Inside the orbit of planet Mercury, a little over one-third as far from the Sun as Earth, lies a region of space which appears to be entirely empty. This is an unusual feature of the inner solar system, most of which possesses a variety of small to mid-sized asteroids and other objects. Indeed, until about a century ago, astronomers believed that there must be a ninth planet within this region of space after all, and even gave it a name: Vulcan.
Today we know through analysis of Mercury’s orbit that there is no ninth planet orbiting even closer to the Sun, but the search for asteroids within this region, known as vulcanoids, does continue. Even studying Mercury is difficult because of its proximity to the Sun. Observation from Earth can only occur at certain intervals, and if these observations are made with the human eye (either naked or aided by a telescope), special precautions must be taken to avoid being harmed by the overwhelmingly bright light of the Sun. These problems are magnified even further when one begins to look within the orbit of Mercury, to see whether any objects lie between the Sun and its innermost known planet.
At the same time, the existence of objects within the orbit of Mercury, even large planet-sized ones, would not violate any current laws of astronomy or physics. Mercury is too small, and too close to the Sun, to keep moons of its own, and the Sun’s gravitational influence even causes trouble for probes orbiting Mercury, as Messenger soon will be. However, a planet could still exist within this space. It would likely have no moons, and its surface would certainly be fried to a crisp by solar radiation – but it could exist.
In the past, many astronomers were convinced that such a planet must exist. In the 1840s and 1850s, French astronomers calculated that Mercury’s orbit was perturbed slightly from where it ought to be according to Newtonian physics. The most logical explanation for the discrepancy, they reasoned, was that there was a previously unseen planet tugging Mercury slightly off its natural course. Urbain le Verrier, the mathematician who made the calculations, decided to call the hypothetical planet Vulcan, reasoning that any planet so close to the Sun would be aptly named after the Roman god of fire.
Discovering planets through theoretical mathematical analysis was not entirely unheard of; indeed, Le Verrier had recently been part of the discovery of Neptune by just such a process (and that planet was subsequently located by astronomers, once they had been given some idea of where to look). Le Verrier’s optimism seemed to have been borne out in 1859, when another French astronomer, Edmond Lescarbeault, claimed he had witnessed a mysterious object transit across the surface of the Sun within the orbit of Mercury while studying it with his telescope. Le Verrie proclaimed the discovery of his planet Vulcan.
Subsequent searches, however, failed to confirm the existence of such a planet. The search for Vulcan continued for more than fifty years, decades after Le Verrier himself passed away, and produced large numbers of false positives eventually attributed to sunspots or other stars seemingly close to the Sun. This theory persisted until Mercury’s orbit was analyzed from the perspective of Albert Einstein’s revolutionary theory of relativity in 1915. Einstein’s analysis showed that the perturbations in Mercury’s orbit could be accounted for as a result of the Sun’s own gravitational field. There was no planet Vulcan. Thanks to the theory of relativity, scientists know today that there is no Vulcan. If there were, it would cause even greater perturbations in Mercury’s orbit than those predicted by relativity, and these would have been detected.
However, this still leaves researchers with the original mystery: why space within Mercury’s orbit is, to the best of our knowledge, empty. Today astronomers are not searching for planets within Mercury’s orbit, but they are searching for asteroids, known as vulcanoids after the mystery planet once thought to lie there. So far, no such vulcanoids have been found – which only deepens the mystery. Any object smaller than about 70 metres across would have been pulled in by the Sun’s gravity, and any object larger than several dozen kilometres in diameter would certainly have been detected by our solar observation satellites. This strange empty space remains an unsolved mystery.