The Planet Vulcan (No, Not THAT One …)

“Vulcan” is best-known as the fictional homeworld of Mr Spock from “Star Trek”. But for a time, Vulcan was thought to be a real planet that existed within our own solar system.

Urbain_Le_Verrier
Urbain Joseph Le Verrier                               photo from Wikicommons

In 1687, Sir Isaac Newton published Philosophiae Naturalis Principia Mathematica, which laid out for the first time the mathematical laws of gravity. Using Newton’s basics (and the orbital laws worked out by Johannes Kepler), the astronomer Pierre-Simon de Laplace then was able to work out the orbital mechanics of all the seven known planets, and also develop a theoretical framework for the formation of the solar system (the “nebular hypothesis”) which is still considered to be largely correct today. Between 1798 and 1825, LaPlace published a five-volume work titled Traité de Mécanique Céleste (“Treatise of Celestial Mechanics”) which described his findings in detail. It was one of the first scientific works to be based solely on science and mathematics, without any religious or biblical inclusions.  (The Emperor Napoleon Bonaparte, upon meeting LaPlace, is said to have remarked that the book describes the entire universe without once mentioning its Creator, to which the scientist famously replied, “I had no need of that hypothesis.”)

But there were two minor details of LaPlace’s system which did not quite fit—the calculated orbits of Mercury and of Uranus did not exactly match the observations that astronomers were making, and as telescopes became more and more powerful, it became clear that something was wrong.

So, in 1846, the matter was taken up by the French astronomer Urbain Joseph Le Verrier, who was serving as Director of the Paris Observatory. After careful calculating, Le Verrier decided that the inconsistencies in Uranus’s orbit were being caused by the gravitational influence of another planet. After some more math, Le Verrier determined what orbit this hypothetical new planet would need to follow, and then projected its future path. And in September 1846, when the German astronomer Johann Gottfried Galle trained his telescope on the designated spot, he indeed found the predicted planet, within one degree of the position calculated by Le Verrier. The new planet was named Neptune. It was a stunning achievement.

Then, Le Verrier turned to the problem of Mercury. After his success with Uranus, it was quite natural for Le Verrier to assume that Mercury’s orbit must be also perturbed by another unknown planet. After much math, Le Verrier published in 1859 what he thought to be the predicted orbit for the new planet, lying between Mercury and the Sun.

Almost immediately he got a response. An amateur French astronomer named Edmond Modeste Lescarbault, who was a doctor in the village of Orgères-en-Beauce about 40 miles outside of Paris, reported that he had seen the shadow of a planet on the Sun earlier that year in March which did not match with Mercury or Venus. Le Verrier, elated, dubbed the new planet “Vulcan”, after the Roman blacksmith god of fire, and announced the discovery at the next meeting of the French Academy of Science. Lescarbault was awarded with the Legion of Honor. 

But confirmation of the finding proved to be enormously difficult. If it existed, Vulcan would be very close to the Sun and very hard to observe directly. The best option was to wait for times when the planet would pass between the Earth and the Sun and cast a dark spot that could be seen moving across the solar surface, a phenomenon known as a “transit”. Another good time to look was during a solar eclipse, when the light from the Sun was blotted out.

And so, for the next 60 years, astronomers would search for it. Most found nothing. A few reported success, but the results were always ambiguous—they could be accounted for by sunspots that mimicked a transit, or by nearby stars during an eclipse, or some other phenomena. Le Verrier periodically updated his calculations and announced new orbits with a new predicted time of transit, but none of them panned out, and although most astronomers accepted that Vulcan was really there, nobody could seem to find it.

It took Albert Einstein to finally solve the mystery. Einstein published a series of papers in 1915 and 1916 that laid out his General Theory of Relativity, which remains today the basis for nearly all of modern cosmology. The new theory demonstrated that a body with mass warps the spacetime around it, and this change in the geometry of space is where “gravity” comes from. Using complex math, Einstein further calculated that the mass of the Sun would warp the spacetime around it in such a manner as to influence the orbit of Mercury—exactly matching the irregularities in the planet’s orbit that had already been observed. In 1919, British astronomer Sir Arthur Eddington measured the deviation of light during a solar eclipse and determined that it matched Einstein’s predictions. Einstein’s math was correct, and the theory of relativity could explain the orbit of Mercury all by itself without any need to postulate another planet to account for it. “Vulcan” could not be observed by astronomers, Einstein showed, because it did not really exist.

But then, decades later, Vulcan was resurrected—sort of.

In 1964, when Gene Roddenberry was working on the concept for his proposed scifi TV series “Star Trek”, he included the character of “Mr Spock”, who came from the planet Vulcan. It’s unclear why he chose that name: there are several possibilities. As a science fan, he almost certainly knew about the once-postulated planet that didn’t really exist. Roddenberry also liked Roman mythology (and had named another of his fictional planets “Romulus” after the mythical founder of Rome). Another possibility comes from his previous TV work: he had earlier worked with actor William Shatner on an episode of “The Outer Limits” which featured “Project Vulcan” as a plot point.

For whatever reason, Roddenberry chose the name “Vulcan”. Further, in 1990 just before his death, while compiling his “series bible” which set out the backstory for the “Star Trek” universe, he noted that Spock’s Vulcan was located around the star “40 Eridani A”, which actually exists: it is part of a triple star system about 16 light years away from Earth in the southern constellation Eridanus. 

And then, years later, “Vulcan” reappeared again, sort of. In September 2018, astronomers from the Dharma Planet Survey, using an earth-based 50-inch telescope, found a “super-Earth” orbiting 40 Eridani A. The rocky planet is eight times the mass of Earth and orbits the star every 42 days.

Officially, the planet is catalogued as HD26965b. But, naturally, it has been given the unofficial nickname of “Vulcan”.

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