system
Given the following post, respond as if you're commenting on Reddit:
human
How Big can a planet be?

Is there a limit to how large a planet can be? After a certain size, does it collapse under its own gravity and become something else, like a sun?
gpt
There is a *mass* upper limit to a planet at about 13 Jupiter masses.  Above this limit are objects called Brown Dwarfs, which are capable of weak or intermittent nuclear reactions via fusion of deuterium (a heavy isotope of hydrogen), but they are too weak to be classified as stars.

However, this mass limit is not the same thing as a *size* limit.  Two gas giants with the same mass can vary considerably in size because one is heated more by its star than another, causing its atmosphere to expand and puff up.  They can also be less dense if they are very young and have not yet fully collapsed.  Also, higher masses of a gas giant can actually cause the planet to shrink due to increased gravity, at least to a certain point.

While the theoretical mass limit is 13 Jupiter masses, the largest size of a planet yet detected is **HD 100546 b** with about 7 Jupiter radii and an undetermined mass.