Over-dense planets

On topic of strange (in every sense) white dwarfs, Zim has something on J132800.85–145019.95, and the gaps in the cloud orbiting same. The star, being a WD, is basically Earth-sized. Its cloud(s) can be measured in fractions of the solar-volume: the inner planetesimal must be located between 0.14 R_⊙ and 0.34 R_⊙ and the outer one between 0.54 R_⊙ and 1.93 R_⊙ (pdf). The former 0.14 might be better expressed as 15.27 Earth radii, or 97.4 Mm.

Usually a ring-cloud with gaps implies some gravitational-exerting mass. They don't see the planets.

To that, a WD is not Earth-heavy. It's still a good fraction of the star it was in life, which was probably A or F. Paper estimates 0.47 ±0.09 solar. That point mass should be exerting a wider Roche than 2 R_⊙. Most planets in here should be sheared, to join the rings. The paper has some large estimates for density, which Zim is showcasing, although these only apply if the bodies lack internal strength (see below).

On the assumption that Young Modulus in fact exists, the outer planet could be rocky, they say minimum 3.5 g/cm³. Earth for contrast is 5.513; Psyche 16 isn't as dense, because it's considered to be fractured. So it could be a dwarf planet like those we have here (an expanding family, I hear).

Back to J1328–1450, the inner one is the problem. It would have to be 45-172 km in diameter. That is if it is pure iron and unfractured.

If internal strength is taken away, densities rise to beyond-possible levels... for normal matter. We do, however, have some papers venturing into abnormal matter. Abudushataer Kuerban et al. proposed such in 2019, in a paper taken more-or-less seriously since. The reasoning was that if quark stars exist, quark planets might also exist. GP Com[ae Berenices] b is mooted along with some others.