Astronomers Inventing Planets Based on Circumstantial Evidence

It happened three times in the past that theoretical astronomers invented a new planet on the basis of indirect and circumstantial evidence. The first time was in 1845 when Adams and Leverrier independently deduced the existence of the planet Neptune from the perturbations which it had produced in the motion of Uranus. One year later, Neptune was duly discovered in the predicted region of the sky. The successful prediction of the presence of an unseen planet was one {31} of the great events of nineteenth-century science. It impressed the educated public of that time as a spectacular demonstration of the power of human reason to uncover Nature's secrets. The second prediction of a new planet was in 1859 when the same Leverrier, now world-famous from his prediction of Neptune, turned his attention to Mercury. He looked at the unexplained deviation of the motion of Mercury from its theoretical orbit, and deduced the existence of one or more planets circling the Sun inside the orbit of Mercury. This time the end of the story was not so happy. The name Vulcan was given to the undiscovered planet. Many people observed Vulcan, either as a bright speck near the horizon at sunset or sunrise, or as a dark speck silhouetted against the disk of the daytime Sun. Unfortunately, the various observations could not be reconciled with any consistent Newtonian orbits. As the techniques of observation improved, belief in Vulcan's existence slowly faded. Finally in 1915 Einstein showed that the deviation of Mercury's motion was explained as a consequence of general relativity without any help from Vulcan. The third prediction of a planet was also in 1915, when Lowell deduced from residual perturbations of Uranus the existence of another planet beyond Neptune. This time the planet Pluto was found in the predicted place, but subsequent observations showed the mass of Pluto to be too small to produce measurable perturbations of Uranus. The discovery of Pluto appears to have been a happy accident, less impressive than the discovery of Neptune as a demonstration of the power of human reason. Adding up the score, we may say that out of three theoretical predictions of new planets, one turned out right, one turned out wrong, and one turned out to be a fluke.