Precision Requirements for Robust Small Planet Classification

Abstract

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 classifying a planet using 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. Among the 37 planets, we find 12 already possess sufficiently precise mass and radius measurements to be confidently classified as Mini-Neptunes or Super-Mercuries. The remaining planets require approximately a ~60% reduction in the combined mass and radius uncertainties before they can be reliably classified. We find that 1-1,000 new observations are necessary to achieve these lower uncertainties, and these are likely lower limits. We argue that, while possible, this level of intense observation is likely not feasible for many of these planets and that mass-radius models alone are not likely to allow us to classify all but those planets with outlier densities. We therefore recommend a staged strategy in which improved mass and radius measurements are used to identify and prioritize ambiguous systems, followed (where feasible) by atmospheric observations with JWST or future facilities as a complementary test of whether selected planets retain substantial atmospheres.