How dense and compact does Kepler-5b need to be to become a black hole ?
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There are two ways to answer this.
The first is to argue that to become a black hole an object must become sufficiently dense that it disappers inside it's own even horizon. For a star of a given mass. This means it's radius must become smaller than the Schwarzchild radius $R < 2GM/c^2$, where $M$ is the mass of the object, $G$ is the gravitational constant and $c$ is the speed of light. Or for an object of given radius then it's mass must exceed $M > Rc^2/2G$. I am not sure why you home in on Kepler 5b since we know both it's mass and radius - about twice the mass of Jupiter and 1.3 times Jupiter's radius, so cannot be a BH.
If we assume the measured radius is wrong, it would need to be only 8.4 metres in radius to be a BH and its density would have exceeded $10^{23}$ kg/m$^3$. There is currently no known astrophysical mechanism to produce such small (low mass) BHs.
The second way is to use general relativity and what we know about the equation of state of the material (how pressure depends on density and temperature) to work out a critical density beyond which the material will not be "hard" enough to support itself against gravity. This latter density will probably occur at a radius a little bigger than the Schwarzchild radius, but it will be similar.