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We present the discovery of strongly variable emission lines from a gaseousdisc around the DA white dwarf SDSS J1617+1620, a star previously found to havean infrared excess indicative of a dusty debris disc formed by the tidaldisruption of a rocky planetary body. Time-series spectroscopy obtained duringthe period 2006-2014 has shown the appearance of strong double-peaked Ca IIemission lines in 2008. The lines were weak, at best, during earlierobservations, and monotonically faded through the remainder of our monitoring.Our observations represent unambiguous evidence for short-term variability inthe debris environment of evolved planetary systems. Possible explanations forthis extraordinary variability include the impact onto the dusty disc of eithera single small rocky planetesimal, or of material from a highly eccentricdebris tail. The increase in flux from the emission lines is sufficient thatsimilar events could be detected in the broadband photometry of ongoing andfuture large-area time domain surveys.

Variable Emission from a Gaseous Disc around a Metal-Polluted White Dwarf