Are planet sizes and spacings within a system truly correlated?
In plain words
Planets in the same system often look alike in size and are evenly spaced, like peas in a pod. Some argue the pattern is real and records how they formed, others that it comes from which planets telescopes can detect.
Precise statement
Weiss et al. 2018 (AJ 155, 48) found correlated sizes and period ratios of adjacent planets in Kepler multiplanet systems; Zhu 2020 (AJ, arXiv:1907.02074) argued the correlations arise mainly from detection thresholds varying with stellar properties and period. Determine whether the intra-system size and spacing correlations survive full forward modeling of Kepler detection efficiency, and their magnitude. The answer is yes or no with the bias-corrected correlation coefficients.
What would settle it
Forward modeling of the full Kepler detection pipeline on synthetic populations with and without intrinsic correlations, plus an independent test with radial-velocity-detected systems that have different selection effects.
Status in the literature
The 2023 Protostars and Planets VII review by Weiss et al. treats the uniformity as astrophysical, while the detection-bias argument of Zhu 2020 has not been fully answered.