  Planet formation is expected to be severely limited in disks of low
metallicity, owing to both the small solid mass reservoir and the low opacity
accelerating the disk gas dissipation. While previous studies have found a weak
correlation between the occurrence rates of small planets ($\leq$4R$_\oplus$)
and stellar metallicity, so far no studies have probed below the metallicity
limit beyond which planet formation is predicted to be suppressed. Here, we
constructed a large catalog of ~110,000 metal-poor stars observed by the TESS
mission with spectroscopically-derived metallicities, and systematically probed
planet formation within the metal-poor regime ([Fe/H] $\leq$ -0.5) for the
first time. Extrapolating known higher-metallicity trends for small,
short-period planets predicts the discovery of ~68 superEarths around these
stars (~85,000 stars) after accounting for survey completeness; however, we
detect none. As a result, we have placed the most stringent upper limit on
super-Earth occurrence rates around metal-poor stars (-0.75 < [Fe/H] $\leq$
-0.5) to date, $\leq$ 1.67%, a statistically significant (p-value=0.000685)
deviation from the prediction of metallicity trends derived with Kepler and K2.
We find a clear host star metallicity cliff for super-Earths that could
indicate the threshold below which planets are unable to grow beyond an
Earth-mass at short orbital periods. This finding provides a crucial input to
planet formation theories, and has implications for the small planet inventory
of the Galaxy and the galactic epoch at which the formation of small planets
started.
