by Kathryn Boast in Astrophysics Tags: astrophysics, Einstein, Neptune, Newton, planets, Urbain Le Verrier, Vulcan

Vulcan: the original ninth planet of the solar system. Discovered by Urbain Le Verrier in 1860, it was seen crossing in front of the sun by a number of contemporary astronomers, and, with an orbit smaller than Mercury’s, hailed as a solution to one of the mysteries of astronomy.

Except that it doesn’t exist.

Le Verrier was continuining his earlier success in predicting and discovering the existence of planets. The orbits of the planets in the Solar System are determined by all the mass in the Solar System. Most of that mass is the Sun, so that has the biggest effect, but each of the planets also affects the motion of all the other planets. You can figure out the orbits from the masses, or the masses from the orbits, using nothing more complicated than Newton’s laws of gravity. (Well, maybe that’s a bit simplistic; the calculations are not terribly straightforward, but really there’s no magic involved.)

In the 1840s, it was well known that Uranus had a wobble in its orbit, and Le Verrier wondered if it might be being pulled about by an extra planet, one that no-one had spotted yet. So he set to with his calculations, and made a prediction for where this new planet ought to be found. Sure enough, astronomers pointed their telescopes to the skies and Neptune was discovered within one degree of where Le Verrier had said it would be. This was some very impressive work.

Having solved the problem of Uranus’ orbit, Le Verrier tried the same trick for Mercury. Again, the observed orbit didn’t seem to quite match with what was expected from Newton’s laws. Another planet, another prediction, another discovery. Vulcan was born.

Wait, what? An extra planet near Mercury? In 1860? Where is it now?

Well, it was never there. Mercury doesn’t follow the orbit predicted by Newton’s laws, but the solution is not to add more mass in the shape of a new planet and recalculate. Instead we must make the move from Newton to Einstein. Einstein’s theory of General Relativity makes some very subtle changes to Newton’s predictions, which are borne out in the orbit of Mercury. In fact, the beautiful match between Einstein’s predition and the observed orbit was one of the first strong pieces of evidence for General Relativity.

Sadly for Le Verrier he shouldn’t have tried killing two birds with one stone. A similar problem need not have the same solution. But the mysterious observations remain. Time and again, astronomers reported seeing something answering to Vulcan’s description either during solar eclipses or crossing the face of the sun. Although a lot of those looking were amateur observers, some were not. In combination with Le Verrier’s earlier success, these sightings led many to believe in a planet that was not there.

However the numbers never really added up – different observations suggested different orbits for Vulcan – and once Einstein got rid of the original motivation for another planet, its popularity faded. We’ll probably never know what, if anything, the astronomers spotted when they were looking for and finding Vulcan. It’s testament to the fallibility of scientists that wanting a certain result can blind us to our own observations, and it’s a bias we must always take care to avoid. Seek and ye shall find, whether it’s there or not.