@wildgrass
I think they tell the size of the planet by the up and down swing of the light emission measured by telescopes. Then they know how far from the parent sun it is and the orbital velocity.
So they can deduce the density from that.
My guess is further measurements will reveal the density figures to be bogus but of course that is just my opinion.
Maybe it is a collapsed star, a remanent of a nova.
What sticks out to me is the density figure is less then double that of the heavies like Tungsten.
If the density was say 10 times that of lead or uranium there could be a better argument for nova leftover but the numbers read out, maybe 50% higher than tungsten is definitely puzzling. Maybe a very small core of a collapsed star surrounded by asteroids and such crashing into the core over eons where most of the stuff is just rock and iron, the stuff of asteroids.
For now, I would go with measurement error.
I would think it would not be a core of uranium since that would seem to me to lead to an explosion.