A Modest Proposal for Naming a Hypothetical Distant Planet in the Solar System

Abstract

The need to choose appropriate and meaningful names for the objects of scientific inquiry, in the spirit of Michael Faraday and, on a different level, of the ancient Chinese doctrine of rectification of names (正名, Zhèngmíng), is illustrated here in the case of the so-called Planet Nine. Since before the discovery of Neptune, the fascinating hypothesis of the possible existence of a new, distant planet in the solar system, yet to be discovered, has regularly surfaced in the pages of astronomy journals in various guises. Its most recent incarnations have been tentatively given names such as Planet X, Planet Y, and, most famously, Planet Nine. Such labels are unsatisfactory because they reveal no significant physical or orbital properties of the object which they are attributed to. I propose here the name Telisto, from the ancient Greek word τήλɩστoς for ‘farthest, most remote’ which captures a feature common to all versions of this scenario that seems destined to remain at the forefront of astronomical research for a long time to come: its supposedly great heliocentric distance, estimated at several hundred astronomical units. By exploring the history of astronomy, I also respond to some criticisms that might be leveled at this proposal. Among other things, I also draw a comparison with the naming of the so-called axions, which are hypothetical elementary particles proposed almost fifty years ago and which continue to be an active object of research.

1. Introduction

The fascinating possibility that a distant, still undiscovered planet may lurk in the remote outskirts of our solar system has been around for a long time since the pioneering work by1 Percival Lowell in 1915 [2]. In it, he postulated the existence of a trans-Neptunian planetary body, which he referred to as ‘Planet X’, responsible for what were then thought to be irregularities in the motions of Uranus and Neptune. For the record, such a planet should have had a mass of 7 Earth masses, an inclination to the ecliptic of 10 degrees and a distance from the Sun of 43 astronomical units. It is worth noting, with a pinch of irony, that if it had been discovered, it would have been, at the time, the ninth planet of the solar system: a sort of Planet Nine ante litteram! Thus, the character ‘X’ in the denomination by Lowell pointed just to its still unknown status, like the unknown in an equation, and not to the Roman numeral 10. Instead, things turned out differently. Lowell died a year later, and in 1930, Clyde Tombaugh discovered what for the next 76 years would be officially considered the actual ninth planet in the solar system: Pluto. The events leading to its discovery were recollected by Tombaugh himself in [3,4,5]. It is worth remembering that what is now only the first of many Kuiper Belt objects to have been discovered does not have the physical characteristics predicted for Lowell’s Planet X.

In the years that followed, the original motivations that led Lowell to postulate the existence of a distant planet faded. This was not the case for the Planet X scenario, having undergone several reincarnations for various reasons mainly related to the structure of the Kuiper Belt; see, e.g., the references in [6,7,8], as well as Schilling [9] for a popular account. It is worth noting that Planet X, after the discovery of Pluto, could well have been considered the tenth planet in the solar system, being, now, ‘X’ synonym of ‘ten’.

Of all these versions of Planet X, the most recent and, at the moment, the most famous also in the mass media and in the general press, is the one that envisages a massive perturber causing the particular grouping of certain orbital features of some trans-Neptunian objects [10,11]. This time, the new alleged planetary tenant of the solar system would have a mass of about 5–8 Earth masses, an inclination to the ecliptic of ≃11–21 degrees, and a perihelion distance ranging from about 240 to 385 astronomical units [11]; a very distant object indeed! Given the difficulty of revealing it with optical and/or infrared surveys [12,13,14,15], currently fruitless apart from some possible candidates to be further investigated, it would seem that this intriguing scenario is destined to remain current in astronomical research for a long time to come. Since Pluto was downgraded to the status of a simple dwarf planet in 2006, the aforementioned putative celestial body, proposed for the first time in 2016 [16], would represent a new incarnation of the ninth planet of the solar system, hence the provisional name ‘Planet Nine’ with which it became famous both among researchers and the general public.

But that is not all, because Planet X has recently been resurrected in the form of a possible lighter, less inclined, and closer object than Planet Nine [8]. Indeed, the new planetary candidate, motivated by certain clustering properties of the orbits of a set of trans-Neptunian Objects, should have a mass of about 3–5 Earth masses, an inclination to the ecliptic of ≃2–12 degrees, and a perihelion distance of ≃150–270 astronomical units [8]. In this case, ‘X’ should be understood as ‘unknown’, and not as ‘ten’ since it could not co-exist with Planet Nine.

Finally, the latest addition to the large family of distant planetary candidates seems to be Planet Y [17]. The role played by it would be warping the mean plane of the distant Kuiper Belt. Its mass would lie between the masses of Mercury and the Earth, its inclination to the ecliptic would be larger than 10 degrees, and its semimajor axis would be in the range of ≃100–200 astronomical units. The existence of Planet Y would not be in conflict with either the latest incarnation of Planet X by Siraj et al. [8] or Planet Nine since they would induce different effects on the Kuiper Belt objects. Interestingly, if Planet Nine or the Planet X of Siraj et al. [8] really existed, then Planet Y would become the tenth planet of the solar system. Therefore, some may be tempted to dub it Planet Ten or, as such, Planet X!

I humbly feel myself part of that respectable and time-honored tradition counting among its leading exponents Michael Faraday which holds that choosing appropriate names for scientific terms is an integral part of active scientific research. To this aim, it is worth recalling that Faraday, who could not boast of a formal academic education, devoted great care and attention to coining from ancient Greek, with the help of the polymath William Whewell, names that reflected as accurately as possible the characteristic physical properties of the new concepts and effects he was discovering from time to time. Thus, I can only consider the names proposed so far for the hypothetical planet(s) to be unsatisfactory. The reasons for that, along with a modest proposal for improving the situation, will be explained in the following. Such an effort might also be viewed, in some ways, as a modern application of the ancient Chinese doctrine of rectification of names (正名, Zhèngmíng), although it, actually, had mainly a political and social nature.

2. ‘Telisto’ as a More Appropriate Name for a Distant Planet

Names such as Planet Nine, Planet X or Planet Y have no informative content since they do not point to any physical and/or orbital feature of the planetary candidates which they refer to; they are simply anodyne labels that could be replaced with any other equally devoid of informative content. As the previous analysis should have elucidated, they are extremely confusing since they are subject to any unessential classification problem that could, overnight, render them obsolete. Furthermore, Planet X may also be intrinsically ambiguous since it is unclear if ‘X’ should refer either to the Roman numeral 10, which would be the case should a ninth planet exist, or to the unknown character of the planet named by it. In the former case, the role of Planet X, meant as Planet Ten, would be played by Planet Y.

As previously noted, it would be much better and meaningful if a name addressing some peculiar features of the object at hand, both of physical and/or orbital nature, were chosen. To this aim, I unpretentiously suggested using the name ‘Telisto’, coming from the ancient Greek word2 τήλɩστoς, -η, -oυ (‘farthest, most remote’), for the first time in3 [7] for a generic distant planet and, subsequently, in [18] specifically for Planet Nine. One advantage of this name is that it, actually, does refer to an unquestionable orbital feature common to all the planetary candidate(s) of interest here, namely their great heliocentric distance. It is also immune from any possible future classification problems. Furthermore, it has a certain degree of flexibility since it can be combined with appropriate prefixes to form names able to meaningfully distinguish different candidates. In this respect, the latest version of Planet X by Siraj et al. [8] and Planet Nine may be conveniently dubbed Microtelisto and Macrotelisto, respectively, in view of the smaller values of the key physical and orbital parameters of the former one with respect to the latter one. Last but not least, ‘Telisto’ is reusable since it may well be attached to any new planet that is actually discovered as long as it is very far from the Sun, as if to say that ‘one name fits all’; it may also be used to designate a class of distant planets, the Telistos, should more than one be finally discovered.

Finally, given that the motivation for Planet Y resides in the warping of the plane of the Kuiper Belt, it may be appropriate to rebrand it ‘Lygizon’, from the ancient Greek verb λυɤίςω (‘to bend; to twist as one does a withe’), derived from the noun λῦɤος, -oυ, ή (‘willow-twig, osier, withe’).

3. Some Possible Objections to the Use of ‘Telisto’

Here, in a procataleptic style, I preemptively address several possible objections to the proposal raised in this opinion article.

First and foremost, one may wonder whether this discussion is really that necessary, given that one is still talking about an object for whose existence there are so far only indirect clues, as it has not yet been directly revealed in any band of the electromagnetic spectrum by telescopes. I think the answer is yes. Indeed, while the Planet Nine scenario has now steadily grabbed headlines in many general astronomical press outlets, the flood of academic articles devoted to it suggests that it is destined to remain a hot topic in current astronomical research for a long time to come. In any case, there seems to be no contraindication to being prepared for the moment of its possible discovery.

Against my naming suggestion, it may be argued that only the actual discoverers of what I call Telisto would be entitled to propose a name for it. On the other hand, history of astronomy demonstrates that not always the names proposed by the discoverers of some planets actually became of common usage, and it can even happen that the discoverers themselves renounce their privilege of naming the planets they decisively contributed to spot. The historical account by Gingerich [19] on how Uranus and Neptune finally took their current names after their discoveries is enlightening in this regard. Indeed, after William Herschel discovered the seventh planet of the solar system in 1781, he proposed the name ‘Georgium Sidus’ for it to honor King George III, noted patron of sciences who, among other things, granted him a pension. Nonetheless, such a denomination did not gain much favor in the astronomical community of the time among whose members a controversy around the name of the new planet arose. It was destined to last for about fifty years until a general consensus was gradually consolidated around the name of the ancient Greek deity Uranus, first proposed by Johann Bode, who, incidentally, had no role in its discovery. It is worth noticing that, at a certain point of the long-lasting debate, Jérôme Lalande proposed the name ‘Herschel’. In the case of Neptune, discovered in 1846, the name ‘Oceanus’ was first proposed for it by the British astronomical community in recognition of the contribution of John Couch Adams to its discovery for which Urbain Le Verrier was, instead, fully credited. Both astronomers theoretically predicted its position in the sky, but it was the Le Verrier’s calculations that ultimately led to the actual discovery at the telescope by Johann Galle4 with the aid of Heinrich d’Arrest. Instead, the French astronomers converged on the noun of the ancient Greek deity Neptune, likely at Le Verrier’s suggestion. After the name ‘Le Verrier’ was subsequently proposed by François Arago, who was persuaded by his friend Le Verrier himself to take over the discoverer’s privilege of naming the planet, Neptune was finally internationally accepted. Interestingly, the French almanacs reintroduced the name ‘Herschel’ for Uranus during the time the name ‘Le Verrier’ was in use for Neptune. For Pluto, things went as follows [20]. Mrs Venetia Phair was an 11-year-old Oxford schoolgirl passionate for Greco-Roman mythology when she learned of the recent discovery of the then ninth planet of the solar system while reading a newspaper with her grandfather. As soon as she told him her suggestion about the Roman god of Underworld, he went to see Herbert Hall Turner, his friend and professor of astronomy at the University of Oxford. Ironically, the latter was attending a conference in London at the time, where, among other things, there was a vibrant discussion about the name to give the new planet. Fortunately, nothing was decided, and when the grandfather of Mrs Phair had finally the opportunity to speak to Turner, the latter was enthusiastic about the name Pluto and telegraphed the Lowell Observatory. About one month later, the name Pluto was formally adopted at the suggestion of an ordinary person who took no part in its discovery. Be that as it may, it may well not be a priori excluded that the actual discoverers of Telisto/Planet Nine, if any, will take into account my modest proposal in the end.

Another objection which may be raised is that the name suggested in this opinion article could be confusing with Telesto, a small irregular moon of Saturn known also as Saturn XIII or Tethys B. Actually, the latter comes from the ancient Greek proper noun Τελεστώ, -oῦς, ή of an Oceanid, a group of water-nymphs of the Hellenic mythology. Therefore, it seems unlikely that anyone may ever confuse a major planet with such a minor body whose name has, among other things, a completely different origin.

One may also want to remark that the name ‘Telisto’ presently denotes both a popular fruit-flavoured drink sold in powdered form in Venezuela and two different pharmaceutical companies. It can well be argued that, if on the one hand, it is just a simple coincidence without any meaning, on the other it is unclear why any mythological character, Greek or otherwise, or animal should be deemed as more respectable or preferable than some harmless commercial products which, purely by chance, would bear the same name as the one I coined from philological arguments. Furthermore, it is worth remembering that the axions, a class of hypothetical light bosons that have long been the subject of both theoretical and experimental studies so far, derive their name, given them intentionally and consciously by the Nobel laureate in physics Frank Wilczek and later accepted by the Nobel laureate in physics Steven Weinberg [21], from a dishwashing liquid product. It was chosen to stress that such putative particles would have ‘cleaned up’ the specific theoretical problem for which they were purposely introduced for the first time in 1978 [22,23]. In turn, the name of the detergent itself originated from the ancient Greek adjective äξιoς, -ία, -oυ (‘worthy, goodly, estimable; meet, fit, due; sufficient for’) to emphasize its efficacy in accomplishing its cleaning task.

With a certain malice, one could even insinuate that, with my suggestion, I am creating a sense of false prestige. Although I will most likely not be among the direct discoverers of Telisto, if any, I have nonetheless contributed to the characterization of its parameter space using the planetary precessions of some other known planets in the solar system [6,18]. Furthermore, by exploring the possibility that they may have left a signature in the telemetry of some interplanetary probes, I have contributed to quantifying the potential level of such an alleged bias [7]. All of this might indicate that, after all, I too may have some standing to say on the naming issue.

Finally, it is worth noting that the rules for naming of solar system objects and features by the International Astronomical Union (IAU) do not say anything about possible new major planet(s), as it can be noted at https://iauarchive.eso.org/public/themes/naming/#majorplanetsandmoon accessed on 5 December 2025, being limited only to dwarf planets, satellites of planets in the solar system, minor planets, meteor showers, and comets.

4. Conclusions

Properly naming of physical concepts and objects should be deemed as fully part of the scientific research activity, as should have been clear since the shining example of Michael Faraday.

In this respect, the provisional names currently in use to denote one or more hypothetical planets allegedly residing in the remote peripheries of the solar system seem unsatisfactory. Indeed, anodyne labels as Planet Nine, Planet X, Planet Y and any other such names that may be coined in the future does not convey any useful information on either the physical or the orbital features of the objects they are attributed to. A discussion on this aspect does not appear inappropriate, considering that more or less well-founded speculation on the possible existence of such a distant planet-sized body has been going on even before the discovery of Neptune and that the Planet Nine scenario is likely to remain a hot topic in astronomical research for a long time to come.

Given that a feature common to all the recently proposed planetary candidates is their likely huge heliocentric distance, amounting to up to a few hundred astronomical units, I suggested to naming them ‘Telisto’, from the ancient Greek word for ‘farthest, most remote’. Such a name, coined philologically and representative of a key feature of the object it is attached to, namely its remarkable distance, is immune to any classification issues that could arise in the future making a name like Planet N, $N\in \mathbb{N}$, obsolete overnight. Furthermore, it is not restricted to any specific incarnation of the distant planet scenario, being possible to attribute it to any planet that is actually discovered one day, as long as it is much further from the Sun than the other planets known to date. Last but not least, it is rather flexible, being possible to combine it with appropriate prefixes to better specialize it to some specific features of the object that should bear it.

It is unlikely that anybody may really be confused regarding the name I suggested and Telesto, one of the small irregular moons of Saturn named after a minor deity of the ancient Greek mythology.

The purely casual fact that a commercial drink and two pharmaceutical companies are also named ‘Telisto’ should be considered nothing more than a funny if irrelevant coincidence. After all, the name of the dish detergent ‘Axion’, intentionally attributed in 1978 by the Nobel laureate in physics Frank Wilczek to a class of hypothetical light bosons to stress that its introduction would have ‘cleaned up’ certain theoretical problems arisen in the field of elementary particles, has been around for 47 years now.

Regarding the privilege of naming newly discovered planets in the solar system, the history of astronomy shows that the titles initially proposed by their actual discoverers were not always accepted by the astronomical community. In some cases, the name that finally prevailed was put forth by someone who had no role in the discovery of the body in question. The cases of Uranus and Pluto are emblematic in this regard. Furthermore, credit for a discovery was not always given to those who materially spotted a celestial object through the telescope, as in the case of Le Verrier and Galle for Neptune. There have also been cases in which those who had the privilege of naming the planet decided of their own free will to transfer it to someone else, as in the case of le Verrier and Arago for Neptune. Interestingly, the current rules for naming of objects in the solar system by the International Astronomical Union do not treat the case of a possible new major planet.

Considering that I too, in my own small way, have made some, albeit modest, theoretical contributions to the characterization of the parameter space of the hypothetical distant planets in the solar system, the previous considerations may suggest that I too have some say in suggesting a name.