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The planet Vulcan: a cautionary tale that deserves to be better known

The planet Vulcan was first allegedly sighted in 1859, having been proposed to orbit the Sun inside the orbit of Mercury. Further sightings were made over the next forty years or so. The properties of Vulcan were described in detail, despite the fact that astronomers are now firmly convinced that it does not exist. What actually was sighted and attributed to Vulcan remains an open question.

That scientists were confident of something that later turned out to be incorrect is hardly breaking news. After all, the inherent vulnerability of scientific theories to falsification is largely what gives the scientific method its credibility. However, the story of Vulcan is an cautionary one, providing a number of lessons that contemporary scientists and science students would do well learn.

Incidentally, this article is not about Mr Spock's home-world. Presumably, the choice of the name Vulcan for this other fictional planet is a coincidence. But it would nice to think it was intentional.

The birth of Vulcan

Our story starts in 1821, when French astronomer Alexis Bouvard published detailed observations of the orbital behaviour of the planet Uranus, which had been discovered about fifty years previously. Almost immediately, astronomers who examined Bouvard's work realised that the orbit of Uranus was not entirely in agreement with predictions made by Newton's laws of motion and universal gravitation. These laws were were already solidly established, and considered to be more-or-less unchallengeable at the time.

Over the next twenty years or so, Bouvard and other astronomers came to appreciate that the anomalous orbit of Uranus could be explained if there were another, as-yet unidentified planetary body exerting a gravitational influence on it. Predicting the orbital path of such a body -- if, indeed, it even existed -- was a mathematical, rather than an astronomical, problem -- and an extremely difficult one. French mathematician Urbain Le Verrier is generally credited with performing the calculations that allowed the new planet, now known as Neptune, to be indentified. It was definitively observed in 1846, within one degree of the location that Le Verrier had predicted. This was, and remains, a strikingly successful application of Newton's mathematics to the modelling of planetary motion.

Whilst working on the mathematics of the orbit of Uranus, Le Verrier also began studying the orbit of Mercury. He published a detailed theory of the behaviour of Mercury, again based on Newton's laws, in 1859. His results were a very close match to the observed behaviour of the planet but, as had been the case for Uranus earlier, not a perfect one. In particular, the precession of Mercury's perihelion -- the variation in the point at which Mercury comes closest to the Sun -- was larger than predicted, by the vanishingly-small amount of 43 arc-seconds per century. That's about one ten-thousandth part of a degree per year.

Given the success of Le Verrier's theoretical work leading to the discovery of Neptune, it's entirely understandable that he proposed a similar mechanism to explain the behaviour of Mercury. Le Verrier gave the name Vulcan to this putative planet, and astronomers immediately began looking for it.

The rise of Vulcan

Vulcan was "discovered" almost immediately. Le Verrier was officially recognized as the discoverer in 1860, although he did not make the confirmatory observations. The clincher -- or so it must have seemed to Le Verrier -- was an observation of Vulcan's transit of the Sun (that is, the passage of the planet across the sun's disc). However, even Le Verrier must have realized that this observation had been made in non-ideal conditions: a smallish telescope and crude timekeeping equipment. Still, over the next forty years many sightings of Vulcan were published, some by reputable astronomers working with state-of-the-art equipment. In particular, respected US astronomers James Craig Watson and Lewis Swift -- already credited with the discoveries of numerous astronomical bodies -- both reported sightings of Vulcan in 1878. Both men reported that Vulcan was red in colour, and appeared as a definite disc. This last point is highly significant because, in principle, it rules out the mistaken observation of a star. Even in state-of-the-art modern telescopes, a star usually appears only as a point of light; only bodies within our solar system are close enough appear as a disc.

In the end, at least twenty sightings of Vulcan were published by at least ten astronomers with some standing. It seems probable that a huge number of less reliable sightings were made as well. The consensus in the astronomical community at the end of the nineteenth century was almost certainly that Vulcan existed, even if was observed only fleetingly.

The fall of Vulcan

It has to be admitted that the existence of Vulcan was somewhat contentious right from the start. French astronomer Emmanuel Liais asserted that he had been observing the Sun at the same time as Le Verrier's initial reported transit, and with better equipment -- and he'd seen nothing. There's little doubt that some of the alleged transits were, in fact, sunspots; and some of the reported bright objects just stars.

Moreover, the various reported sightings did not always confirm Le Verrier's mathematical model perfectly. He continued to adjust the parameters of the model to accommodate all the sightings, but the agreement with observation was never strikingly good. Nevertheless, Le Verrier was able to publish detailed values for the radius of Vulcan's orbit (about thirteen million miles), its period of revolution (about 20 days), and a number of other orbital properties.

It's probably fair to say that nobody now believes that there is a planet with the properties that Le Verrier predicted, although the existence of some planet within the orbit of Mercury can't be ruled out. In fact, the International Astronomical Union has registered the name "Vulcan" for such a body, should one ever be discovered.

In the end, though, it wasn't lack of evidence that killed Vulcan -- there was plenty of evidence of its existence, from many different sources. Nor was imperfect agreement with Le Verrier's theory a fatal blow -- remember that the measurements of Mercury's orbital parameters only differed from Newtonian prediction by incredibly small amounts. Astronomers were trying to work out from these tiny discrepancies the orbital behaviour of a putative planet millions of miles away. Perfect agreement with theory, or even perfect agreement between observations, could hardly have been expected, with the technology available at the time.

Vulcan's nemesis turned out, in fact, to be Albert Einstein. Einstein's model of gravitation, expressed in his general theory of relativity, simply offered a better explanation of Mercury's orbit. Einstein did not need to appeal to a new planet to explain why Mercury's orbit took the form it did -- it was already perfectly in line with his own predictions. It's entirely possible that, without Einstein's contribution, we'd still be looking for Vulcan today, and perhaps finding it.

So why is any of this important?

The tale of the rise and fall of Vulcan can be considered cautionary for contemporary scientists, and science students, for at least three reasons.

First, it illustrates the problems caused by refusing to accept that a successful theory might be inadequate. Of course, we shouldn't reject an effective theory on a whim, but we have to be ready to revise our theoretical understanding in the light of new data.

By the time Le Verrier published his observations of Mercury. Newton's law of universal gravitation had been firmly established for nearly two hundred years. Together with his laws of motion, Newton's theory of gravity was able to explain with great precision a huge range of physical and astronomical phenomena. This was, of course, before Einstein upset the gravitational apple-cart with his general theory of relativity.

The long-term success of Newton's theory meant that, in the nineteenth century, it's accuracy was essentially undisputed. This meant that it was far more acceptable to propose an entire new planet -- a clear violation of Occam's razor -- than try to modify Newtonian physics.

The second -- somewhat related -- reason for caution that Vulcan highlights is a bit more subtle. Just because a hypothesis adequately resolves theory and observation on one occasion, that doesn't mean it will do so again, even in similar circumstances. In this case, the presence of Neptune was successfully invoked to explain the orbit of Uranus. This actually strengthened scientists' confidence in Newton's theory of gravity -- not that anybody really doubted it. I think it's unlikely that Vulcan would have been proposed, had Le Verrier not made such a good call with Neptune. However, with hindsight we can see that Le Verrier was over-confident here. Understandably so, of course -- Neptune had been a big win.

The third reason for caution is much simpler: Vulcan shows only too clearly that, when we look hard enough for something, we tend to find it -- whether it exists or not. It wouldn't be of much interest if one or two crackpots had "found" Vulcan -- the disturbing fact is that a number of perfectly reputable astronomers and mathematicians found it, when it was never actually there. How can that be? Presumably, the explanation lies in a mixture of wishful thinking, over-confidence, and sub-optimal experimental methodology.

It's commonplace in science for phenomena to be predicted before they are observed. We frequently allow theory to guide our observations, and it's proper to do so. After all, it's much easier to find something if you're looking for it in roughly the right place.

Consider just one of any number of examples from particle physics. The entity that we now call the Higgs boson was first proposed to exist back in the 1960s. Such a particle had never been observed, and the technology of the day would have made observation very difficult -- ambiguous at best. However, the Higgs was (and remains) necessary to explain certain features of the prevailing model of subatomic physics. The Higgs had to exist -- if it did not, then one of our most powerful scientific models would be cast into doubt.

In fact, had we not found all the novel subatomic particles that our theories predicted, unpalatable questions would have arisen about the value for money of the colossally expensive equipment we built to look for them (the Large Hadron Collider is the most expensive piece of scientific apparatus every constructed).

The Higgs particle was first detected back in 2012. It's been detected many times since then, although agreement between measurement and theoretical prediction of its properties is currently not perfect.

Does any of this sound familiar at all?

For the avoidance of doubt, I should point out that I'm not proposing that the Higgs boson doesn't exist. Increasing amounts of compelling observational data suggests that it does. All I'm suggesting is that the story of Vulcan should make us take a step back occasionally, and ask: are we sure about this?

Closing remarks

The story of Vulcan is not an edifying one, and none of the main actors emerges with much credit. Key features of the history of Vulcan are a hidebound, unquestioning attitude to theory, and a willingness to accept sloppy, inconsistent data as confirming that theory. Vulcan shows how a dogged refusal to put observation ahead of theory leads to wasted time and effort at best, and damage to the credibility of science at worst. Most importantly, the story of Vulcan is a salutary reminder that we tend to find what we're looking for, particularly when money and reputation are at stake. Anybody who cares about the scientific endeavour should think about Vulcan from time to time.

[ Last updated Tue 22 Feb 19:28:25 GMT 2022 ]