Hypothetical Planets

Vulcan, the intra-Mercurial planet, 1860-1916, 1971

During the 19th century, astronomers were puzzled over unexplained deviations in the motion of Mercury. The French mathematician Urbain Jean Joseph Le Verrier, who (along with John Couch Adams) had predicted the position of Neptune based on deviations in the motion ofUranus, believed similar forces were at work. During a lecture on January 2, 1860, he announced that the solution to Mercury's deviations could be explained by assuming the existence of an intra-Mercurial planet, or possibly a second asteroid belt, inside Mercury's orbit.The only possible way to observe intra-Mercurial bodies was when they transited the Sun, or during total solar eclipses. Professor Wolf at the Zurich sunspot data center found a number of suspicious "dots" on the Sun, and a second astronomer found additional ones. A total of two dozen spots seemed to fit the pattern of two intra-Mercurial orbits with periods of 26 and 38 days.

In 1859, Le Verrier received a letter from the amateur astronomer Lescarbault, who reported having seen a round black spot on the Sun on March 26, 1859. Lescrabault thought the object was a planet transiting the Sun. He had seen the spot for about 75 minutes, during which time it moved a quarter of the solar diameter. Lescarbault estimated the object had an orbital inclination of between 5.3° and 7.3°, a longitudal node of about 183°, an "enormous" eccentricity, and a transit time across the solar disk of 4 hours, 30 minutes. Le Verrier investigated this observation, and computed the following orbit:

• a period of 19 days, 7 hours;
• a mean distance from Sun of 0.1427 AU;
• an inclination of 12° 10'; and,
• an ascending node at 12° 59'.

The diameter was considerably smaller than Mercury's and its mass was estimated at one-seventeenth of Mercury's mass. This was too small to account for the deviations of Mercury's orbit. However, Le Verrier theorized that this might be the largest member of an intra-Mercurial asteroid belt. He named it Vulcan.In 1860, there was a total eclipse of the Sun. Le Verrier mobilized astronomers throughout the world to find Vulcan. No one did. Wolf's suspicious 'sunspots' now revived Le Verrier's interest, and additional 'evidence' found its way into print just before Le Verrier's death in 1877. On April 4, 1875, German astronomer H. Weber saw a round spot on the Sun. Le Verrier's orbit indicated a possible transit on April 3 that year. Wolf noticed that his 38-day orbit also could have performed a transit at about that time. That 'round dot' also was photographed by astronomers in Greenwich and Madrid.
There was one more flurry of sightings after the total solar eclipse on July 29, 1878. Two observers claimed to have seen small, illuminated disks in the vicinity of the Sun, objects which could only be small planets inside Mercury's orbit. J.C Watson, professor of astronomy at the University of Michigan, believed he had found two intra-Mercurial planets. Lewis Swift (co-discoverer of Comet Swift-Tuttle, which returned in 1992), also saw a 'star' he believed to be Vulcan. However, it was in a different position than either of Watson's two 'intra-Mercurials.' Neither Watson's nor Swift's sightings could be reconciled with Le Verrier's or Lescarbault's 'Vulcan.'
Nobody ever saw Vulcan again, in spite of several searches at different total solar eclipses. In 1916, Albert Einstein published his General Theory of Relativity, which explained the deviations in Mercury's motions without the need to invoke an unknown intra-Mercurial planet. In May 1929, Erwin Freundlich photographed the total solar eclipse in Sumatra; the plates showed a profusion of star images. Comparison plates were taken six months later. No new objects brighter than 9th magnitude were found near the Sun.
But what did these people really see? Lescarbault had no reason to lie, and even Le Verrier believed him. It is possible that Lescarbault happened to see a small asteroid passing very close to the Earth, just inside Earth's orbit. Such asteroids were unknown at that time, so Lescarbault believed that he saw an intra-Mercurial planet. Swift and Watson could, during the hurry to obtain observations during totality, have misidentified some stars as Vulcan.
"Vulcan" was briefly revived around 1970-1971, when a few researchers thought they had detected several faint objects close to the Sun during a total solar eclipse. These objects might have been faint comets. Comets have been observed to pass close enough to the Sun and eventually collide with it.