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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?

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The recently discovered asteroids 2019 LF6 and 2020 AV2, each taking 151 days to orbit the Sun, have the shortest periods currently listed in the JPL Small Body Database.

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.

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  • $\begingroup$ 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. $\endgroup$
    – user15381
    Dec 18 '20 at 21:22
  • $\begingroup$ @BenCrowell Maybe it would take 100M years, but they think Poynting-Robertson drag at 0.2 au would decay the orbit of a 30 m asteroid. $\endgroup$
    – Mike G
    Dec 19 '20 at 1:25
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There is a problem with very high velocity dust particles orbiting the Sun at distances much closer than Mercury with masses of micrograms and below. These particles can do damage to spacecraft that move close to the Sun in order to study it. With the Sun's standard gravitational parameter of 1.327E+20 m^3/s^2 and the vis-viva equation we see that at 0.1 AU for example the speed of a circular orbit is 100,000 m/s, $\sqrt{10}$ larger than Earth's orbital velocity at 1 AU.

Below is from What is the origin of the dust near the sun? and the answer(s) about how this dust got there are quite informative and the answer is worth a read and an up vote!

note: Solar Probe+ is now officially Parker Solar Probe

...Reading a 2008 report I saw that there is an elevated exposure to high velocity dust near the sun...

4.3.5 Micrometeoroid and Dust. Solar Probe+ will encounter dust particles ranging in diameter from submicron up to several hundred microns and consisting of highly refractory carbon and silicate species with a typical bulk density of ~2.5 g/cm2. The particles will be traveling at relative speeds as high as 350 km/s. To define the shielding requirements for Solar Probe+, a dust model was developed based primarily on the work of Mann et al. (2004). The model employs the following assumptions...

Figure 4-6. Predicted dust environment at 0.1 AU (20 Rs) (from Mann et al. 2004 and Ishimoto, 2000)

above: screen shot from SolarProbePlus2008

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    $\begingroup$ The OP ask specifically about bigger than planetesimals, not micrometeoroids $\endgroup$ Dec 17 '20 at 2:46
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    $\begingroup$ @planetmaker ya, I saw the title "Is there anything orbiting the Sun with a shorter orbital period than Mercury?" and answered it without reading the fine print. Rats! $\endgroup$
    – uhoh
    Dec 17 '20 at 2:49
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    $\begingroup$ I think your answer is on point with respect to the OP's original question. +1 and thanks for the information! $\endgroup$
    – Connor Garcia
    Dec 17 '20 at 5:26
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A hypothetical asteroid that orbited the sun inside the orbit of Mercury would be called a vulcanoid. No vulcanoids have so far been discovered, although there is a population of asteroids that cross Mercury’s orbit, and some pass very close to the sun indeed.

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