Asteroids are the most abundant minor objects in the Solar System and most of them are distributed within the main asteroid belt between Mars and Jupiter, and in the two groups of Trojan asteroids located 60 degrees ahead of and behind Jupiter in its orbit around the Sun. Asteroids in these regions of the Solar System can reside stably over billions of years. Apart from these regions, are there other regions of the Solar System where long-lived stable belts of asteroids can possibly exist? In this article, I will describe the possibility of a long-lived stable belt of asteroids existing close to the Sun, in a region of space that is interior to Mercury’s orbit around the Sun.
Mercury is the closest planet to the Sun and it orbits the Sun at an average distance of 57.9 million kilometres or 0.387 AU, where 1.0 AU is basically the mean distance of the Earth from the Sun. The asteroids that belong to this hypothesized belt of asteroids that exists interior to Mercury’s orbit are referred to as the Vulcanoids. The Vulcanoids are a population of intra-Mercurial asteroids that exist in a region between 0.09 AU and 0.20 AU from the Sun. The inner edge of the Vulcanoid belt is approximately 0.09 AU from the Sun as any asteroid orbiting closer to the Sun than this will have an unstable orbit due to perturbation by the intense radiation of the Sun. On the other hand, the outer edge of the Vulcanoid belt is approximately 0.20 AU from the Sun as any asteroid located outside this boundary will be perturbed by Mercury’s gravity.
As the asteroids in the hypothesized Vulcanoid belt orbit the Sun at high velocities in a relatively small volume that is entirely interior to Mercury’s orbit, evolution through the mutual collisions of asteroids will be much more frequent than in the main asteroid belt between Mars and Jupiter. Models of the collisional processes have shown that for the Vulcanoid belt, only a few hundred asteroids larger than one kilometre in size could have survived until the current epoch. Because collisional evolution of asteroids proceeds fastest at smaller distances from the Sun, it has been proposed that a favourable location to search for existing Vulcanoids is near the outer edge of the Vulcanoid belt.
It is estimated that the region between 0.16 AU and 0.18 AU is most likely to contain surviving kilometre-sized Vulcanoids since objects near 0.20 AU from the Sun can get perturbed by the gravity of Venus. The closeness of the Vulcanoids to the Sun makes them extremely challenging to observe from the Earth. However, this closeness is expected to cause the Vulcanoids to be hot enough to give off a significant amount of infrared emission, making infrared detection methods the best choice for detecting the elusive Vulcanoids. To date, no Vulcanoids has yet been discovered and the Vulcanoid belt remains merely as a hypothesis.