  When and how planets form in protoplanetary disks is still a topic of
discussion. Exoplanet detection surveys and protoplanetary disk surveys are now
providing results that allow us to have new insights. We collect the masses of
confirmed exoplanets and compare their dependence with stellar mass with the
same dependence for protoplanetary disk masses measured in ~1-3 Myr old
star-forming regions. The latter are recalculated by us using the new estimates
of their distances derived from Gaia DR2 parallaxes. We note that single and
multiple exoplanetary systems form two different populations, probably pointing
to a different formation mechanism for massive giant planets around very low
mass stars. While expecting that the mass in exoplanetary systems is much lower
than the measured disk masses, we instead find that exoplanetary systems masses
are comparable or higher than the most massive disks. This same result is found
also by converting the measured planet masses into heavy-element content (core
masses for the giant planets and full masses for the super-Earth systems) and
by comparing this value with the disk dust masses. Unless disk dust masses are
heavily underestimated, this is a big conundrum. An extremely efficient
recycling of dust particles in the disk cannot solve this conundrum. This
implies that either the cores of planets have formed very rapidly (<0.1-1 Myr)
and large amount of gas is expelled on the same timescales from the disk, or
that disks are continuously replenished of fresh planet-forming material from
the environment. These hypotheses can be tested by measuring disk masses in
even younger targets and by better understanding if and how the disks are
replenished by their surroundings.
