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ASASSN-21qj is a distant Sun-like star that recently began an episode of deepdimming events after no prior recorded variability. Here we examine archivaland newly obtained optical and near-infrared data of this star. The deepaperiodic dimming and absence of previous infrared excess are reminiscent ofKIC 8462852 (``Boyajian's Star''). The observed occultations are consistentwith a circumstellar cloud of sub-micron-sized dust grains composed ofamorphous pyroxene, with a minimum mass of$1.50~\pm~0.04\times10^{-9}~M_{\oplus}$ derived from the deepest occultations,and a minimum grain size of $0.29^{+0.01}_{-0.18}~\mu$m assuming a power lawsize distribution. We further identify the first evidence of near-infraredexcess in this system from NEOWISE 3.4 and 4.6~$\mu$m observations. The excessemission implies a total circumstellar dust mass of around $10^{-6}M_{\oplus}$, comparable to the extreme, variable discs associated withterrestrial planet formation around young stars. The quasiperiodic recurrenceof deep dips and the inferred dust temperature (ranging from 1800 to 700~Kacross the span of observations) independently point to an orbital distance of$\simeq$0.2~au for the dust, supporting the occulting material and excessemission being causally linked. The origin of this extended, opaque cloud issurmised to be the breakup of one or more exocometary bodies.

Sudden extreme obscuration of a Sun-like main-sequence star: evolution of the circumstellar dust around ASASSN-21qj