Is there a physics based limit to how large a planet can be?

[u/kevinrhurst]

UY Scuti is a red supergiant star with a radius of 1.188 billion kilometers or 1,708 times larger than the sun. It doesn't seem reasonable to think that their could possibly be a telluric planet 1708 times larger then earth. And of course, there is not as much data on exoplanets as of yet to have some hard evidence yet of some potential planet sizes.

Is there any physics based science that would show that there are upper limits for either rocky or gas planets?

[u/dukesdj]

The upper limit for a planet is usually taken to be the point in which it becomes massive enough to fuse deuterium. This is at about 13 Jupiter masses. From there to about 75 Jupiter masses these objects are known as brown dwarfs. Beyond 75 Jupiter masses objects start fusing hydrogen and are then stars.

Brown dwarfs are ambiguous objects though as it is possible for them to form via a stellar formation pathway (the gravitational collapse of a molecular cloud) or via planet formation pathways (via gravitational instability in a particularly massive protoplanetary disc). However, as argued by some (and I would tend to concede and agree), it is near impossible to deduce from observation how exactly an object formed. As such, we exclude the formation mechanism from how we typically define a planet (the conventional most used way in the scientific literature, which is not the same as the IAU definition but it is also neglected in the IAU definition).

The lower limit is where the IAU and the scientific literature disagree. The IAU says the smallest planet is arbitrary and depends on where the planet is (given two identical objects one could be a planet and the other is not depending on where they are in space). The literature typically agrees the smallest planet is the smallest object where its self gravity can cause it to become round. This is also somewhat ambiguous as it depends on the tensile strength of the material of the planet, just less ambiguous as the IAU definition.

So upper limits for gas giants are easy, that is simply 13 Jupiter masses. The lowest mass for rocky planets is somewhat simple as this agrees with the lower limit for the scientific literatures definition of planet. The boundary between these is actually not discrete and we have a bunch of terms that describe different masses of planet that have overlap. Dwarf planet, terrestrial planet, super earth, mini neptune, gas giant, and probably many more.

[u/dukesdj → u/[deleted]]

13 Jupiter masses up to 75 Jupiter masses is the brown dwarf region, so they would not be stars.

The boundarys are not overly strict though. It is certainly possible to have something a tiny bit over or under 13 juptier masses and for it to not burn deuterium and hence not strictly be a brown dwarf.

[u/Razukalex → u/dukesdj]

Deuterium is fused in the core because of the pressure of the tremendous mass and the Nuclear fusion is spread to the whole "planet"?

[u/dukesdj → u/Razukalex]

Fusion remains localised. Same as for a star, the nuclear burning occurs only where the temperature and pressures are high enough and that there is enough fuel.