Vulcanoid

Any vulcanoids must be between about 100 metres (330 ft) and 6 kilometres (3.7 mi) in diameter and are probably located in nearly circular orbits near the outer edge of the gravitationally stable zone between the Sun and Mercury.

The vulcanoids, should they be found, may provide scientists with material from the first period of planet formation, as well as insights into the conditions prevalent in the early Solar System.

[1] 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.

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.

[3] Vulcanoids, should they exist, would be difficult to detect due to the strong glare of the nearby Sun,[4] and ground-based searches can only be carried out during twilight or during solar eclipses.

[9] Later attempts to detect the vulcanoids involved taking astronomical equipment above the interference of Earth's atmosphere, to heights where the twilight sky is darker and clearer than on the ground.

A Black Brant rocket was launched from White Sands, New Mexico, on January 16, carrying a powerful camera named VulCam,[13] on a ten-minute flight.

[14] The MESSENGER space probe took a few images of the outer regions of the vulcanoid zone; however, its opportunities were limited because its instruments had to be pointed away from the Sun at all times to avoid damage.

[18] All other similarly stable regions in the Solar System have been found to contain objects,[8] although non-gravitational forces such as radiation pressure,[9] Poynting–Robertson drag[18] and the Yarkovsky effect[5] may have depleted the vulcanoid area of its original contents.

Objects more distant than this are unstable due to interactions with Mercury and would be perturbed into Mercury-crossing orbits on timescales of the order of 100 million years.

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.

[13] Vulcanoids, being an entirely new class of celestial bodies, would be interesting in their own right,[24] but discovering whether or not they exist would yield insights into the formation and evolution of the Solar System.

[24] If vulcanoids are found not to exist, this would place different constraints on planet formation[24] and suggest that other processes have been at work in the inner Solar System, such as planetary migration clearing out the area.