  Quasars are generally divided into jetted radio-loud and non-jetted
radio-quiet ones, but why only 10% quasars are radio loud has been puzzling for
decades. Other than jet-induced-phenomena, black hole mass, or Eddington ratio,
prominent difference between jetted and non-jetted quasars has scarcely been
detected. Here we show a unique distinction between them and the mystery of jet
launching could be disclosed by a prominent excess of radio emission in
extremely stable quasars (ESQs, i.e., type 1 quasars with extremely weak
variability in UV/optical over 10 years). Specifically, we find that $>$ 25% of
the ESQs are detected by the FIRST/VLASS radio survey, while only $\sim$ 6-8%
of the control sample, matched in redshift, luminosity, and Eddington ratio,
are radio-detected. The excess of radio detection in ESQs has a significance of
4.4 $\sigma$ (99.9995%), and dominantly occurs at intermediate radio loudness
with R $\sim$ 10 - 60. The radio detection fraction of ESQs also tends to
increase in the ESQ samples selected with more stringent thresholds. Our
results are in contrast to the common view that RL quasars are likely more
variable in UV/optical due to jet contribution. New clues/challenge posed by
our findings highlight the importance of extensive follow-up observations to
probe the nature of jets in ESQs, and theoretical studies on the link between
jet launching and ESQs. Moreover, our results makes ESQs, an essential
population which has never been explored, unique targets in the burgeoning era
of time domain astronomy, like their opposite counterparts of quasars
exhibiting extreme variability or changing-look features.
