Occurrence–metallicity relation for cold low-mass planets beyond the snow line

Characterize how the planet occurrence–host metallicity relation extends to cold low-mass planets beyond the snow line, which are largely inaccessible to transit, radial-velocity, and astrometric surveys.

Background

The paper explains that the occurrence–metallicity relation is established for several classes of planets, but that its dependence on planet mass and orbital period leaves cold low-mass planets beyond the snow line as an unmeasured region of parameter space. Microlensing is identified as the principal foreseeable method capable of detecting such planets, particularly around M-dwarf hosts in the Galactic disk and bulge.

Determining this relation would test how metallicity affects the formation of low-mass planets at large orbital separations and would provide the missing empirical constraint needed to compare planet-formation models across planet mass and orbital period. The proposed photometric–microlensing metallicity method is intended to enable the measurements required for this question, but the paper does not itself determine the relation.

References

Within the parameter space currently probed, the occurrence--metallicity relation therefore tends to steepen toward larger planets and shorter orbital periods. However, how this trend extends to cold low-mass planets beyond the snow line, which constitute the currently missing quadrant of occurrence--metallicity measurements, remains unclear, as such planets are largely inaccessible to transit, radial-velocity, and astrometric surveys.

— Enabling Metallicity Measurements of Microlensing Lenses through Multi-band Lens Photometry  (2609.05265 - Zhang et al., 4 Sep 2026) in Section 1, Introduction