Is it likely that any of Jupiter's small moons were formed in orbit around Jupiter or are they all captured asteroids? I thought of this question because I wondered whether the planetary-system of a sub-brown dwarf would have any asteroids or just round planets.
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Short Answer:
It seems reasonable to suppose that many brown dwarfs are orbited by many objects with masses ranging from giant planets down to tiny asteroids.
Long answer:
Planets, brown dwarfs, and stars are somehwate similar but also different types of astronomical objects. What is true for one type may or may not be true for another type.
The Sun had a circumstellar disc of gas and dust when it was young.
A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place, and around white dwarfs, they indicate that planetary material survived the whole of stellar evolution. Such a disc can manifest itself in various ways.
https://en.wikipedia.org/wiki/Circumstellar_disc[1]
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, because gases or other material may be falling from the inner edge of the disk onto the surface of the star. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds.
In July 2018, the first confirmed image of such a disk, containing a nascent exoplanet, named PDS 70b, was reported.33
https://en.wikipedia.org/wiki/Protoplanetary_disk[4]
The nebular hypothesis of solar system formation describes how protoplanetary disks are thought to evolve into planetary systems. Electrostatic and gravitational interactions may cause the dust and ice grains in the disk to accrete into planetesimals. This process competes against the stellar wind, which drives the gas out of the system, and gravity (accretion) and internal stresses (viscosity), which pulls material into the central T Tauri star. Planetesimals constitute the building blocks of both terrestrial and giant planets.[18][19]
https://en.wikipedia.org/wiki/Protoplanetary_disk#Planetary_system[5]
So planets were formed out of the dust and gas of protoplanetary circumstellar discs.
How are moons around planets formed?
Some of the moons of Jupiter, Saturn, and Uranus are believed to have formed from smaller, circumplanetary analogs of the protoplanetary disks.[20][21] The formation of planets and moons in geometrically thin, gas- and dust-rich disks is the reason why the planets are arranged in an ecliptic plane. Tens of millions of years after the formation of the Solar System, the inner few AU of the Solar System likely contained dozens of moon- to Mars-sized bodies that were accreting and consolidating into the terrestrial planets that we now see. The Earth's moon likely formed after a Mars-sized protoplanet obliquely impacted the proto-Earth ~30 million years after the formation of the Solar System.
Some of the moons of Jupiter, Saturn, and Uranus are believed to have formed from smaller, circumplanetary analogs of the protoplanetary disks.[20][21] The formation of planets and moons in geometrically thin, gas- and dust-rich disks is the reason why the planets are arranged in an ecliptic plane. Tens of millions of years after the formation of the Solar System, the inner few AU of the Solar System likely contained dozens of moon- to Mars-sized bodies that were accreting and consolidating into the terrestrial planets that we now see. The Earth's moon likely formed after a Mars-sized protoplanet obliquely impacted the proto-Earth ~30 million years after the formation of the Solar System.2
So some planets form circumplanetary discs that some of their moons or natural satellites form out of.
In astronomy, a regular moon is a natural satellite following a relatively close and prograde orbit with little orbital inclination or eccentricity. They are believed to have formed in orbit about their primary, as opposed to irregular moons, which were captured.
There are at least 57 regular satellites of the eight planets: one at Earth, eight at Jupiter, 23 named regular moons at Saturn (not counting hundreds or thousands of moonlets), 18 known at Uranus, and 7 small regular moons at Neptune (Neptune's largest moon Triton appears to have been captured). It is thought that Pluto's five moons and Haumea's two were formed in orbit about those dwarf planets out of debris created in giant collisions.
I don't know if any of the four inner planets of our solar system had circumpanetary discs that moons formed out of. If Mercury and Venus ever had moons they secaped into interplanetary space.
The origin of Earth's Moon is something of a mystery. The prevailing theory is that a planet about the size of Mars collided with Earth and formed a debris ring around teh Earth that the Moon formed out of. If Earth ever had any mmons formed out a circumplanetary disc they must have been lost.
The origin of Mar's two tiny moons is also uncertain. https://en.wikipedia.org/wiki/Moons_of_Mars#Origin[6]
Thus i don't know whether any terrestrial type form circumplanetary discs that moons form out of. Perhps only giant planets have circumplanetary discs that moons form out of.
In astronomy, an irregular moon, irregular satellite or irregular natural satellite is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They have been captured by their parent planet, unlike regular satellites, which formed in orbit around them. Irregular moons have a stable orbit, unlike temporary satellites which often have similarly irregular orbits but will eventually depart. The term does not refer to shape as Triton is a round moon, but is considered irregular due to its orbit.
https://en.wikipedia.org/wiki/Irregular_moon[7]
Since Jupiter has 8 egular moons (only 4 of them giant moons), Saturn has 23 plus hudnrds of tiny moonlets, 18 at Uranus, and 7 small regular moons at Neptune, It seems probable that giant planets can form not only large regular moons but also smaller regular moons small enough to be cosnidered asteroid sized.
And a brown dwarf is intermediate in mass between a planet and a low mass star. Thus it seems probable that many brown dwarfs have equivalents of circumstellar and circumplanetary discs and that larger and smaller astronomical objects form out of those discs.
At the prsent time, astronomers find it much easier to detect giant planets around other stars than to detct Earth sized planets. But they have discovered a few exoplanets which are much smaller than Earth.
Astronomers also hope to detect exomoons orbiting exoplanets. But of course the first exomoons to be detected would probably be much larger than any moons in our solar system, let alone any astroids in our solar system.
Here is a link to a list of exomoon candidates, none being confirmed yet:
https://en.wikipedia.org/wiki/Exomoon#Candidates[8]
The first discovery of a low mass planet orbiitng a brown dwarf was in 2008.
MOA-2007-BLG-192Lb, occasionally shortened to MOA-192 b,2 is an extrasolar planet approximately 3,000 light-years away in the constellation of Sagittarius. The planet was discovered orbiting the brown dwarf or low-mass star MOA-2007-BLG-192L. At a mass of approximately 3.3 times Earth, it is one of the lowest-mass extrasolar planets at the time of discovery. It was found when it caused a gravitational microlensing event on May 24, 2007, which was detected as part of the MOA-II microlensing survey at the Mount John University Observatory in New Zealand.1
The system's primary is small as well. At roughly 6% the mass of the Sun, it is probably too small to sustain fusion reactions, making it a dimly glowing brown dwarf.3 Also, the estimated projected distance between MOA-2007-BLG-192Lb and its primary is approximately 0.62 astronomical units.1 That means the planet probably formed with much ice and gases, more like Neptune (an ice giant planet) in composition than Earth (a terrestrial planet), according to astronomer David Bennett of the University of Notre Dame.3
https://en.wikipedia.org/wiki/MOA-2007-BLG-192Lb[9]
So brown dwarfs are not restricted to having giant planets as satellites. They can have satellites much less massive than giant planets. Considering the vast range in masses of the planets and astroids orbiting the Sun, and in the masses of moons orbiting planets in the solar system, it seems reasonable to deduce that objects orbiting brown dwarfs range in size down to asteroid and meteoroid size.
But it will take much more advanced observational techniques to detect asteroid sized objects around other stars or brown dwarfs.
