This question was inspired by:
- This recent article about an exoplanet "10 times the size of Jupiter"
- Prof. David Taylor's website about the "Life and Death of Stars" (in particular, the first page's discussion of massive planets)
- Exoplanet Mass-Radius Diagram
Is the boundary distinct between planetary mass and stellar mass?
In other words, if one takes a planetary body brown dwarf with nearly 75 $M_{J}$ (PDF link), added e.g. one kilogram to it, then would it start up nuclear fusion as if "someone flipped a light switch"? (For lack of a better analogy.)
Yes, I am aware of many other factors, e.g. the composition of the (exo)planet brown dwarf, stratification layers, internal structure, etc.
Conversely, if this isn't a sharp boundary, then what does happen inside planetary bodies brown dwarf objects in the 70-75 $M_{J}$ mass range?
Addendum
After reviewing the PDF link I gave above (from Prof. Taylor) and Rob's answer below, I noticed that my original terminology was flawed. "Brown dwarf" is the appropriate term instead of "planetary body". Also note that the "10 times the size of Jupiter"-exoplanet is approaching the size limit of a "planet".