Period-radius diagram of all known USP planets with radii constrained to a precision better than 30% (source: NASA Exoplanet Archive, accessed on 24/03/2025). The colour code indicates the radii of the stellar hosts, with the blueish and reddish tones indicating radii above and below R_★ ≃ 0.7 R_☉, respectively. USP planets with one detected neighbour are highlighted with hexagonal symbols, while those with two or more neighbours are shown with triangular symbols.