Planet 9 – The Controversial missing planet?

By | 05/04/2023

We know a lot about the solar system – but we also know so little. Over the last century or so there have been several theories of additional planets inhabiting both the inner and the outer solar system.


During the 19th century it was discovered the planet Mercury was not sticking to the predictions made using Newtonian physics – it appeared to wobble – and so it was proposed that there may be a planet inferior to the orbit of Mercury, thus closer to the Sun and virtually impossible to see.

Edmond Lescarbault, a young doctor with a deep love of astronomy, built an observatory in a stone barn with a little dome on top of it. One day in 1859, he saw a patient and then retired to his observatory, climbs up and looks through his telescope. As he projects an image of the Sun through his telescope, he sees what he believes to be a spherical object crossing the face of the Sun. Lescarbault was a precise man, being a doctor this was important, so he times the mystery object as it crossed the face of the Sun. He tracks it continuously, taking notes on its path until it finally moves off the face of the Sun and disappears.

He is unaware of the work carried out by Urbain Le Verrier on Mercury, so he does not report his sighting and continues his observations.  It is reported that some months later he was made aware of the work Le Verrier had completed on Mercury, and at this point he reported his observations to the him. Le Verrier checked his observations, and it is reported that he too saw a spherical object crossing the disk of the Sun, in fact it became reasonably common for people to report such observations.

There was a very real problem though, because everyone accepted that Newtonian physics had all the answers, the only explanation was a planet nearer to the Sun than Mercury that had a gravitational influence on Mercury, the science community convinced itself that Vulcan was real. This belief was supported because work on Uranus had led to the discovery of Neptune by Galle and D’Arrest in September 1846, based on the mathematical work of Urbain Le Verrier some years earlier.

At the end of the 19th Century entered a man who would revolutionise our understanding of the natural world – Albert Einstein – when he published his Theory of Relativity in the second decade of the 20th century, with his theory of gravity – Vulcan was no longer needed to explain the movement of Mercury – in one fell swoop, Einstein had revolutionised our understanding of gravity and also destroyed a planet – take that Darth Vader!

We know that no planet orbits the Sun inside the orbit of Mercury, although we are now finding asteroids there.


Nemesis is the ancient Greek God of retribution for hubris before the Gods – now the name is more commonly linked to a questionable theory of a dwarf star companion to the Sun.

In 1984 two palaeontologists, Professor David Raup and his University of Chicago colleague, Jack Sepkoski, published a paper in which they speculated about an apparent pattern in the extinction record around 26 millions years in periodicity, they speculated that the possible causes of this was  connected to the motion of the solar system around the galaxy or asteroid impacts because they could see no other explanation for such periodicity, this also coincided with the revelations of the Walter and Louis Alvarez for the extinction of the dinosaurs being linked to an asteroid impact – we now know as the K-T event.

A short time later, a former post doc student of Walter Alvarez, physicist, Richard Muller, published a theory that the Sun may have an unseen dwarf companion, possibly a M-Class red dwarf – the thinking being that this companion had an elongated and very long period orbit that caused it to gravitationally interact with the Oort cloud and even possibly the scattered disk objects in the outer solar system – flinging many into the inner solar system, thus accounting for the impact cratering periodicity seen on Earth, however, this was soon ruled out after IRAS (Infra-Red Astronomy satellite) of the 1980s and then by the 2MASS infra-red survey carried out between 1997 and 2001 which concluded that the sun had no M-Class companions, however, if the Sun had a sub-stellar companion, below around 40Mj (Mass of Jupiter), otherwise known as a brown dwarf, the 2MASS survey was not sensitive enough to detect it – leaving a slim window of possibility for such a companion. Later, the Wide Field Infra Red Survey Explorer (WISE), which was able to detect brown dwarves as cool as 150°K to a distance of 10 light years also failed to detect any sub-stellar objects in the outer solar system, leading to David Morrison, a senior scientist with NASA to conclude there was “no confidence in the existence of any sub-stellar or stellar mass companions to the Sun in the solar system”

Nemesis was dead, another solar system family member sent to the bin of disappointment.

Planet X

Shortly after the discovery of Neptune in September 1846, it was discovered that the orbit did not conform to predictions exactly, the conclusion was that another planet lay beyond Neptune, the gravity of this unseen planet was the cause of the discrepancy in Neptune’s path.

This became known as the Hunt for Planet X – not 10 as some often ascribe, but X for unknown.

Anyway, the 18th and 19th centuries were thick with astronomical discoveries, not least were the discovery of the planets Uranus and Neptune. The latter, in particular, was predicted by comparing observed perturbations in the orbit of Uranus to what was expected. This suggested the gravitational influence of another nearby planet. the English mathematician, John Couch Adams and Frenchman Urbain Le Verrier, had the calculated probable the orbit of Neptune by comparing these perturbations in Uranus’ orbit to those of the known planets. As noted above, Neptune was subsequently discovered almost in the predicted location in September of 1846.

sometime in the mid 1850’s, the French mathematician, astronomer and physicist Jacques Babinet proposed the existence of an even more distant planet, which he suggested should be named  Hyperion. Le Verrier wasn’t convinced, stating that there was “absolutely nothing by which one could determine the position of another planet, barring hypotheses in which imagination played too large a part”. This statement seems hypocritical considering Le Verrier had undertaken the same task with the perturbations in the orbit of Uranus.

Anyway, for almost 80 years numerous searches were undertaken by amateurs and austere giants of the astronomy world, including the eminent Percival Lowell who had set up the observatory near Flagstaff Arizona – who is more well know for his claims about a civilisation on Mars than his hunt for planet X.

In 1916, Percival Lowell employed a farmhand with a keen interest in astronomy to take images of the night sky and use a blink comparator, a simple device that allowed an observer to view two photographic plates take on different nights so the eye would pick up any movement of any objects imaged – this young man’s name was Clyde Tombaugh. For 14 years he imaged the same part of the sky on different nights along a predetermined path, he then compared these images and in 1930, he found something moving against the background stars – he had found the world that today we call Pluto – but was this planet X?

Despite the clamour, Planet X was already dismissed in much of the science community for the same reason Vulcan was relegated to the science bin – Einstein’s theory of gravity cleared up the perceived perturbations in the orbit of Neptune and there was no need for a planet further out to account for the errors which had been “seen” in earlier observations.

Researchers soon realised that even if they were wrong and there was another planet out beyond Neptune, Pluto could not be it – although is would be several decades before we knew how small Pluto actually is, let alone image it – it was clear it was too small to have any gravitational influence on Neptune, regardless of Einstein’s improved theory.

Despite the clamour following the change in classification by the IAU in 2006, there were many researchers over the decades who questioned whether Pluto was actually a planet or just a large asteroid or unknown type of object because even the largest telescopes of the day were unable to resolve Pluto as a disk, it was also discovered that whilst Pluto was further than Neptune at the time of discovery, between 1979 and 1999 the diminutive world would be closer to the Sun that Neptune – it certainly did not behave like a “normal planet”

The orbit of Pluto showing how it comes closer to the Sun for around 20 years of it’s 248 years journey about the Sun

Plutinos, the Scattered disk and the Kuiper Belt.

You may be reading this thinking what are Plutinos, what is the scattered disk, what is the Kuiper belt, and what does that have to do this planet 9  – actually, it has quite a lot to do with the proposed planet 9 – but I am getting ahead of things, let me go back to the beginning – back to the start.

In 1978, astronomer James Christy, working with the 155cm (61″) telescope installed at the US Naval Observatory, discovered Charon, the dwarf planet sized moon of Pluto – this was a game changer, now with a body discovered orbiting it, they could use normal orbital mechanics to work out the mass of both bodies – and they shock was that clearly Pluto was a lot smaller than anyone had dared to fear, in fact, Pluto was at least 50% smaller than the smallest confirmed planet – Mercury, the estimates at the time constrained Pluto to no larger than 2500km, Mercury was known to be 4800km (at the time), from the orbital motions of Charon the mass of Pluto was also tied down, 1.33×1022 kg which is more than 24 times less than Mercury at 3.30×1023 kg – thus you would need 24.81 Pluto’s to make one Mercury – that is a huge difference, it started the questions again about what Pluto actually was.

In the late 1970’s and into the 1980’s researchers started to find objects orbiting the Sun out beyond Jupiter, with orbits that stretched as far as Neptune and crossed the orbits of the outer planets – sound familiar? The first of these was named 2060 Chiron, being discovered on the 1st November 1977 by Charles Kowal. The size of Chiron has been a matter for debate for sometime, with estimates ranging from as little as 130km to as high as 300km, the most accepted size is 220km+/- 25km

Chiron started a raft of discoveries, and pretty soon the outer solar system was not such a lonely place – and many researchers noted the similarity between the orbit of Pluto and many of the smaller bodies being discovered in those cold vast outer reaches.

Things really heated up when Mike Brown and his team discovered Eris in the early 1990s, this body was reported as being larger than Pluto, but was clearly not even a Kuiper belt object, it was part of a family that is now called the Scattered Disk – bodies who had a gravitational tussle with Neptune and lost.  This led to the debate over whether Pluto was or was not a planet and precipitated the creation of the new designation of Dwarf Planet . This article is not going to reopen that debate, suffice it to say the debate also encouraged greater research into the region beyond Pluto and the Kuiper belt which then led to the discovery of numerous bodies in the outer solar system – many of which have been classified as Dwarf planets and many which likely will be.

Many objects appear to orbit in resonance with Neptune, including Pluto, and these have been dubbed Plutinos, the scattered disk is a family of bodies that are not in resonance with Neptune, but have been flung into elongated orbits, outside the plane of the solar system and these orbits are considered unstable over geologic time. Many will likely be flung into other orbits over time. The Kuiper belt, first proposed by the Dutch-American astronomer Gerard Kuiper in the 1950’s, is a family of objects beyond around 40AU (1 AU=149.6 million km) stretching out to possibly several hundred AU. The bodies here were theorised to be building blocks of the solar system that, due to their slow orbital rate at this distance, failed to coalesce into the larger body, and some may have even been thrown out of the inner part of the protoplanetary disk by the Ice and Gas giants as they formed. What we have since found is that there is a number of larger bodies in the inner Kuiper Belt that are the size of Dwarf planets and Mike Brown and Konstantin Batygin discovered there appears to be a clustering of these objects that is very low on the coincidence scale -implying that a larger body may lay in the outer solar system and it’s gravity is causing the odd clustering being witnessed.

Planet 9

Mike Brown and Konstantin Batygin started to notice the apparent clustering of the objects in the Inner Kuiper Belt as they discovered more of these objects – I won’t delve to deeply into the technical discussion here, but suffice it to say that the orbital behaviour of these objects rang and alarm bell that had no explanation to the team when they investigated further.

They first looked to see if this was observational bias, meaning that due to technological constraints they were only finding the larger bodies and only those bodies who happened to be closest enough to the Sun in their orbit for them to be detected – a reasonable and high probability occurrence and not dissimilar to the discovery of the first asteroids back at the start of the 19th century and similar to the detection of Exoplanets which at first were all the giant planets close in to their star making them easier to find with the technology we had at the time.

Anyway, as far as could be determined, it was not observational bias – although only a lot more discoveries will kill that possibility off completely, then Mike Brown, Konstantin Batygin and their team did a statistical analysis to calculate the probability that these orbits just happened to be similar at this point in their orbits or history – whilst the possibility was not zero, it was so low as to be statistically significant, strongly implying that there is a mass in the outer solar system that is influencing the orbits of these bodies in a way that results in the orbital mechanics we witness.

The current thinking is that this planet, Planet X, has about the mass of 10 Earths, will be about the Size of Neptune and will have an eccentric orbit that sees it come no closer than around 380AU from the Sun – that would be some find if it turns out to be true.

However, lets real this in and look at it logically.

We already know that 2MASS and WISE ruled out sub-stellar masses out to 10 light years between them, and both should have discovered a planet of this scale in the outer solar system, however,  WISE also eliminated Saturn-sized planets within 10,000 AU, and Jupiter-sized planets within 26,000 AU. WISE has also done a more sensitive search, which would pick up Neptune-sized objects, but that search has so far covered only a limited part of the sky. In 2011 WISE was put into hibernation as it’s primary mission was completed, however, in 2013 it was woken up and started a new mission as NEOWISE looking for Near Earth Objects – it is scheduled to halt operations in June 2023 unless the mission gets an extension from NASA – it’s replacement will not launch before 2026.

So where do we stand – it is conceivable that objects are in the 2MASS and WISE data which has been missed, although this seems unlikely given the continued scrutiny the data gets, there are also other reasons to question the idea of a single 10 Earth mass planet in the outer solar system.

A planet out this far would need to be dark – very dark – blacker than coal dark – or it would have been spotted by now, unless it is more massive but significantly further away than predicted. We know that Venus has volcanoes, we certainly know Earth does – in fact, in recent research it has been suggested that whilst Venus does not have plate tectonics, it does have about 850 volcanoes and many of these appear to be active – this is an energy source – so we can reasonably assume that a planet with several times the Earth’s mass will also have active volcanoes because the planet has to dissipate the heat generated in its core. Given that the outer solar system in horrifically cold, it is unlikely that a planet would have must of a gaseous envelope as it would freeze onto the surface of the planet – the core may be warm, but the cloud tops would get little to no heat from the distant Sun so they would form ices very easily unless they had a heat source below them, and we know that both Uranus and Neptune generate heat from gravitational contraction – the question is would a 10 Earth Mass planet – I suppose the answer to that question will depend on the characteristics of the planet – If it was a metal and silicate planet – like Mercury, Venus, Earth and Mars, and it had a density similar to Earth at 5.5g/cm³, then the planet may not be as large as thought, given that Uranus has a density of only 1.27g/cm³ and Neptune has a density of 1.64g/cm³, although we now believe both has solid metal/silicate cores up to 4 Earth masses.

This is a difficult one, on the one hand we assume a planet with the mass of a mini-Neptune would be of a similar size – but we have no experience of a major planet this far from it’s star, let alone one in our own solar system – determining it characteristics is a crap shoot – it may be an ice giant similar to Uranus and Neptune, but then it could be a terrestrial type world whose atmosphere long ago froze onto its surface and only have a diameter 3-4 times that of Earth – it would also be dark, interstellar radiation tends to make thing a very deep red so bodies out that far are extremely dark.

Could we have a situation where the planet is in an unexpected orbit, we have found systems with planets at very high inclinations to the plane of the system and other planets within that system – an example of this type is HD 3167, which has 4 planets, two orbit the poles, two orbit within the plane of the stystem. It is unlikely that we have a planet orbiting the poles of the Sun, but a larger body could orbit at very high inclination, how would this impact the gravitational influence of the body?

There is another potential option – it’s not a single body, but perhaps  a pair of bodies or even a system that contains the mass required to have the impact seen but inherently harder to detect. We have two systems in the solar system where the secondary is a significant percentage of the primary – The Moon is around a 1/5th the diamter of Earth, but 81 times less massive, Pluto and Charon are far closer is both size and mass, but they and Earth demonstrate that double planets are concievable and are likely more common that we may have seen to date. We see multiple star systems, with two larger stars orbiting closely whilst a third, smaller one, orbits the pairing – Alpha Centauri A-B being orbited by Proxima centauri is a good example of this – it is conceivable that Planet X could be two 4 Earth mass planets in a close binary that are orbited by a third planet of 2 Earth masses – it is unlikely, but not impossible – detecting such a family would be virtually impossible with current equipment.


Humans have a habit of seeing things that are not there, that a misobservation or faulty equipment gives us a result we exepct to see and we then continue to convince ourselves the result was real – and we can do this for a long time -history is full of humans falling into such traps. We have several examples above of how even intelligent, normally dilligent, observant researchers can fool themselves into seeing what is later clearly not there – so where are we with Planet 9 – in reality, Planet 9 likely does not exist, we are likely falling victims to observational bias which will onl;y be clarified as we discover more members of the Kuiper belt family or those within the scattered disk.

The 26 Million year impact record does not hold water when looked at in the cold light of day – but this is difficult to reach a meaningful conclusion on becase Earth is an active planet with oceans – it does not take a lot of hide or erase impact craters, even large ones, so it is highly probable we have a vastly impcomplete record of impacts on our home planet b- regardless of their size -this raises questions about bodies being pushed in the inner solar system, removes the need for an unseen companion – largely confirmed, as we saw with the 2MASS and WISE data sets, but this does not help us with Planet 9.

The  clustering of the orbital points of the inner Kuiper belt bodies we have found to date is likely the result of circumstances we have yet to discover or an, as yet, misunderstood area of gravitational theory we need to clear up – highly improbable.

The Jury remains out on Planet 9 – part of me hopes we have more to the solar system than we can currently observe, but when you weigh up all the potential criteria – it is unlikely.


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