Context. Substructures such as rings, gaps, and cavities are commonly observed in protoplanetary discs and are thought to play a key role in dust evolution and planet formation. However, a fraction of the extended discs (68% dust radii >30 AU) in nearby star-forming regions remain unresolved, leaving their substructure content uncertain and thereby limiting our understanding of dust evolution and the initial conditions for planet formation across the full disc population. Aims. We aim to investigate the presence of substructures in previously unresolved, extended discs to assess whether all extended protoplanetary discs in the solar neighbourhood exhibit substructures. This enables a statistical evaluation of substructure occurrence among protoplanetary discs and provides statistical constraints for disc evolution models and comparisons with exoplanet populations. Methods. We present new high-resolution (∼0.12′′) ALMA (Atacama Large Millimeter/submillimeter Array) Band 6 continuum observations at 1.33 mm of 26 previously unresolved, extended discs within 200 pc. This completes the sample of high-resolution observations of extended discs in the nearby star-forming regions Taurus, Ophiuchus, Chamaeleon, Lupus, Upper Scorpius, Upper Centaurus-Lupus, and Lower Centaurus-Crux. We analysed radial intensity profiles using Frankenstein and Galario to detect substructures. Results. Seventeen discs show clear substructures, while nine appear compact and structureless, smooth, or ambiguous due to inclination or possible binarity or late-stage infall. We detect 12CO J=2-1 emission in 15 discs, where extended CO emission is observed in four discs. Combined with literature data, our complete sample of 730 protoplanetary discs reveals that nearly all extended discs exhibit substructures, ∼91% detected in the full sample, and up to ∼98% when correcting for high-inclination systems where substructures may be hidden. Conclusions. Substructures are a near-universal feature of extended protoplanetary discs. Substructures are more commonly detected and, it is proposed, more prevalent in larger, massive discs and around higher-mass stars, and structured discs retain their dust mass over time. This is consistent with the hypothesis that dust traps, possibly induced by giant planets, are key in shaping the dust disc morphologies.
Bosschaart et al. (Fri,) studied this question.