You could be forgiven for thinking that, provided you accept Earth is roughly spherical, the two options for understanding the Solar System are either that the Sun and planets orbit us or we orbit the Sun. Both of these seem like quite simple solutions for how the Universe, or this bit of it at least, works. However, these are not in fact the only options. Tycho Brahe, for example, thought this was an accurate portrayal of the state of affairs:
Tycho held that Earth is too heavy and slow to be in motion, and therefore that it must be stationary, but accepted that the movements of the planets when they went retrograde was best explained by the idea that they orbited the Sun, so he concluded that the Sun orbits Earth but the other planets orbit it, except for our own satellite. Before that, the geocentric system included the idea that everything orbited us but also described little circles in its own orbit to explain how planets go retrograde.
Two thousand years before Tycho, Πυθαγορας (Pythagoras) founded a school or cult which had a whole host of unusual ideas, while including elements which have persisted to the present day. The trouble with the figure of Pythagoras is that it isn’t entirely evident that he existed, something he has in common with many of his near-contemporaries. Nonetheless, the ideas associated with him and his cult are quite clearly delineated, and one of those was the first model of the Solar System which dislodged Earth from the centre and asserted that we were in orbit like the other planets. The Pythagoreans also believed that all things were made up of numbers, so if you happen to believe in the simulation theory, that’s kind of what they thought too, in their own way, which just shows how old the ideas expressed in ‘The Matrix’ really are. Not wanting to go too far off track though, Φιλόλαος (Philolaos) came up with the actual cosmology. The initial idea was that the planets were in spheres whose motion produced a sound inaudible to human ears but which was in perfect harmony, meaning that the ratios of their orbits had to be harmonious in musical terms. The Cosmos, which is spherical, expanded out from a central point at a steady speed in all directions. At this centre is situated the central fire of the Cosmos, which is both an unlimited element and central. The Sun is a mirror, reflecting this fire, and Earth rotates once a year as it orbits, meaning that the central fire is constantly visible in the antipodes but not from Greece. Also, and this is where the Counter-Earth comes in, there is a twin Earth on the other side of the fire which is also invisible due to the way Earth rotates.
Why is there a twin Earth though? Two reasons: it makes the number of orbiting bodies plus the central fire up to ten, which according to Pythagoras is a perfect number, and because only Earth and Counter-Earth were massive, enabling the Solar System to be balanced. All the other orbiting bodies are made of fire and therefore fairly insubstantial. So the reason is partly numerological. Although this theory is wrong, it’s not completely wrong, and it might also be noted that according to this our planet is round. You have to go back an extremely long way before the idea of Earth being flat was dominant among the intellectual élite of Western culture. This planet, in any case, is referred to as “Antichthon”.
This idea of a Counter-Earth has been very persistent. It does at first seem to make sense to think that for all we know, there’s another Earth on the other side of the Sun which we never see because it takes exactly the same amount of time to orbit and is always behind it. It’s also a very appealing idea. It is, however, impossible without enormous forces being deployed to keep it or us hidden, and it must be them because we can’t detect any. Here’s an illustration of the problem:
We orbit the Sun in a slightly elliptical path with the Sun at one focus, or rather, the barycentre of the Earth-Sun system there. To a very limited extent, the Sun is also orbiting us but not significantly – it’s just slightly off-centre. Although it takes roughly 365¼ days to get round the Sun, it doesn’t happen at a constant speed. Kepler’s laws of planetary motion include the most influential of all, the third, which was to give rise to Newton’s theory of universal gravitation. This states that the square of the sidereal period (year) of a planet is directly proportional to the cube of its mean distance from the Sun. This is easiest to work out with Saturn, since it takes about thirty years to orbit and is about ten times the distance of Earth from the Sun. The cube of ten is a thousand and that’s square root is close to thirty, thirty squared being of course nine hundred. Now Earth is between 1.01675 and 0.9832899 AU from the Sun-Earth baycentre, so it can be calculated to reach a maximum velocity of 30.29 kps and minimum of 29.29 kps, meaning that if Earth was at aphelion and Antichthon perihelion and a line drawn through their centres passed through the barycentre of the Sun-Earth system, Antichthon would be due to appear from behind the Sun after about six hours. Also, the barycentre would be at the centre of the Sun rather than to one side because, as the Pythagoreans correctly surmised, the system would be balanced between the two planets pulling the Sun in opposite directions.
The stable locations in an orbit between two bodies are referred to as Langrangian Points, and they do not correspond to where one might at first expect them to be. L1 is where the pulls of the Sun and Earth are equal, and since the surface gravity of the Sun is 109 times ours, it will be about 1.5 million kilometres above the equator at noon on an equinox. Likewise L2 on the other side at midnight, at the same distance. Jupiter has a collection of asteroids situated at L5 and L4 known as the Trojans, sixty degrees ahead and behind. It’s also possible that there are dust clouds in cis lunar space in the same places, L4 and L5, referred to as Kordalewski Clouds. If Antichthon was the same size and mass as Earth, it would have influenced the trajectories of spacecraft aimed at Venus and Mars to the extent that they would not have taken their predicted courses and would have failed to enter orbit, failed to land or crashed into their targets. At this distance from Antichthon, the gravitational influence would be equivalent to 0.46 parts per thousand million, but it’s possible to land rovers and landers precisely on the surface, which would not be the case if Antichthon was of the same size as Earth. The L3 point on our orbit is also unstable in this star system because Venus would come within about thirty million kilometres of it and has a gravitational pull similar to ours. This also influences the position of Earth itself. Hence there can be no Antichthon in the sense of being a planet the same size as ours. However, so far as I can think nothing rules out the possibility of a much smaller body or perhaps a dust cloud in that position, which is too small to detect from here and is difficult to detect due to the Sun’s glare, and in fact I think such an object very probably does exist. It could be an asteroid, a group of asteroids, a rubble pile or a dust cloud, but the chances are it is there, just as there are likely to be objects in the other four locations. The same applies to the other planets, meaning that there are forty such spots in the Solar System, plus innumerable others associated with moons.
Antichthon has cropped up a lot in fiction. Probably the most famous example is Mondas, the Cybermen’s home world, which is in ‘Doctor Who’. In the final William Hartnell adventure, ‘The Tenth Planet’, an Antarctic tracking station finds that the Zeus IV spacecraft is being pulled off course by an unknown force. This is in fact what would happen if Antichthon actually existed, and it’s also described as having left its previous orbit due to the arrival of Cynthia (“The Moon”), which led to inhospitable conditions and the need to use prosthetics on the inhabitants. Mondas is shown as identical to Earth. This is actually quite plausible for a ‘Doctor Who’ story and it also explains why Mondas hasn’t been detected, except that nowadays it’s not where the Whoniverse ended up going because the revival decided to put them in a parallel universe instead. This is one use for Antichthon in fiction: it enables a single “parallel timeline” compared to factual history, though only one. It’s also feasible that a previously stable orbit could be disrupted by the arrival of a large moon on this side.
The 2011 film ‘Another Earth’ is based on the parallel universe premise, in that so far as I can tell it seems to posit the idea that the new Earth encountered in the film is absolutely identical down to individuals, whose lives may however have taken different courses, so this is a kind of “what if?”/”if only” scenario. Back in the Anderson ‘verse, there’s a film called ‘Doppelgänger’, also known as ‘Journey To The Far Side Of The Sun’, which (spoilers follow but this is pretty obscure nowadays) involves the discovery of a mirror world on the other side of the Sun, where everything, including apparently organic molecules, has reverse chirality. As I haven’t seen it, I don’t know if this means food on the other planet turns out to be poisonous or non-nutritious to the astronaut, or much about it at all.
I have vague memories of a children’s sci-fi book where Antichthon was still populated by non-avian dinosaurs, but I haven’t tracked it down.
The most notorious use of Antichthon is in John Norman’s ‘Gor’ series, which raises all sorts of political and ethical questions due partly to the writing itself and also due to the public response to it. I’m probably going to go into this in greater depth in a post of its own, but for now I’ll just cover it sketchily. In the ‘Gor’ series, Antichthon is called Gor, which apparently means “rock” in the lingua franca, and has been populated by advanced aliens abducting humans through history and forcing them into a technologically primitive state. There are other aspects which I’m reluctant to mention, although I will in future, but the whole thing strikes me as a bit of a wasted scenario because of what Norman did with it. Here once again, though, is an attempt to portray a single alternate history and in this case an alternate scenario which the author very disturbingly regards as utopian. There’s so much to say about this that it deserves a post of its own, but for now I will say that, the appallingly extreme sexism notwithstanding, I can empathise with an attempt to anchor a fantasy in reality and plausibility this way, although even saying that feels like I’m being too kind to him about these atrocious novels. I also get the impression that the fact that there is just one alternative suits Norman quite nicely.
That, then, is all I want to say about the matter. I think it would be interesting to send a space probe to our Lagrangian points to discover what’s actually going on there. It would be particularly nice to know that there was a Pluto-sized body there, although I’m not sure what the maximum diameter and mass possible for it not to be detected would be. Even a few pebbles would be good. But what are the chances of that getting paid for?
A Box Of Chocolates 