The main problem with the Asteroid Thicket trope in Real Life is, of course, the sheer size of space. Even if the number of known asteroids is in the hundreds of thousands, down to those that have sizes comparable to that of a car, and if one includes even smaller bodies the numbers could very well be in the millions or more the space is so vast that if you were to cross the Solar System you'd be very lucky to find something larger than specks of space dustnote , and as stated in the main page you'd be very lucky to traverse the asteroid belt and find an asteroid that looked like anything but a star (unless your trajectory was designed with that in mind, as per some probes like Galileo with the asteroid Gaspra). The belt during the good ol' days when the Solar System was forming was much more rich in stuff than now, but even then as we'll see below the Asteroid Thicket was averted. The Kuiper belt and especially the Oort cloud are thought to be far more massive than the inner asteroid belt, where most asteroids are confined, but there's a whole lot of space to fill and thus are even more sparsenote note .
To give an example of how empty the real asteroid belt is, imagine Ceres -the largest body there- is a small rock with a diameter of 1 centimeter. The asteroids (remember Ceres is now considered a minor planet) would be from a couple of pebbles to finer and finer sand grains that would require a microscope to spot them. Now, the asteroid belt is estimated to have between 700,000 and 1,700,000 bodies down to a size of 1 kilometer. Does sound impressive, right? Well, in one hand the main part of the asteroid belt at this scale would extend between a radius of 3.2 kilometrs and 5.1 kilometers from the Sun, that would be a ball with a diameter of 14.5 meters -ie, as tall as a tree- and in the other it's estimated that a good handful of sand may contain several hundred thousand of grains. Scatter all those pebbles as well as a few of those handfuls of sand in that area and have fun finding one in a casual walknote . It's easy to understand why no space probe sent to cross the asteroid belt has ever been lost colliding with one, and especially why NASA's Dawn spacecraft's controllers did not have to emulate Han Solo.
A planetary system in formation would be closer to this trope at least while it lasted (see next paragraph). However it would probably look like more as "lots of stars more or less bright depending of closeness and size moving in front of the background, real, stars and some large bodies that, when close, would appear with some luck when using a telescope or (much rarely) to the naked eye as more than just points of light, with everything in front of a diffuse background formed by the combined light of all those bodies too far away to be seen individually that would resemble the Milky Way as seen from Earth", instead of rock fragments as far as the eye can see, as it was a big planetary ring.
Alas, as mentioned on the main page the rocks within a belt so dense would -in astronomical terms- soon begin to collide with themselves, smashing into dustnote or merging to eventually form a larger body, as happened in the early days of the Solar Systemnote .
In planetary ring systems, things are somewhat different since in one hand the gravity of the planets they surround stops them of coalescing to form a larger body and in the other circularizes the orbits of the space debris that forms them, and this without mentioning the likely existence of shepherd moons. All together minimize the collisions between the bodies that form them at least to a pointnote
Nonetheless, there's still some room to have a dense asteroid field. One belt where what has been stated above that occurs in Saturn's rings, with rubble piles forming and being broken down later, happens, is massive (for example because of a planet that got broken apart for whatever reasons), and orbits very close to a star (preferably a very small one as a white dwarf, rather than a red dwarf or a brown dwarf, as there's less space to fill) could have Asteroid Thicket-like density. However besides being very hot there, orbiting so close to its star, and small with a size of "just" a couple million kilometers at the largest it would rather look like a planetary ringnote but surrounding a star instead of a planet -especially in the case of the white dwarf, as they have a size comparable to that of Earth.-
Last but not least a pair of things that are overlooked when dealing with this trope:
- A rather unpleasant side effect of dense asteroid belts is that those collisions among large asteroids that break them producing lots of smaller ones, that may end up being expelled from the belt into orbits that would carry them to bad places for both the health of those smaller asteroids as well as the one of those who live there, would be quite frequent.
- The view from a planet of a system that had an Asteroid Thicket would be quite impressive, even if someone there would have far more serious issues to deal with as explained above. From the inside, it would look like this◊, resembling the Milky Way (perhaps even outshining it; astronomers would logically be very pissed, with so much space debris messing with the night sky). From the outside, it would resemble this◊, a bit as a galaxy seem from far away.