Type Ia supernovae that interact with close circumstellar material — SNe Ia-CSM — are extremely rare, comprising roughly 0.04% of all Type Ia events. Their rarity has been used as evidence against the core-degenerate formation scenario, in which a white dwarf merges with the degenerate core of an asymptotic giant branch star inside its planetary nebula.
Soker (arXiv:2603.16810) argues the rarity is not evidence against the scenario but a prediction of it. The planetary nebula surrounding the merger system disperses on a timescale of roughly 10,000 years. The white dwarf explodes on a much longer timescale — typically millions of years after the merger. By the time the explosion occurs, the nebula is gone. No circumstellar material remains to interact with.
The 0.04% rate corresponds to the fraction of core-degenerate systems where the delay between merger and explosion is short enough that the nebula hasn't fully dispersed. This is a selection effect: we see Ia-CSM only in the rare cases where the timing is right, not because the formation channel is rare.
Soker proposes a name for this class: SNIPs — supernovae Ia inside planetary nebulae. The name unifies two phenomena previously treated as distinct: the circumstellar interaction (an observational property of the supernova light curve) and the planetary nebula (a pre-existing structure from the progenitor's evolution). They are the same object seen at different times.
The rarity of SNe Ia-CSM, which seemed to constrain the core-degenerate scenario, instead becomes consistent with it. The constraint was on the observation rate, not the formation rate. The nebula disperses. The evidence disappears. The formation channel is invisible except in the narrow window where explosion and dispersal overlap.