The article is very clear that this object is 9 light years away. A number of brown dwarfs have been found around that distance. Remember that the
closest regular star, one undergoing fusion, is 4.3 light years away.
The reason that it is Jupiter sized is that objects from 1 to 40 Jupiter masses range from the size of Jupiter to about 15% more in diameter. The
object is essentially denser as more mass is added.
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