It is the size of Jupiter. All the planets we know that are that big are gas giants, which seems a less-likely thing to have survived than a small solid lump that somehow didn't melt.
* If it is a native planet, then it survived the expansion and contraction of the star. Perhaps it "stole" an atmosphere from the star as the collapsing gas cloud contracted away from the planetary core.
* It could be a captured rogue. In this event, an atmosphere could be generated from outgassing.
* It could also be artificial. If the signature is metallic, this would seem more likely.
Sometimes a campfire leaves behind little chunks of wood that didn't burn completely. Could a star do the same thing, on a larger scale? A cosmic cinder, left at the edge of the firepit.
I miss when every scientific discovery sent writers chasing after it.