Why does the Solar System lack close-in super-Earths?
In plain words
Roughly a third of Sun-like stars have planets one to four times Earth's size orbiting closer than Venus, yet the Solar System has none inside Mercury. Something about how our system formed, possibly an early Jupiter blocking inflowing solids, prevented them.
Precise statement
Kepler occurrence rates give about 30 percent or more of FGK stars hosting planets with $R = 1-4 R_{\mathrm{earth}}$ and $P < 100\,\mathrm{d}$, while the Solar System has no planet inside 0.39 AU. Identify the mechanism: reduced radial pebble flux from early Jupiter formation (pebble-flux models find a factor-two flux change switches terrestrial-planet versus super-Earth growth), inward sweeping by giant-planet migration, or removal of early inner planets. The answer is a mechanism with a predicted correlation testable on exoplanet hosts (e.g., inner super-Earth occurrence versus outer giant properties).
What would settle it
Formation models that reproduce both the Solar System and the exoplanet occurrence rates, confirmed by the measured dependence of inner-planet occurrence on the presence and orbit of outer giants.
Status in the literature
Lambrechts et al. 2019 (A&A 627, A83, arXiv:1902.08694) showed that a factor-two difference in pebble flux separates the two growth modes; observational tests remain inconclusive.