options are 1) that it somehow managed to form where it currently orbits or 2) that it formed further in, closer to the Sun, and then migrated outwards. That first option is unlikely, as it’s not very dense at the edge of the Solar System, so 4.5 billion years is still not enough time to bring together all of those far-flung tiny clumps of rock to form a planet that large. The second option is also problematic, because you need an event that kick-started the migration but also one that stops it in its current orbit, which could perhaps be an interaction with a passing star again, but that seems unlikely. So with those two ideas out, the hypothesis of a captured Planet 9 is currently the most favoured.

Planet-system formation models have shown that during the chaos of planet formation around stars, with bits of rock colliding and clumping together under gravity or perhaps slingshotting around each other, many planetesimals (i.e. baby planets) get flung out of the melee into interstellar space. We think one such object, dubbed ‘Oumuamua’, travelled through the Solar System back in 2017, passing within just 24,200,000 km of us here on Earth. That’s about 15,040,000 miles, or about 16 per cent of the Earth–Sun distance. Given how big space is (think about the huge distances involved and then remember that space is three-dimensional, so you need to cube whatever huge number you just thought of) we think these events are incredibly rare, and the gravitational capture of such an object by the Sun even rarer. However, that likelihood of capture doesn’t change whether the object is a rocky planet or an incredibly dense primordial black hole.

The beauty of this hypothesis – that Planet 9 is a black hole – is that it also explains why we haven’t found it. Not just in recent searches, like with the Zooniverse, but in previous searches throughout the last few decades which found other Kuiper Belt objects. Not only would we not get any light from a black hole but nothing would ever get close enough to be directly impacted by it. If the black hole Planet 9 turned out to be five times the mass of the Earth, its event horizon would be just 9 cm across; about the size of a tennis ball.

A circle 9 cm across. This could be the size of a primordial black hole five times the mass of the Earth lurking at the edge of the Solar System.

Now, as much as I desperately want this hypothesis to be true, the problem with Planet 9 turning out to be a primordial black hole would mean that it would be incredibly difficult to find evidence for it. Although, if it was a black hole that has existed since the very early days of the Universe, in the past 13 billion years or so, it will have collected a little halo of matter around it. Not in an accretion disk necessarily, just a clump of matter that is shepherded along by its movement through space, making the region around it much denser than normal. If that area is denser, it increases the chances that some of that matter will encounter some very rare anti-matter. Thankfully, we have a lot more matter in the Universe than anti-matter, otherwise nothing you have ever seen in your life, including the stars themselves, would ever have existed. Because when matter meets anti-matter it turns back into pure energy released as gamma rays; the most energetic type of light.

So, if the Solar System has its very own pet black hole, we should be able to detect that radiation with the gamma-ray telescopes that we currently have in orbit around Earth. The Planet 9 search isn’t just one that optical and infrared astronomers are involved with, the gamma ray astronomers have now been caught up in the furore as well. Because the idea that our nearest black hole could be light hours away rather than light years is enough to capture the heart of even the most hard-headed astrophysicist. To me, the theoretical evidence is very compelling, but perhaps I’m slightly biased as a black hole scientist; a black hole right on my doorstep would be the best present the Universe could ever get me.