Live long and prosper: the search for Vulcan and other hypothetical Solar System objects

Tuesday 20th June, 2017

Our speaker was Dr Ann Bonell from Leicester Astronomical Society and her talk was entitled "Live Long and Prosper: The Search for Vulcan and Other Hypothetical Solar System Objects". She began by saying something that is fairly unusual for the majority of the Society's talks in that we were "not going to be able to see anything that I am talking about".

The first hypothetical planet she mentioned was Vulcan (not the one featured in the Star Trek franchise). She said that in 1840 Francois Arago, who was then the director of the Paris Observatory, wrote to the French mathematician Urbain Le Verrier proposing that he use Sir Isaac Newton's physical laws to model the orbit of Mercury. He suggested this as Mercury's orbit did not match Newton's laws of motion as its closest point to the Sun did not stay constant but advanced a little every orbit.

Vulcan was thought to be within the orbit of Mercury and so very close to the Sun, making it extremely difficult to observe. Attempts were made to view this new planet when it was predicted to cross the Sun; appearing as a miniscule black dot or beside the Sun when the Sun's disc would be blotted out at the time of a total solar eclipse. Over the years some observers said they had spotted Vulcan and others refuted their observations. Finally, in 1915, Einstein published his scientific paper on General Relativity which explained the mystery of Mercury's orbit. It turns out that the huge mass of the Sun actually distorts the fabric of space and time surrounding it and this is most noticeable near the Sun — just where Mercury orbits.

Dr Bonell then told us about the search for a moon around the planet Mercury. This search was triggered when NASA's Mariner 10 spacecraft flew by Mercury in March 1974. Two days before the flyby the spacecraft's ultraviolet instrument recorded large amounts of ultraviolet radiation (or UV) that then disappeared the next day, only to reappear again three days later. Sadly, the scientists soon discovered that it was no moon but a background star, 31 Crateris, and that UV radiation can get through the gas and dust between star systems in a galaxy. In 2011 any moons of Mercury were definitely ruled out when NASA's Messenger mission began its orbital study of Mercury.

Dr Bonell then continued by saying that claims for other moons also applied to our own planet Earth. In 1846 the French astronomer Frederic Petit, who was director of the Toulouse Observatory, announced that he has found a moon in an elliptical orbit around Earth. Unfortunately, he should have looked more carefully at his predicted orbit as it would have sent the moon through the Earth's atmosphere at an altitude taken by modern airliners.

She added that there are quasi-satellites of Earth but there is only one true natural satellite — the Moon. One quasi-satellite is the asteroid 3753 Cruithne which, technically, orbits the Sun yet makes a horseshoe-shaped movement around Earth. However, the search for hypothetical objects has not ended as, in 2014, astronomers proposed the existence of a large planet in the outer Solar System similar to Uranus or Neptune.