  When a protoplanetary disc loses gas, it leaves behind planets and one or
more planetesimal belts. The belts get dynamically excited, either by planets
('planet stirring') or by embedded big planetesimals ('self-stirring').
Collisions between planetesimals become destructive and start to produce dust,
creating an observable debris disc. Following Kenyon & Bromley (2008), it is
often assumed that self-stirring starts to operate as soon as the first ~1000
km-sized embedded 'Plutos' have formed. However, state-of-the-art pebble
concentration models robustly predict planetesimals between a few km and ~200
km in size to form in protoplanetary discs rapidly, before then slowly growing
into Pluto-sized bodies. We show that the timescale, on which these
planetesimals excite the disc sufficiently for fragmentation, is shorter than
the formation timescale of Plutos. Using an analytic model based on the Ida &
Makino (1993) theory, backed up by numerical integrations, we find the
excitation timescale to be $T_{\rm excite} \approx 100 x_{\rm m}^{-1}
M_*^{-3/2} a^3$ Myr, where $x_{\rm m}$ is the total mass of a protoplanetary
disc progenitor in the units of the Minimum-Mass Solar Nebula, $a$ its radius
in the units of 100 AU, and $M_*$ is the stellar mass in solar masses. These
results are applied to a set of 23 debris discs that have been well resolved
with ALMA or SMA. We find that the majority of these discs are consistent with
being self-stirred. However, three large discs around young early-type stars do
require planets as stirrers. These are 49 Cet, HD 95086, and HR 8799, of which
the latter two are already known to have planets.
