Meaning of the Planck Temperature I don't understand what makes the Planck Temperature the "absolute hot". To my understanding Temperature is just a measure of the kinetic energy of the particles, so is the Planck Temperature the temperature at which the particles are moving at a speed so close to the speed of light that their behavior can no longer be understood?
If not, what are the formulas that break down as an object is simulated above the Planck Temperature?
 A: The meaning of the Planck Temperature is precisely this: IF there exists in the Universe a natural temperature, which is determined by the interplay of the scales of all of quantum mechanics, relativity, gravitation, and thermodynamics - all of whom are understood respectively by the physical constants $\hbar$, $c$, $G$, and $k_B$, then the equation to calculate that temperature from a theory that gives its exact value must be able to be written in the mathematical form
$$T_\mathrm{fundamental} = K \cdot \underbrace{\sqrt{\frac{\hbar c^5}{k_B^2 G}}}_\text{"Planck temperature"}$$
where $K$ is an unknown dimensionless constant. That's it, nothing more. The whole reason is that, given all those scales are involved, the only way you can dimensionally consistently put them all together is that.
But it says nothing else because of that $K$: the actual value of this temperature could be anything. The idea that the Planck temperature is the fundamental temperature is a tact assumption that $K = 1$ or at least $K \approx 1$, but there is nothing about it that says that must be the case. "Heuristically", we'd "like" to think that because "most numbers tend to be about one", but there is no guarantee there at all of anything. $K$ could be 1, but it could also be $10^9$. It could be $10^{-9}$. Heck, it is entirely possible that $T_\mathrm{fundamental}$ is less than one kelvin, that is, that $K \lessapprox 10^{-32}$! There could be some unknown quantum-gravitational effect that only kicks in when things are very cold, colder than we've gotten them and/or only when involving the right substances or looking at the right aspects. That's not "expected", but that's just our prejudice, however "reasonable" we might want to think it.
Our current best model, the Standard Model, is only empirically tested out to temperatures of around $10^{17}\ \mathrm{K}$, or $K \approx 10^{-15}$, and mathematically, the pile of hacks (technically it's not truly a well-founded quantum theory, even!) it is tends to fall apart some ways before we get close to $K = 1$, so we cannot deduce anything useful from tested theory about whether $K \approx 1$ is likely to have real meaning. And finally, it could also be that there are multiple important quantum-gravity temperatures, each of which has an expression like the above, but with a different $K$.
A: The short answer is "We don't know"  if there is an "absolute hot" or if there is, what it is.
This column by Peter Tyson : https://www.pbs.org/wgbh/nova/zero/hot.html
is what I point people to when they want to know why I can't explain it better.
But here goes my attempt: as the thermodynamic temperature rises from absolute zero, where particles don't exhibit significant movement, matter changes. First the classic phase changes from solid to liquid to gas, then as temperature continues rising, molecules can no longer exist, atoms are broken down until eventually at the Hagedorn temperature hadronic matter (ordinary matter) "evaporates"  for lack of a better word. Current theories predict that a similar boundry exists at about $10^{30}K $ where quarks/gluons will similarly no longer exist, although obviously we have no way of actually testing this.
The Planck temperature, ~ 1.42 x $10^{32}K$ is where the models and theories run into the wall. We literally have nothing yet to predict how the universe behaves beyond this point, although what models there are predict that at this point particle energies would be enormous. Gravitational forces would become as strong as the other fundamental forces. In short someone has to come up with a quantum theory of gravity to start taking a crack at the problem - Holy Grail anyone?
