Does anything orbit the Sun faster than Mercury?
Mercury's orbital period around the Sun is about 88 days. Comets and other things have gotten closer to the Sun than Mercury does. But has there ever been an asteroid or some other body discovered that has a shorter orbital period? Are there theoretical constraints on if such a body can exist?
The IAU defines a planet as one that has swept out its orbit. Does the neighborhood of Mercury extend to the Sun?
Vulcanoids are difficult to detect from Earth; none are known yet.
To remain in such an orbit, Evans and Tabachnik 1999 estimate a minimum diameter of 100 m and a semimajor axis between 0.09 and 0.21 au.
There are various competing definitions for a planet's neighborhood.
Soter 2006 says:
Two bodies share an "orbital zone" if their orbits cross a common radial distance from the primary and their periods are nonresonant and differ by less than an order of magnitude.
A Sun-grazing asteroid with perihelion 0.01 au and aphelion 0.31 au (Mercury's perihelion) would orbit in 23 days, well above the 9 day minimum for that definition.
I was curious about the minimum diameter. Evans says there are two effects that impose this limit: radiation pressure and evaporation. I don't really understand how radiation pressure (Poynting-Robertson effect) can be significant for anything but a dust grain.