  Supernova (SN) explosions have been sought for decades as a possible source
of dust in the Universe, providing the seeds of galaxies, stars, and planetary
systems. SN 1987A offers one of the most promising examples of significant SN
dust formation, but until the James Webb Space Telescope (JWST), instruments
have traditionally lacked the sensitivity at both late times (>1 yr
post-explosion) and longer wavelengths (i.e., >10 um) to detect analogous dust
reservoirs. Here we present JWST/MIRI observations of two historic Type IIP
SNe, 2004et and SN 2017eaw, at nearly 18 and 5 yr post-explosion, respectively.
We fit the spectral energy distributions as functions of dust mass and
temperature, from which we are able to constrain the dust geometry, origin, and
heating mechanism. We place a 90% confidence lower limit on the dust masses for
SNe 2004et and 2017eaw of >0.014 and >4e-4 M_sun, respectively. More dust may
exist at even colder temperatures or may be obscured by high optical depths. We
conclude dust formation in the ejecta to be the most plausible and consistent
scenario. The observed dust is radiatively heated to ~100-150 K by ongoing
shock interaction with the circumstellar medium. Regardless of the best fit or
heating mechanism adopted, the inferred dust mass for SN 2004et is the second
highest (next to SN 1987A) inferred dust mass in extragalactic SNe thus far,
promoting the prospect of SNe as potential significant sources of dust in the
Universe.
