Removed on request
In 2006 CE, the International Astronomical Union declared a new definition of “planet” which excluded Pluto because it didn’t satisfy the new criteria. These were:
- It had to orbit the Sun (or presumably another star or it’s very silly).
- It had to be almost round (so no doughnut-shaped planets?).
- It had to have cleared the neighbourhood around its orbit.
They did this because a number of large new objects had recently been discovered which were round and two, I think, were more massive than Pluto, but they didn’t want to call them planets because it would’ve led to a very large number of bodies ending up being called that. They also introduced a new category of “dwarf planet”, which included Ceres, previously regarded as an asteroid, and also Pluto and others. It does make sense to do this, although I don’t understand why they didn’t just carry on with the term “minor planet”, referring mainly to asteroids, or perhaps “planetoid”, which they’d also used a lot.
The least clear of these three criteria is “clearing the neighbourhood”. This means that a body has no other bodies of comparable size other than its moons or other bodies under its gravitational influence such as Trojan asteroids. These are asteroids which orbit 60° ahead of or behind a planet in the same orbit which are pulled there by the gravity of the Sun and the planet concerned, examples being Achilles and Hector with Jupiter. Arguably this criterion either makes Cynthia a planet or Earth not a planet, and whereas I’m fine with the former I don’t think the latter is sensible.
The word “planet” has been applied differently during different times in the history of astronomy. When the large Galilean moons of Jupiter were discovered in the early seventeenth century, they were referred to as planets, and this also happened when Ceres was discovered in 1801. A similar process to the one leading to Pluto’s demotion then ensued, with lots more “planets” being discovered until it was decided to call them minor planets or asteroids.
It’s actually quite nice to think of Cynthia as a planet because it increases the number of known planets in our Solar System to nine again, and also means the Apollo astronauts landed on another planet rather than just a moon, and it also bolsters the idea that it should have its own name. It’s the largest body within the asteroid belt which isn’t considered a planet. Leaving that aside though, one issue with Pluto not being a planet is that most people have grown up with the idea that it is one, and it’s hard to let go of apparent certainties arrived at in childhood. Its demotion is akin to the youth of today liking different music or something. To quote Abe Simpson, “I used to be with ‘it’, but then they changed what ‘it’ was. Now what I’m with isn’t ‘it’ anymore and what’s ‘it’ seems weird and scary. It’ll happen to you!”. And it did. It happens to all of us.
I exploited this idea in my Caroline Era alternate history with the discovery of Persephone and subsequent visit by Voyager III. This body is in fact either Eris or Sedna, I can’t remember which. There is also an eleventh planet according to the Caroline Era astronomers, which is whichever one this isn’t, and this could’ve happened. It isn’t an alteration to the solar system, just to what we call things, and the name Persephone has been hanging around waiting to be attached to a new outer planet for a very long time now.
When Neptune was discovered, its mass and position explained some of the vagaries of the Uranian orbit but not all. Neptune also takes more than a gross years to orbit the Sun, so it was too slow-moving to plot its orbit accurately for quite some time after its discovery. Therefore, it was conjectured that a further planet must exist beyond the orbit of Neptune. Two planets were proposed, one by the well-known Percival Lowell who elaborated the Martian canals. He proposed a planet seven and a half times Earth’s mass with a mean distance of around 6 500 million kilometres from the Sun and a period of 299 years. It would have had a diameter of around 25 600 kilometres. Those figures, which turned out to be very wrong for Pluto, are worth remembering because they suggest something else, but I’ll be coming back to this. The other proposal was from Edward Charles Pickering. He suggested a planet with a mean distance of 8 200 million kilometres from the Sun and a period of 409 years. Obviously it couldn’t be both. Incidentally, this is where “Planet X” comes from. It was Lowell’s name for this planet while it was still undiscovered. Then, after a lot of searching using photographic plates to detect the movement of the body against the background of the stars, Clyde Tombaugh detected something moving in approximately the right position. After a competition, the eleven year old Venetia Burney decided it should be named Pluto, because it was far out, dark and gloomy and therefore appropriately named after the god of the underworld, which also happened to begin with Percival Lowell’s initials.
Both astronomers had predicted a highly elliptical orbit in comparison to the other planets, and in fact its orbit is indeed considerably more elliptical than any of them apart from Mercury, and was still quite a bit more eccentric even than that. For a long time, Pluto’s satellite Charon remained undiscovered due to being very close to Pluto in both distance and size, and consequently there was no easy way to calculate its mass, so it seemed that in order to yank Uranus around sufficiently from that distance it had to be practically a solid ball of iron, probably the densest element found in large enough quantities to make up an entire planet. If Charon had been found earlier, its orbital period would’ve indicated that Pluto was in fact not very dense at all and mainly made of ices, so when it was discovered in 1978, or more likely somewhat later when its month became known, it was realised that Pluto was not nearly massive enough to account for it. Its density is only 1.88 grammes per cm3 rather than more than four times greater as it had had to be assumed. So it looks like Pluto was actually just discovered by chance and has nothing to do with perturbing Uranus. Astronomers just happened to be looking really hard at the patch of sky it was by chance crossing at the time. It was in fact fainter than expected too, because they thought it would be larger, and the size of Pluto was also overestimated for a while for the same reason as its mass. In fact, to fulfil requirements it would actually have had to be more than twice as dense as the densest atomic materials in existence. Note that that doesn’t mean “known”. The densest elements are already known because the strength of the nuclear strong force compared to the other forces in atomic nuclei allows the heaviest stable elements to be determined, and they’ve already been discovered in the form of osmium and iridium.
Pickering believed that his planet and Lowell’s were not the same, and that both existed. To return to his “Planet P” as he called it, it’s of a type which is nowadays referred to as a “Super-Earth” or “Mini-Neptune”, and these are notable by their apparent absence from our Solar System. Of all the planets discovered in the Galaxy by the current rather flawed method, the most common of all are of this type: considerably larger than Earth and considerably smaller than Neptune and Uranus. It is in fact an unresolved problem in astronomy that the apparently most common type of planet also seems to be completely absent from our own system. Some have suggested that at some point a Super-Earth did indeed orbit with us but was slung out of the system entirely, or way too far out to be easily detected, æons ago, which is why we seem so atypical.
Before I go on to the next bit, I want to talk about Uranus and Neptune, both of which were “precovered”, i.e. noted before it was realised they were planets. William Herschel published his ‘Account Of A Comet’ in 1781, where he thought he’d found a comet but it turned out to be Uranus. This planet is actually just about visible to the naked eye and could easily be mistaken for a star. Neptune is too faint for this to happen, although I wonder if nocturnal animals can see it as well as Uranus, so the idea of it being discovered when it was may be preceded by perhaps 200 million years or more, although that would only be an early mammal happening to notice a light in the sky rather than a genuine discovery. It is, though, possible that Neptune was recorded as a star by various astronomers before it was actually found to be a planet.
And this brings us up to date, because as you probably know, a ninth (tenth‽) planet may have been discovered through old telescope photographs. The IRAS project, from a satellite launched in 1983, was an infrared sky survey operating for nine months. As seen highlighted in the image at the top of this post, it may have found a new solar planet. The object in question is in roughly the right place for Planet 9 but may not be a planet at all because it’s close to the galactic plane, where there’s a lot of dust and stars, making observations rather difficult. If it is a planet, it’s about 225 AU from the Sun (33 750 million kilometres or one light day and seven light hours from it) and has a mass at least five times Earth’s. If that difference is average it would take more than three millennia to orbit the Sun and the last time it was in the position it was in 1983 would’ve been in the late Bronze Age. It may well not be a planet at all.
The reason Planet 9 might exist is that the Pluto-like bodies orbiting between 150 and 300 AU out – those are average distances by the way and the orbits are far from circular – seem to be clustered on one side of the Sun but are too far out to have their movement disturbed significantly by the gas giants we know about, so the idea is that there is a planet even further out which influences their motion. Although I’m in the Dunning-Kruger zone with this, I have my doubts because it seems to me that the bodies we know about are all currently near their closest approach to the Sun because otherwise they’d be too dim and slow to be detectable, and it could be an artifact of a small sample size. I may well be wrong about this. If it exists, the planet in question would be about five times Earth’s mass, as stated above, but also 400 to 800 times further out than us as opposed to 225. However, Pluto was discovered because of looking in the right place accidentally, so although the hypothesised planet is too close, it doesn’t mean it isn’t there. Presumably it could mean there’s yet another one further out. Some people are uncomfortable calling it “Planet 9” because they see it as insulting to Clive Tombaugh. I feel a strong urge to call it Persephone. It isn’t the hypothetical Tyche, because that would be larger than Jupiter and has been ruled out by observation at any distance closer than 10 000 AU. Tyche would actually be fairly warm incidentally, because it would be large enough to heat itself – it would be only slightly cooler than Saturn.
A super-Earth at that distance, though, would be very cold. I’m not sure how cold exactly, but it would be between -270°C and -195°C. Planets of this type are either water worlds or “gas dwarfs”. At that distance it seems unlikely it would have oceans because they’d be frozen solid, but one depiction of a gas dwarf is that it would be like this:
It could also have moons, which I find interesting because they could be warmed by tidal forces and if not, might have neon-rich atmospheres if they’re large enough.
The subject of Super-Earths and/or Mini-Neptunes is worth holding over for a post in itself, so I won’t go into more detail here, and I really think this is going to turn out to be nothing, but it’d be nice to discover another planet in our Solar System and perhaps resolve the problem of why we don’t seem to have one of this type. Alternatively, maybe a planet at that distance is far enough out to have been a rogue planet wandering between the stars or to have belonged to another solar system entirely which passed too close to the Sun and had one of its planets captured, which is exciting as well because it means we’d be able to study a planet from another star at relatively close range. It’s still over a thousand times closer than the nearest star though.
So to conclude, because good science always goes for the most boring option, I don’t think this is Persephone, but it’d be nice if it was.
A Box Of Chocolates 