Not all asteroids in the solar system reside in the asteroid belt. The main asteroid belt between Mars and Jupiter does holds most of our solar system’s asteroids, at least several million, but there are many other smaller groups of asteroids. Almost every gravitationally stable region of the Solar System has at least one known object orbiting there – all of them, except one.

For example, there are the thousands of known “Near-Earth Asteroids” that orbit, as you might expect, near Earth. Then there the “trojan asteroids” that trail ahead of and behind several of the solar system’s major planets. Jupiter alone has over 10,000 known trojans, but every major planet from Venus through Neptune has at least one that we’ve discovered. And beyond Jupiter, there are icy objects like the centaurs, the Kuiper belt, the scattered disk, and the Oort cloud – though these are technically not considered asteroids.

But there’s one place in the Solar System where we think there could be asteroids, but still haven’t found any. That place is inside the orbit of Mercury: the Vulcanoid zone.

The Vulcanoid zone isn’t (alas) named for Spock’s species from Star Trek. Instead, it’s named for the once-theorized planet Vulcan, which was supposed to orbit very close to the Sun and which was proposed to explain why Mercury’s orbit didn’t perfectly follow the predictions of Isaac Newton’s theory gravity. The proposed planet would have named for the Roman god of fire and the forge. Vulcan was never directly observed, however, and the hypothesis was made unnecessary when Albert Einstein explained Mercury’s orbit using General Relativity.

But even though we no longer think that there’s a full-sized planet closer to the Sun than Mercury, there still could be asteroids in that same region. If we could identify one and examine it through a telescope, it could tell us loads about the formation of Mercury and the inner Solar System – including Earth.

So why haven’t we found any vulcanoids? One possibility is that something other than gravity has depleted all the asteroids from the Vulcanoid zone. The prime suspect here is the Sun. The sunlit side of Mercury is nearly 800 °F (or about 420 °C), and the Vulcanoid zone would be even hotter. Any vulcanoids that survived that heat for the 4.5 billion years the Solar System has existed would probably have to be largely metallic in order to resist the Sun’s heat.

That said, we know of plenty of metallic asteroids in farther-out orbits. We’ve also found several asteroids that dip down into the Vulcanoid zone for part of their orbit – the first known one being the aptly named “1566 Icarus”, discovered in 1949. So the Sun’s heat alone shouldn’t have gotten rid of all the vulcanoids.

Another possibility goes back to General Relativity: “Poynting–Robertson drag”. Relativity states that light itself carries a small amount of momentum. From the perspective of an asteroid speeding along on its orbit, the asteroid runs into more photons of light on the side facing forward along its orbit than the side facing backward – like how more rain hits your front windshield than your back windshield when you’re driving along a highway. But when the asteroid re-emits that energy, it emits it evenly in all directions.

The net effect is that everything orbiting around the Sun is slowly losing momentum and falling closer into the Sun. This effect destabilizes the orbits of very small vulcanoids, under about 100m across, and any vulcanoids closer in than about 0.06 AU – about 1/6th the distance from the Sun to Mercury.

One last problem is collisions. At the high speeds required for stable orbits near the Sun, even a collision with a small impactor could knock an asteroid into an unstable orbit. Alan Stern and Daniel Durda estimated in 1999 that only a few hundred vulcanoids could plausibly remain.¹

Still, a small population of vulcanoids – metallic, about 1 km across, and about 0.2 AU from the Sun – could very well exist. If they do, there’s a much more mundane reason we haven’t found them yet: we can’t point our telescopes that close to the Sun.

A vulcanoid at its farthest from the Sun is only about 10 degrees of angle away from the Sun as viewed from Earth. From the ground, any Vulcanoids that do exist would be lost in the glare of the Sun’s light, even at dawn and dusk. And it’s too risky for an ordinary space telescope to look that close to the Sun.

Our best bet to find a vulcanoid would be to use a spacecraft with a dedicated sun shade to make artificial solar eclipses, like the European Space Agency’s PROBA-3,² or a space probe much closer to the Sun than Earth, like NASA’s Parker Solar Probe.³ Both of these specific spacecraft have limitations that make it difficult for them to find vulcanoids, but difficult doesn’t mean impossible. Perhaps someday soon, we’ll finally find one of these most elusive asteroids, or prove they can’t exist. And either possibility would be, to quote one famous Vulcan, fascinating.