In Search of Planet Vulcan

Astronomers used to believe that there is a planet closer to the Sun than Mercury that affected the planet’s orbit before Einstein’s “General Relativity Theory” made such a planet’s existence unnecessary. Many years on, does planet Vulcan really doesn’t exist?


By: Vanessa Uy


Back in the day’s when astronomers used to believe that Kepler’s Laws of Planetary Motion are part and parcel of Newtonian Physics, astronomers were at a loss in explaining on as to what have caused the advancing perihelion of the planet Mercury’s orbit around the Sun. Then came French astronomer Urbain Jean Joseph Leverrier, who’s prior fame included correctly predicted the existence of planet Neptune in 1846. Through his calculations based on the orbital irregularities of the planet Uranus’ orbit years before Neptune’s existence is proven by being seen through a telescope, Leverrier also theorized that the planet Mercury’s advancing perihelion was caused by a henceforth yet undiscovered planet. Leverrier dubbed the unknown planet Vulcan after the Roman god of volcanoes probably due to its closeness to the Sun and therefore having a very hot surface.

But as time went on, our recently updated knowledge of the natural world necessitates the reevaluation of what we’ve known before. According to Kepler’s laws, the planets move in ellipses, with the Sun near one common focus. This is not exactly the prediction of Newtonian theory. The planets, in addition to being attracted by the Sun, also attract each other, although to a lesser extent since their individual masses are much less than that of the Sun. If these small mutual perturbations are taken into account, then the accurately observed planetary motions agree closely with the predictions of the Newtonian theory, except in a few small particulars. The most notorious and most accurately observed discrepancy between theory and observation is the so-called advancing perihelion or perihelion motion of the planet Mercury.

Perihelion is the point of closest approach of a planet to the Sun. On account of the perturbations by other planets, the perihelion position changes slightly with each passage of the planet around its respective orbit. However, the observed perihelion motion of the planet Mercury has been known since the 19th Century to be much larger compared to the figure predicted by the Newtonian theory by 42 seconds of an arc per century. Quite small, but observable nonetheless.

Various explanations were put fourth, including the theoretical existence – some say in the astronomical community as “invention” – of an intra-Mercurial planet named Vulcan. Some even proposes the modification of Newton’s Law of Universal Gravitation which for all intents and purposes seem like a return to the days when the Catholic Church labeled the Copernican model of the Solar System as a “fiction convenient for calculation”. But each proposal produced fresh conflicts with observation. Albert Einstein was able to show that the additional perihelion motion was predicted directly by his theory without any further assumption. And the discovery of a particular solution of his equation – which is more commonly known as the Schwarzschild solution – made even more direct and elegant calculations possible, that lead to the correct prediction.

By about 1950, a corresponding but much smaller correction to the motion of the planet Earth’s perihelion – which is also predicted by Einstein’s theory – had been established by observation. In the case of the other planets, the effect is too small to have been observed thus far. Looks like this effect is much easier to observe near the Sun’s gravity well. Latter theories say the perturbations of Earth crossing asteroids is sufficient to explain the perihelion motion without resorting to the yet proven General Relativity of Einstein. Some even talk about the presence of “dark matter” in our solar system caused the perihelion motion. To me, all of these are enough to make every astronomer’s “day job” extremely interesting.

Despite it’s existence being continuously endangered by Einstein’s General Relativity theory, the planet Vulcan being theorized by Urbain Jean Joseph Leverrier steadfastly refuses to die out. Planet Vulcan even gained a new lease of life in the years after World War II during which it infiltrated the science fiction literate pop culture of America. There’s an episode in the original Star Trek back in the 1960’s where Captain Kirk and his Science Officer Spock must “beam in” a US Air Force pilot after their tractor beam “accidentally” wrecked his F-104 Starfighter. As the American airman inquired about Spock’s homeworld, he told Captain Kirk that planet Vulcan just lies beyond the planet Mercury. Even in the mid-1960’s, the existence of Leverrier’s planet Vulcan is more or less common knowledge despite Einstein’s General Relativity relegating it to the mythical realm.

But in the mid-1990’s, the credence of planet Vulcan’s existence gained a renaissance when amateur astronomers armed with appropriate telescopes saw “chunks of rock” other than the planet Mercury transiting the Sun. But later, those rocks were later proven to be just asteroids whose highly elliptical orbits bring them closer to the Sun than Mercury. Though some of them concluded that Vulcan therefore exists as opposed to some faraway planet created by the mind of Gene Roddenberry.