If Leverrier’s Vulcan existed, it would not normally be seen in the brilliant blue of the daytime sky (any more than Venus is) — but it would blaze out during solar eclipses as the brightest object in the sky. Since solar eclipses are seen from some part of the world virtually every year, Vulcan’s existence would have been known long ago.
What Swift and Watson saw — if it was real at all — was probably a small comet passing near the Sun. Great comets blaze brilliantly, and grow long tails; but small comets look like faint ‘woolly’ balls of gas. In Watson’s telescope, one of these would have appeared more like the tiny disc of a planet than a point-like star.
At the end of the last century, no professional astronomer had yet confirmed Vulcan’s existence. Its leading proponent,
Leverrier, had died in 1877, and the general mood was highly sceptical. The name `Vulcan’ dropped out of the indexes of popular astronomy books. But the orbit of Mercury did undoubtedly swing round. Although Simon Newcomb had successfully denounced the supposed observations of Vulcan, his meticulous analysis of Mercury’s motion showed that the swinging of its orbit was real, and even a little larger than Leverrier had calculated. No one could explain it. No one, that is, until Einstein.
For 35o years scientists had calculated what the planets’ orbits should be using Newton’s law of gravitation. In 1915, Albert Einstein proposed a new theory of gravitation, which he called general relativity. Although it looks a lot more complicated than Newton’s law, general relativity does turn out to give the same answer for the gravitational pull of one body on another when that pull is quite weak. When gravity is very strong, though, differences appear between the predictions of the two theories.
The strongest gravitational pull in our solar system occurs close to the Sun. Einstein worked out what his theory predicted for a planet in Mercury’s orbit: according to general relativity, Mercury should not follow the same oval over and over again, but the oval orbit should gradually swing round. Einstein calculated the rate of swing to be 43 seconds of arc per century (a second of arc is 1/3600 degree of angle). This is exactly the rate at which Mercury’s orbit does swing forward.
So what had appeared to astronomers as a `perturbation’ of Mercury by another planet was not that at all. Mercury’s motion disagreed with theory because astronomers had been using the wrong theory of gravitation.
With the publication of the theory of general relativity, the theoretical foundation for believing in Vulcan was knocked away. In fact, there can be no massive planet within. Mercury’s orbit, or its gravitation would upset the agreement between Mercury’s motion and Einstein’s theory (which has been verified thoroughly by other tests).
But there may be smaller bodies orbiting the Sun more closely than Mercury. These could be small rocky worlds only 6 to 3o miles ( so to 50 kilometres) across — very like some of the smaller asteroids, which orbit the Sun between Mars and Jupiter. These inner asteroids would be very dim, ten times fainter than the human eye can see.
In 1979, a team of American astronomers calculated that a swarm of a million small asteroids could exist here, the closest circling only 2 million miles (3 million kilometres) above the Sun’s fiery surface. In all, they would constitute a ring of matter, very much like the rings around the planets Jupiter, Saturn and Uranus.
Another American astronomer, Henry C. Courten, has taken photographs during eclipses to look for such asteroids. He believes that an asteroid belt within Mercury’s orbit is dominated by one large asteroid, between 8o and 500 miles (13o and Boo kilometres) in diameter — just as the Mars-Jupiter asteroid belt sports the asteroid Ceres, which is as heavy as all the others put together. If the existence of this possible asteroid belt is eventually confirmed by spaceprobes, no doubt the largest will indeed be named Vulcan — though it will be a world less than a tenth the size of its elusive and infamous predecessor of the same name.