Is This Planet Rocky: Precision Requirements for Robust Small Planet Classification

Classifying small exoplanets as rocky, volatile-rich, or iron-enriched is a prerequisite for efficient allocation of atmospheric characterization resources. The Nominally Rocky Planet Zone (NRPZ) provides a framework for this classification by comparing observed bulk densities against the expected density range for volatile-free rocky planets, calibrated from stellar abundance distributions. However, the efficacy of the NRPZ is critically dependent on the precision of the underlying mass and radius measurements. Here, we perform an analytical propagation of observational uncertainties through the NRPZ classification framework for 37 candidate rocky exoplanets with radii below 2 Earth radii orbiting stars with visual magnitudes <11. Of these 37 planets, we find 11 of them currently have mass and radius uncertainties to definitively classify them as Mini-Neptunes or Super-Mercuries. The remaining planets, however, require extreme reductions in current mass and radius uncertainties, together, to classify them as either leaning rocky or not rocky. We further quantify the number of orbits needed…