If you say “Vulcan” to most people nowadays in an Outer Space context, the chances are they’ll think of Spock, and that’s an entirely valid thing to do. However, if you were to say it to anyone with much knowledge of astronomy in the nineteenth century, it would’ve called something completely different to mind: a planet which orbits the Sun even more closely than Mercury. I’m going to cover both in this post.
Firstly, the ‘Star Trek’ Vulcan, whose Vulcan name is Ni’Var. This is reputed to orbit the star 40 Eridani A, a member of a trinary star system also known as ο2 Eridani (Omicron-2 Eridani – that isn’t an “O”) sixteen and one quarter light years from here, and therefore also quite close to 82 Eridani, which is said to be one of the most suitable nearby stars for life, around which a possibly habitable planet orbits in real life. Of the stars, A is an orange dwarf, B a white dwarf and C a red dwarf which is also a flare star. Because B would previously have been a red giant and exploded, the chances are that any habitable planets orbiting A would have been sterilised by B’s outburst, and since C is a flare star, this is also unsuitable, although there would be nothing to stop an interstellar civilisation settling a planet in A’s habitable zone, which would of course be Vulcan.
As I’ve mentioned, I don’t pay much attention to either ‘Discovery’ or the new ‘Star Trek’ films, but I’m aware that Vulcan has been destroyed in revenge for the destruction of Romulus. I find this a bit annoying and I’m not sure what the point of it was plot-wise, but it doesn’t alter the in-universe fact that Vulcan was the homeworld of the first species to make open contact with humans when Zefram Cochrane first activated the warp drive. I’m also aware that that is inconsistent with the depiction of Cochrane in TOS. It is interesting, though, that any real planet in the habitable zone of 40 Eridani A would have been severely damaged by the 40 Eridani B supernova.
I understand Vulcan to have no moons, higher gravity than Earth and no surface oceans. I’m also aware that Romulans and Vulcans are the same species. It irritates me that they’re humanoid but also interests me that some of their anatomy and physiology is known, such as their copper-based respiratory pigment. Then again, although the in-universe explanation of widespread humanoid aliens is that we are all descended from humanoid ancestors who existed around the time our own Solar System formed, it’s also conceivable that convergent evolution would lead to similar body forms among sentient tool-using species. Back to Vulcan itself though. It has a thinner atmosphere than Earth’s, which I think justifies the copper-based blood pigment, and the sky and much of the surface is red. There are seas, i.e. large landlocked lakes, rather than oceans continuous with each other. Depending on the total surface coverage of bodies of water, I think this would probably make the planet uninhabitable for humans although clearly not for native life. 40 Eridani A is a K-type star, with a longer lifetime than the Sun’s in terms of being able to support a habitable planet, which, if orbiting at the distance necessary to receive the same quantity of light and head from its primary as we do from our Sun as a planet, would have a mean orbital radius of about 0.68 AU, i.e. sixty-eight percent of Earth’s distance from the Sun, and 223-day year. However, Vulcan is supposed to be hotter than Earth and might therefore be closer to its sun or have more greenhouse gases in its atmosphere, or it could just reflect less heat back into space, and in fact it probably would due to less ice on its surface. The difficult thing to account for with Vulcan is the combined higher gravity and thinner atmosphere, but there is another reason than gravity why a body might lose some of the gas surrounding it, which is consistent with what we “know” about Vulcan. Earth’s strong magnetic field is generated by our own large moon, Cynthia, which raises tides in our iron-nickel core and magnetises it like stroking a bar of iron with a magnet does, and that generates our magnetosphere, which traps ionising radiation from the solar wind which might otherwise reach Earth’s surface and strip away our atmosphere. Hence Vulcan, with no pre-existing satellites, would not have this benefit but would on the other hand still be able to hold on to some atmosphere because of its higher gravity, so maybe that is in fact realistic. Venus has no magnetic field but an extremely dense atmosphere, although not one hospitable to life at the solid surface, due to photolysis – the action of light on rocks releasing carbon dioxide gas. However, we’re basically aware that Vulcan’s atmosphere has enough oxygen to support human life without their own oxygen supply, and not enough carbon dioxide to poison us, which is 0.5% at our own atmospheric pressure. 170 millibars partial pressure of oxygen is required for this and CO2 cannot be making a significant contribution to the pressure, so we can surmise that the rest of Vulcan’s atmosphere substantially consists of other gases. It isn’t pure oxygen. In fact, it’s quite likely to be nitrogen if Vulcan physiology is anything like ours and their bodies consist partly of protein, as the nitrogen has to come from somewhere, so I’m going to say the mean surface air pressure is about 0.25 bars. I’ve plucked this figure out of the air, so to speak. There probably is no such thing as sea level there because of the various lakes with different presumed depths and heights, so this would be defined as some kind of mean distance from the centre of the planet or a level at which gravitational pull is close to a particular standard. The boiling point of water on Vulcan is therefore about 60°C, but we know from McCoy’s mouth that Vulcan is very hot compared to Earth, so this puts an upper limit on its surface temperature unless it’s so hot at the equator that it causes water to evaporate.
40 Eridani A is orange. The sky is likely to be close to a complementary colour, such as teal, given that, but because of the dusty surface it’s entirely feasible that it would in fact be pinkish due to small particles high in the atmosphere. Also, the general ruddiness of the planet as shown on screen gives the impression of heat and dryness, so artistically that does seem to be a good decision. The same features make some people think of Mars as a hot planet when in fact it’s often colder than Antarctica. Regarding sparse water cover, a thin atmosphere might make sense here too, particularly if water is regularly evaporating from the surface at the equator, since some might then be lost into space.
Vulcan would also lack plate tectonics if it’s like this, since that’s fuelled by water. The planet has no continents as such, but it does have active volcanoes and lava fields, which is to some extent to be expected as it corresponds to the “hot spot” situation in the centre of the Pacific plate on Earth, where magma seems to need to vent. Here, this results in Hawaiʻi, but on Vulcan a mountain range could be expected because there are no oceans. There would be nothing like the Pacific Ring Of Fire, and also no fold mountains because those are caused by the collision of continental plates.
Vulcan’s colour is depicted differently in different manifestations of the series. In TOS and Enterprise, it’s red. In TAS it’s yellower, and in TNG brownish. However, on Mars there is variation in colour from space due to a dust storm season, and this can be imagined on Vulcan too. Maybe one way to think of Vulcan is as a larger, hotter version of Mars.
The real 40 Eridani A does have a planet. This is, as usual, called “b”, and orbits much closer to the star than the inner edge of the habitable zone. It has a roughly circular orbit 0.22 AU from the star and a mass estimated at 8.5 times Earth’s (both those figures are rounded off). At Earth’s density, this would give it a diameter of around 25 000 kilometres, which is a type of planet unknown in our own solar system at any distance from us, and it’s classed as a “Super-Earth”, but it has a period of 43 days and would be like Mercury on its surface during the day, if it rotates at all. It’s also the closest known Super-Earth. Its orbit differs considerably from Mercury’s, which will become relevant later in this post, in being much less elliptical, which to me, in my probable naïveté, suggests there are no planets larger than it in at least the inner solar system.
This brings me to the other Vulcan. In the nineteenth Christian century, the French astronomer Urbain Le Verrier came up with a particularly accurate model of planetary motion within the Solar System. It had been noted that the most recently discovered planet, Uranus, tended to drift slightly behind and ahead of its predicted position given its distance from the Sun and shape of its orbit. From this, Le Verrier calculated mathematically that there was likely to be another planet further out pulling at it, and predicted its position, which turned out to be correct. In fact he almost had it named after him, but they eventually decided to call it Neptune. This established his reputation and consequently, when he turned his attention to the orbit of Mercury, people paid attention and took his views seriously.
Mercury’s orbit is quite unusual compared to the other planets, particularly if you ignore the period of time when Pluto was regarded as one. It’s the most eccentric orbit by a long way compared to the others, with a variation in distance from the Sun of around twenty percent. Le Verrier also noted that the movement of the “points” of the orbit precessed around the Sun much faster even when compared to its year of eighty-eight days than those of other planets. Just as he had with Neptune, Le Verrier proposed that there was either an as-yet undiscovered planet even closer to the Sun or a number of smaller bodies like asteroids within the orbit of Mercury, and since it would’ve been so close and so hot, he called it Vulcan after the Roman god of fire, Vulcanus. The planet’s existence could be confirmed in two ways. Either it could be detected in transit, as most planets are detected at the moment, or it could possibly be glimpsed during a total solar eclipse. A number of astronomers then reported that they had indeed seen this planet transiting the Sun. For instance, Edmond Lescarbault, a doctor, described a tiny black spot moving across the Sun faster than a sunspot, moving with the rotation of the Sun, would, and also lacking a sunspot’s penumbra. The observations even seemed to confirm Le Verrier’s prediction of Vulcan’s size and orbit. However, it was difficult to predict when these transits would occur because that depended on the tilt of Vulcan’s orbit compared to ours. Mercury, for example, can only be seen to transit the Sun in May or November because only then is the tilt of both its and our orbits aligned such that it can get between us and the Sun. The observations did seem to occur fairly randomly, but at first glance Mercury’s do as well, if you didn’t know anything about its movements already.
There was a total eclipse of the Sun in 1883, shortly after Le Verrier’s death in 1877, during which Vulcan was not observed. It was still possible that the planet was either behind or transiting the Sun at the time, but six further such observations, the last in 1908, also failed to turn it up, making it increasingly improbable that the planet existed. However since that time astronomers have claimed that close ups of the Sun’s surface do sometimes show small black dots which are not sunspots, although these may be imperfections of photographic plates, and there are asteroids which approach the Sun more closely than Mercury does, such as Icarus. It strikes me that it’s not only possible but probable that there are asteroids which orbit entirely within the orbit of Mercury, although they would have to be very small and would be difficult to observe or confirm. These are known as Vulcanoids, and would have to be between six kilometres and a couple of hundred metres in diameter. Every region of the Solar System which is not severely perturbed by the gravity of known objects has been found to contain objects like asteroids or comets, so if the innermost region of the system doesn’t have any this must be due to a non-gravitational effect. It is in fact possible that the light from the Sun is so strong at that distance that it would push smaller bodies away from it over a long period of time, so this may be the explanation. This might sound far-fetched, but it’s been proposed that this effect could be used to divert asteroids which would otherwise crash into Earth by painting them white in order that the pressure of light from the Sun would change their orbits, and this is also the principle used in a solar sail. The MESSENGER probe took photographs of the region but this was limited because damage from sunlight needed to be avoided. Much closer in than Mercury, asteroids are likely to vaporise of course.
Vulcan was considered to orbit 26 million kilometres from the Sun, giving it a sidereal period (“year”) of twenty-six days. At another point, observations appeared to show it had a year of 38.5 days. I think it was also supposed to be very small but I can’t track this down: possibly about a thirtieth the mass of Mercury, which with the same density would’ve given it a diameter of around 1 600 kilometres, probably meaning that if it had been found to exist it would’ve been demoted from planethood by now in the same way as Pluto was. In fact, if it did exist, it would indeed have perturbed the orbit of Mercury but the other factors which turned out to be the explanation for this phenomenon would still be in play, meaning that there would’ve been an even greater anomaly unless the planet happened to be exactly the right mass and in exactly the right place, and possibly retrograde. Some kind of pointless immense astroengineering project could probably achieve that to some extent, but why? Possibly to prevent us from being aware of relativity?
The fact is that the planets don’t simply orbit the Sun alone without influencing one another and the Sun. This is the famous three-body problem, that it’s impossible to work out in almost all cases how three bodies would orbit each other, and even more so the much larger number of massive bodies in the Solar System. It’s possible to work out how much gravitational influence the planets would have on each other if they were the only two bodies in the Universe though, and if initial conditions are known. For instance, Venus and Earth approach each other to within fifty million kilometres and have roughly the same mass, so left to themselves they would orbit each other at roughly twenty-five million kilometres from their centre of gravity once in forty-five millennia if I’ve calculated that correctly, and at the closest approach, which would be during a transit of Venus, that’s the gravitational pull we’re exerting on each other – about forty-five thousand times less than the Sun’s. Mercury is the least massive planet, being just over half the mass of Mars, the next smallest. Pluto is of course far lower in mass, and if Cynthia is considered a planet in its own right, that would be considerably less massive. Anyway, this means that Mercury is pulled around a lot by the other planets. Venus approaches it to within about 38 million kilometres but without doing the maths it isn’t clear if that’s the biggest gravitational influence because of Jupiter being so much more massive than the other planets, even though it’s far further away. Jupiter is over three hundred times the mass of Earth but would get within 4.8 AU of Mercury, which actually gives it roughly the same influence as Venus. But this is not the only reason Mercury’s orbit precesses as much as it does.
Albert Einstein listed a number of ways to test his theory of general relativity, one of which was the orbit of Mercury. The pull of the other planets is insufficient to explain precession in Newtonian terms. There’s still a bit left over if you try to do this. It’s at least seven percent larger than it “should” be. The explanation for this was instrumental in getting general relativity accepted. Einstein made three suggestions about how general relativity could be corroborated. One was that light would be red shifted if it passed through a gravity well. Remarkably, although it took something like four decades, the observation of 40 Eridani B eventually showed that this was so, I’m guessing because of the other stars in its system. Gravity stretches light because it distorts space. The second proposition was that stars observed near the Sun during a total solar eclipse (Again! They’re useful things) would appear to be in a different position because their light would be bent by the solar gravity, and this was indeed found to be so a few years later. However, the world had to wait for these two findings. The other one was that Mercury’s orbit would precess at the rate it did having taken into account the perturbations of all the other planets, and again this was found to be so, but in this case it was already known that this happened because Le Verrier had observed it in the previous century and the existence of Vulcan had been refuted. The reason this happens, I have to admit, I don’t really understand, but I can provide a kind of visual model of it which could show this.
The Rubber Sheet Theory is a model of space as if it’s two dimensional left to itself with weights representing stars and planets which, if placed on such a sheet would create dents in it. Obviously this is not an adequate explanation as such of general relativity for several reasons, one of which is that it uses gravity to explain gravity – that’s what’s pulling down the weights. It also makes space appear to be a substance, something which physicists had worked heavily against when they disproved the existence of the luminiferous æther, which since it was supposed to be extremely rigid wouldn’t work in this situation anyway. It shouldn’t be mistaken for Einstein’s theory itself, but it is a useful way of looking at it. In any case, if you imagine the kind of dent which shows up in the title sequence of Disney’s ‘The Black Hole’:
. . . which is like one of those charity coin collection things, space around the Sun is distorted to a limited extent like that, and attempting to do a “wall of death”-style orbit around it, which would in any case be elliptical rather than perfectly circular because the Universe is imperfect like that, would lead to your bike describing a series of ellipses which were not perfectly congruent with each other but were more like a spirograph pattern. Having written that paragraph with its references to a number of very ’70s things makes me wonder if it’s going to make any sense to someone born after Generation X.
Now I can see that this does happen, but I am also puzzled by it. Whereas I’m sure that I couldn’t aim a coin at one of those charity collection things in such a way that it would just circle around at the same level until friction interfered, and that at best if I could make it describe an elliptical path for a few revolutions, the bits of the ellipse furthest from and closest to the hole would precess, I would put that down to the fact that I, and anyone else to a lesser extent, can’t aim perfectly rather than simply due to the geometry of the hole. Nevertheless, this appears to be what I’m being asked to believe with this: that it isn’t only one’s inability to aim perfectly, or for that matter the friction the coin (or ball bearing – let’s take the instability of the coin out of the picture), that leads to this precession. But apparently not. Apparently, if you were to have too much time on your hands and designed some kind of precision ball bearing throwing machine for charity coin collectors, and it wouldn’t be popular because they want coins, not ball bearings, it would do the wobble thing even if it stayed circular enough not to fall down the hole immediately, and it would wobble more the closer it was to the whole. So they say, and this is what got general relativity accepted.
There have been other Vulcans. For instance, one of the many hypothetical planets in Western astrology is the intramercurial Vulcan, seen as the soul ruler of Taurus and orbiting once every twenty days. This Vulcan would go retrograde more often than Mercury. It’s fiery and urges the individual to look for non-physical knowledge, which makes sense given its history in astronomy. It was also suggested in a poll as the name of one of the moons of Pluto, and actually won the most votes but that was then named Kerberos after the Hadean dog, which was the runner up. Vulcan actually doesn’t seem like a very good name for a moon of an icy planet way out in the outer reaches of the Solar System, but I don’t know the reasons it wasn’t used. Maybe the IAU just didn’t want to be reminded of what they might regard as an embarrassing phase in the recent history of their science. In the Second Doctor story ‘The Power Of The Daleks’, there’s a planet called Vulcan which is settled by humans and highly volcanic with pools of fuming mercury on its surface. Doesn’t sound very nice at all really. There does not, however, seem to be an asteroid named Vulcan, which is quite surprising.
I’ve sometimes wondered if there’s a story behind the naming of the ‘Star Trek’ Vulcan and if it’s in any way connected to the hypothetical planet, but I don’t know. How about you?
A Box Of Chocolates 