From Vulcan to Neptune

From the multitude of medals representing the entire Solar System excels one that depicts not only the usual planets, but asteroids and even sets up memorial to planet Vulcan. But where is this planet Vulcan?

leverrier reverse

Alphee Dubois made a bronze commemorative medal in 1884 in honor of a discovery. The medal was made by the Paris mint. The title BRONZE and the cornucopia struck into the edge refer to this. Its diameter is 68 mm and weighs 166 grams. It is not considered to be a rare thing when you issue a commemorative medal in honor of a discovery. It is much rarer when the same coin anticipates a discovery. What happens if the discovery of which we are sure does not exist? Can we certainly state in the possession of our trusted theories the existence of something we have not found yet? As we will see the answer is no. But let us go in turn. The discovery of Neptune led to the chain of events, which was very adventurous and has become a shining example of the French-English rivalry.

The discoverer of the planet Neptune was Urbain Le Verrier (1811-1877) a French mathematician who - in contrast to previous discoveries, which astronomers made with telescopes - he used paper and pencil at his desk to find the eighth planet of the solar system. The fact that there should be an eighth planet is already became clear in 1841 to John Couch Adams (1819-1892), an English mathematician and astronomer, who - correctly - attributed the deviation of orbit of the planet Uranus from the calculated one to an unknown planet. He sent his calculations to Sir George Airy the English Astronomer Royal, who showed no particular interest and returned the calculations for "further clarification". Adams made the mistake not to pursue the matter, leaving the question of the new planet to lay off.

leverrier obverse

In 1845-46 Le Verrier, entirely independently of Adams came to a similar conclusion regarding the unknown planet. He calculated its position in the sky where it should be. The results were sent to many of the astronomers of the world, but he also had to experience the lack of interest of his colleagues. Finally, in June Airy - seeing that Adams and Le Verrier reported similar coordinates - convinced the head of the Cambridge observatory, James Challis to launch the search. But Challis instead of aiming the telescope at the given coordinates had led a systematic search in August and September at the given part of the sky, without any result.

Meanwhile, Le Verrier wrote a letter to Johann Gottfried Galle the Director of the Berlin Observatory, urging to start the search. Heinrich d'Arrest - who then was a student in the observatory - has suggested to Galle to compare the recent drawings from the area, with what they see in the telescope. After all, if the planet exists, in the meantime it also had to move relative to the stars. On the same evening, September 23, 1846, when Le Verrier's letter was received, they found the Neptune, less than 1 degree from the point Le Verrier calculated. It later became clear that Challis saw the planet twice in August, but had overlooked it. Thus, the final glory went to the French and Germans.

At first glance there is nothing special on the medal. On the obverse Le Verrier's left-facing bust is visible. Legend around is: "U.J.J.LE-VERRIER DE L'ACADEMIE DES SCIENCES. 1811-1877". Over the shoulder of Le Verrier, the year 1884 and the engraver's name in shown. On the reverse the Solar System is drawn by the artist, mostly by allegorical persons. In the middle the Sun on a Quadriga is visible. Around him are the planets in order of their distances from the Sun. After Mercury, Venus, Earth and Mars - at the top of the medal - the asteroids are visible (PETITES PLANETES). The fact that the asteroids were displayed on the coin indicates that an astronomer helped the design of the coin. The number of known asteroids now is more than a hundred thousand. The first was discovered in 1801, and in 1884 when this medal was made their number was already taken over 100. They are not a negligible part of the Solar System, yet most modern coins are missing them.

Continuing the line of distance from the Sun, the picture of Jupiter, Saturn, Uranus and the newly discovered Neptune follows. Do not look for Pluto. It had to wait until 1930 to be discovered. The found of Pluto - as in the case of Neptune - is resulted from the fact that the planet's gravitational field distorted the orbits of other planets, leaving a trail of its existence and inspired the search. But it is not a really big problem that it is missing from the medal, because since 2006 Pluto is no longer considered a planet, but a member and the namesake of another group of celestial bodies - the plutoids. It could be on the medal only as the asteroids, as a group of celestial bodies. However, there was not any indication in 1884 for this.


What is then the planet - the Vulcan - which is placed at the bottom of the medal? There is no such planet, and neither was in 1884 in the Solar System. How could it get on a medal, of which an astronomer certainly was present at the design? The answer can be supported by an interesting phenomenon, the perihelion precession of Mercury's orbit. This phenomenon was already known in 1884. Mercury has not moved in the solar system, as it should have. If all the orbit disturbing effects of the other planets were taken into account, there remained a small part that could not be explained in any other way, just as an undiscovered planet, the Vulcan, which is orbiting even closer to the Sun as Mercury. The idea aroused the interest of many scientists and laymen. Here is an example - a short text of the Hungarian Kassai Raisz Miksa on the planet Vulcan - in the text box.

Kassai Raisz Miksa : The planet Vulcan


Dealing with the mathematics calculation laws of the absolute movement of the planets of the solar system for several years, one of the results of my calculations led to the conclusion that between the Sun and Mercury another planet - suspected by more astronomers -, the planet Vulcan must exist. For this my results are as follows:

The planet Vulcan's diameter (axis) = 724.9752 km; path on orbit 5,502,355 km per day; 229.264 km per hour; Tropical movement of 98,059.16 km in a day; Its distance from the Sun 11,436,932 km. Yearly periodic movement performed during 13.21651 days.


Természettudományi Közlöny XCIII. kötet, 202-ik füzet

June 1886

The fact that the Vulcan so far was not discovered was easily understandable, since Mercury is also difficult to be observed, because it never gets too far away from the Sun. A planet that is orbiting even closer to the Sun easily got unnoticed. But its gravitational effect is betraying it. And if Neptune's existence was revealed by the differences between the calculated and actual orbit of Uranus isn't it logical to assume that the discrepancy in Mercury's orbit is caused by an unknown planet? Logical indeed logical, but unfortunately it is wrong. Vulcan was never found, because it did not exist. For the explanation we had to wait until the early 1900s. Albert Einstein's theory of general relativity gave the correct answer to this phenomenon. The gravitational effect of the Sun's huge mass distorts (curves) the space so the nearest planet, Mercury is not moving according to the rules of classical physics. The proof of the correctness of the theory of relativity was that it exactly described the motion of Mercury. Where classical physics has reached its limits, and failed, there the general theory of relativity prevailed. There is no further need for the Vulcan, but it cannot be erased from the medal. So it is left on it, as a memento, warning posterity that even a seemingly sure discovery should not be advanced, or, as Shakespeare says, there are more things in heaven and earth, Horatio, than are dreamed of in your philosophy.