How are Brown Dwarfs like jovian planets?

Brown dwarfs are not even close to being regarded as planets; they are not precisely planets.

To put it in terms of Jupiter, brown dwarfs are failed stars that were once stars but could not sustain further fusion reactions due to their lack of mass.

In the context of the Jovian Planets, scientists believe that an object roughly 75 times the mass of Jupiter will almost not have enough mass to support a nuclear fusion reaction. After a little while, they actually found objects with the above properties, even a little less massive. These type of stars start out life like normal stars but eventually cool down instead of sustaining fusion.

These stars are cool and black, but they're not really brown; the term "Brown Dwarf" was taken because the name "Black Dwarf" was already used for another kind of object.

Because actual stars burn through their lithium fuel quite quickly, Brown Dwarfs have a unique property that has helped scientists discover them: Lithium.

In terms of makeup, brown dwarfs and jovian planets are similar in that they are made mostly of hydrogen and helium. They are also different from main-sequence stars in that they are too small to support continuous nuclear fusion.