If you are not a Theosophist or a student of the writings of Alice A. Bailey you may not have ever heard of a planet called Vulcan. In the mid 19th century the existence of a planet inside Mercury’s orbit was put forward to explain some peculiarities in the orbit of Mercury.
Starting in 1840 astronomer Francois Arago and mathematician Urbain Le Verrier were trying to come up with a model of the orbit of Mercury based on Newton’s laws of motion and gravity. As it turned out there was a slight peculiarity in the orbit, something called perihelion precession that differed from the value predicted by Newton’s laws. Le Verrier proposed this could be explained by the presence of a small planet inside the orbit of Mercury. He named this planet Vulcan in 1859.
An amateur astronomer by the name of Edmond Modeste Lescarbault claimed to have actually observed Vulcan. Convinced by this Le Verrier in 1860 announced the discovery of the planet Vulcan at the Academie des Sciences in Paris. At this point it was a respectable scientific theory.
Helena P. Blavatsky mentions the planet Vulcan in 1888 explicitly as “intra-mercurial”:
“… our modern astrologers are ignorant of these planets. One is an intra-mercurial planet, which is supposed to have been discovered, and named by anticipation Vulcan …”
Secret Doctrine Commentary (Transactions of the Blavatsky Lodge) by H. P. Blavatsky
Meeting 4 held at 17, Lansdowne Road, London, W., on January 31st, 1888; Mr. T. B. Harbottle in the chair.
Later Alice A. Bailey discovered the Theosophical Society and the work of Blavatsky somewhere around 1917 or 1918. Bailey started writing her own works in 1919 (to 1949). At my count she mentions the word “Vulcan” 133 times in her writings, most often in Esoteric Astrology (95 times by my count). For example:
“They could only “come in” in the middle of the fifth round, and were a group of initiates who arrested their own evolution (technically speaking) in order to take up a special piece of work on the planet Vulcan; therefore, they must return to continue and complete that which has been left undone. Owing to the results of their experience on Vulcan, the physical vehicle necessitated is of such an order that they could not at this time, and in this round, incarnate without disaster.” Esoteric Astrology, A Treatise on Cosmic Fire – Section Two – Division D – Thought Elementals and Fire Elementals
In A Treatise on Cosmic Fire she clearly identifies Vulcan as inside the orbit of Mercury which is consistent with Blavatsky and consistent with Le Verrier.
“Vulcan is within the orbit of Mercury.” A Treatise on Cosmic Fire – Section One – Division E – Motion on the Physical and Astral Planes
The problem though is that science is not necessarily static. As it turns out there really is no need to postulate a planet to explain Mercury’s orbit. In fact if there were such a planet of significant mass then the orbit of mercury would necessarily have to be different than what we observe today.
The necessity for such a planet ceased to exist when Albert Einstein published the theory of general relativity which solved the difficulties that Newton’s laws could not. There may be some asteroids inside Mercury’s orbit, but certainly no planets of any significant mass.