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I know there are some people thinking, "Vulcan, is he talking about the fictional planet Vulcan in Star Trek?" No, there was a real search for a planet named Vulcan in our not so distant past. Today every schoolchild learns that our solar system contains nine planets in orbit around the sun, plus a variety of other bodies such as asteroids. The discovery of these facts was far from straightforward. In some cases, the successes are stunning proof of the veracity of Newtonian mechanics; in others, such as the quest for a hypothetical planet "Vulcan" orbiting well inside Mercury, the fallacies and failures are equally staggering, or are they?
Before it spawned Spock, the "planet Vulcan" was proposed to orbit inside Mercury to account for a chronic deviation in Mercury's predicted orbit. When amateur astronomer Edmond Lescarbault claimed to see the culprit, case closed, right? Nevertheless, try as they might, no one else could observe the fugitive; still, theoretically, its existence suited world-class mathematician Urbain Jean Joseph Le Verrier, so he offered proof. Le Verrier made his name by explaining perturbations in Uranus orbit in terms of an unknown planet, duly discovered as Neptune in 1846. Astronomers tramping to solar eclipses to glimpse Vulcan, the repeated futility of which finished it off, but the problem of Mercury's orbit persisted. Einstein eventually solved it: a curvature of space-time, not a planet, explained Mercury's orbit.
Le Verrier (Director of the Paris Observatory early in the 19th century) in pursuit of his obsession with extending the universality of Newton's Laws, proving the planet Vulcan's existence, and securing his place in history as one of the greatest astronomers of his time was all for naught, as history would have it.
The planet Vulcan? Hey, wasn't that just a made-up planet Gene Roddenberry created for Star Trek? Not at all. For a period of many years in the late nineteenth century, some, if not all, of the world's astronomer's believed in the existence of a planet Vulcan that orbited the Sun inside the orbit of Mercury. Vulcan was actually "observed" quite a few times through the telescope by both professional and amateur astronomers.
The story actually starts in 1781 with the discovery by William Herschel of the planet Uranus. It soon became clear to astronomers that Uranus was behaving badly, it was not moving along the orbit predicted for it by Newtonian physics. What could be the matter? Was Newton wrong? Impossible! If Newton was not wrong, then he had to be right, and something else had to be causing the odd orbit of Uranus, something doing so in obedience to Newton's laws. The obvious answer was that there was another planet beyond Uranus, the gravitational influence of which was causing Uranus to orbit as it did.
Finding this hypothetical planet was a huge challenge. In the early 1840s, two mathematicians, John C. Adams of England and Urbain Jean Joseph LeVerrier of France (shown above on a 1958 French postage stamp, England has never so honored Adams), both started working on the problem independently. It was incredibly complex, for it required taking into account the gravitational influences of the Moon, the Sun, and the known planets on the orbit of Uranus and then using the nature of the unexplained Uranian movement to predict the orbit of the new planet. Adams and LeVerrier solved the problem almost simultaneously. On September 23, 1846, German astronomers in Berlin, using LeVerrier's predictions (Adams had been somewhat shy about publishing his work) discovered Neptune. The discovery was hailed, quite properly, as a great victory for Newtonian theory. LeVerrier and Adams went on to great fame. The discovery of Neptune was, it should be noted, a great embarrassment to astrology, which had never even hinted at the existence of such a planet. The same was true of the earlier discovery of Uranus and the later discovery of Pluto. Not to be fazed, however, astrologers attributed influences to the planets following their discoveries.
Impressive as the discovery of Neptune was, another challenge to the Newtonian view of the solar system remained. Mercury was also orbiting in a fashion that was not predicted by Newton's laws. It was natural to try the same approach to the problem of Mercury's orbit as had been applied so successfully to the case of Uranus. And try LeVerrier did. He spent much time and effort throughout the rest of his productive life calculating where the planet Vulcan, interior to Mercury's orbit, should be. He was occasionally buoyed by supposed reports of sightings of Vulcan where the calculations, sort of, said it should be.
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LeVerrier died in 1877 and so never knew the solution to the mystery of Mercury's orbit. Its orbital deviations were shown by Einstein in 1915 to be due to relativistic effects of the Sun's huge mass bending space-time. These effects are utterly trivial for planets further away from the Sun.
Skeptics will be especially interested to note that while astronomers abandoned Vulcan by the early twentieth century, astrologers, no doubt stung by being caught out when Uranus, Neptune, and Pluto were discovered, have refused to give up on Vulcan.
So the effects of another planet are seen, very much so with Mercury, but since the other planet has not been seen yet, it was presumed to be closer to the sun then Mercury. After a lifetime of people looking for an explanation to the strange behavior of the planet Mercury, it was finally accepted to be a space-time fluctuation. In addition, this space-time distortion is the accepted reason, even today!
Although it is not a point of fact, it is interesting that the fictional planet Vulcan from star trek also shared an orbit with another planet.
“The planet Vulcan is located in the same spiral arm of the Milky Way as the Earth. The Vulcan's solar system is currently comprised of three planets, the largest of which is their home world. Many centuries ago, Vulcan had several small iron-nickel moons. At an early stage in Vulcan space flight, these moons were mined as a source of ore, since the planet itself was poor in ferrous materials. The lack of moons is more than compensated by Vulcan's sister planet, T'Khut. Vulcan (T'Khasi) and T'Khut are unique in that they revolve around a common point in the same orbit. T'Khut dominates the sky and its closeness causes gravitational stress on the planet Vulcan. This is a major factor in planetary tectonic activity and maintains active volcanism."
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