<<Celestial bodies interior to the orbit of Mercury have been hypothesized, and searched for, for centuries. The German astronomer Christoph Scheiner believed he had seen small bodies passing in front of the Sun in 1611, but these were later shown to be sunspots. In the 1850s, Urbain Le Verrier made detailed calculations of Mercury's orbit and found a small discrepancy in the planet's perihelion precession from predicted values. He postulated that the gravitational influence of a small planet or ring of asteroids within the orbit of Mercury would explain the deviation. Shortly afterward, an amateur astronomer named Edmond Lescarbault claimed to have seen Le Verrier's proposed planet transit the Sun. The new planet was quickly named Vulcan but was never seen again, and the anomalous behaviour of Mercury's orbit was explained by Einstein's General theory of relativity in 1915. The vulcanoids take their name from this hypothetical planet. What Lescarbault saw was probably another sunspot.
There is evidence that Mercury was struck by a large object relatively late in its development, a collision which stripped away much of Mercury's crust and mantle, and explaining the thinness of Mercury's mantle compared to the mantles of the other terrestrial planets. If such an impact occurred, much of the resulting debris might still be orbiting the Sun in the vulcanoid zone. The outer edge of the vulcanoid zone is approximately 0.21 AU from the Sun. More distant objects are unstable due to the gravitational influence of Mercury and would be perturbed into Mercury-crossing orbits on timescales of the order of 100 million years. The inner edge is not sharply defined: objects closer than 0.06 AU are highly susceptible to Poynting-Robertson drag and the Yarkovsky effect
, and even out to 0.09 AU vulcanoids would have temperatures of 1,000 K or more, which is hot enough for evaporation of rocks to be the limiting factor in their lifetime. There may be no more than 300–900 vulcanoids larger than 1 kilometre (0.62 mi) in radius remaining, if any. The gravitational stability of the vulcanoid zone is due in part to the fact that there is only one neighbouring planet. In that respect it can be compared to the Kuiper belt.
The volume of the vulcanoid zone is very small compared to the main belt of asteroids. Collisions between objects in the vulcanoid zone would be frequent and highly energetic, tending to lead to the destruction of the objects. The most favourable location for vulcanoids is probably in circular orbits near the outer edge of the vulcanoid zone. Vulcanoids are unlikely to have inclinations of more than about 10° to the ecliptic. Mercury trojans, asteroids trapped in Mercury's Lagrange points, are also possible.
Any vulcanoids that exist must be relatively small. Previous searches, particularly from the SOHO spacecraft, rule out asteroids larger than 60 kilometres in diameter. The minimum size is about 100 metres since objects smaller than 70 m would be drawn into the Sun by Poynting-Robertson drag. Between these upper and lower limits, a population of asteroids between 1 kilometre and 25 kilometres in diameter is thought to be possible. They would be almost hot enough to glow red hot. Although every other gravitationally stable region in the Solar System has been found to contain objects, non-gravitational forces, such as the Yarkovsky effect, or the influence of a migrating planet in the early stages of the Solar System's development may have depleted this area of any asteroids that may have been there.
If they do exist, the vulcanoids could easily evade detection because they would be very small and drowned out by the bright glare of the nearby Sun. Due to their proximity to the Sun, searches from the ground can only be carried out during twilight or solar eclipses. They are most likely to be between 100 metres and 60 kilometres in diameter and located in nearly circular orbits near the outer edge of the gravitationally stable zone.
In 1998, astronomers analysed data from the SOHO spacecraft's LASCO instrument, which is a set of three coronagraphs. The data taken between January and May of that year did not show any vulcanoids brighter than magnitude 7. This corresponds to a diameter of about 60 kilometres (37 mi), assuming the asteroids have an albedo similar to that of Mercury. In particular a large planetoid at a distance of 0.18AU, predicted by the theory of Scale relativity, was ruled out.
In 2000, planetary scientist Alan Stern performed surveys of the vulcanoid zone using a Lockheed U-2 spy plane. The flights were conducted at a height of 21,300 metres during twilight. In 2002, he and Dan Durda performed similar observations on an F-18 fighter jet. They made three flights over the Mojave desert at an altitude of 15,000 metres and made observations with the Southwest Universal Imaging System—Airborne (SWUIS-A).
The MESSENGER space probe may provide evidence regarding vulcanoids. Its opportunities will be limited because its instruments need to be pointed away from the Sun at all times to avoid damage. The spacecraft has already taken a few of a planned series of images of the outer regions of the vulcanoid zone. [The most favourable location for vulcanoids is probably in circular orbits near the outer edge of the vulcanoid zone.]
It is believed that the vulcanoids would be very rich in elements with a high melting point, such as iron and nickel. They are unlikely to possess a regolith because such fragmented material heats and cools more rapidly, and is affected more strongly by the Yarkovsky effect, than solid rock. Vulcanoids are probably similar to Mercury in colour and albedo, and may contain material left over from the earliest stages of the Solar System's formation.
Vulcanoids, being an entirely new class of celestial bodies, would be interesting in their own right, but discovering whether or not they exist would yield insights into the formation and evolution of the Solar System. If they exist they might contain material left over from the earliest period of planet formation, and help determine the conditions under which the terrestrial planets, particularly Mercury, formed. In particular, if vulcanoids exist or did exist in the past, they would represent an additional population of impactors that have affected no other planet but Mercury making that planet's surface appear older than it actually is. If vulcanoids are found not to exist, this would place different constraints on planet formation and suggest that other processes have been at work in the inner solar system, such as planetary migration clearing out the area.>>