Vulcan is a hypothetical small planet that orbited between Mercury and the Sun.

Reasons for Vulcan's existence

Vulcan was proposed to explain a small perturbation in Mercury's orbit from the path predicted by classical mechanics, technically called advancing perihelion.

During Mercury's orbit, its perihelion advances by a small amount each orbit. The measured value is about 43 arcseconds in a century. Even if very small, this result is incompatible with the laws of Kepler and Newton.

This idea and the name "Vulcan" was postulated by the French mathematician Urbain Le Verrier in 1859, closely following his spectacular success in "discovering" the planet Neptune in the same way - using only calculus. Various persons and astronomers around the world attempted to prove the existence of said planet.

The search for Vulcan

Observing a planet inside the orbit of Mercury is extremely difficult, since the telescope must be pointed very close to the Sun, were the sky is never black. Also, an error in pointing the telescope can result in damage for the optics, and injury to the observer. The huge amount of light present even quite away from the Sun can produce false reflections inside the optics, thus fooling the observer into seeing things that do not exist.

The best strategy for observations is to wait for the planet transit on the Sun disk. A small, round dark spot can be seen moving, like it regularly happens with Mercury and Venus.

For half a century, observers tried to find the hypothetical Vulcan. Many false alarms were triggered by round sunspots, that closely resembled a planet transit. During solar eclipses, stars close to the Sun were taken for planets. To reconcile different observations, at some point two planets were postulated.

Search conclusion

In 1877 Le Verrier died, still convinced of having discovered another planet. With the loss of its principal proponent, the search for Vulcan cooled down. After many years of searching, astronomers were seriously doubting the planet's existence.

The final act came in 1915, when Einstein's theory of relativity explained the perturbations as a mere byproduct of the Sun gravitational field. His equations predicted slightly different results than classical mechanics, and exactly in the right amount to explain the different orbit.

The difference should apply to the orbits of all planets, but only Mercury is so close to the Sun that the effect becomes visible.

A resurge of interest in the theory occurred in the 1960s, and planets called Vulcan began appearing in the science fiction of the time. In a Doctor Who story entitled "The Power of the Daleks" the setting is the Earth colony on Vulcan in the early twenty-first century, while the name was used for the fictional home planet of the Vulcan race in Star Trek. According to Gene Roddenberry, Vulcan orbits either the star 40 Eridani, or Epsilon Eridani. In 1998, scientists indirectly detected a Jupiter-sized planet around Epsilon Eridani. It is not known if this planet is capable of sustaining life or not, but many astronomers point to its highly elliptical orbit as a damaging factor.

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