  Dust drifting inward in protoplanetary disks is subject to increasing
temperatures. In laboratory experiments, we tempered basaltic dust between 873
K and 1273 K and find that the dust grains change in size and composition.
These modifications influence the outcome of self-consistent low speed
aggregation experiments showing a transition temperature of 1000\,K. Dust
tempered at lower temperatures grows to a maximum aggregate size of $2.02 \pm
0.06$ mm, which is $1.49 \pm 0.08$ times the value for dust tempered at higher
temperatures. A similar size ratio of $1.75 \pm 0.16$ results for a different
set of collision velocities. This transition temperature is in agreement with
orbit temperatures deduced for observed extrasolar planets. Most terrestrial
planets are observed at positions equivalent to less than 1000 K. Dust
aggregation on the millimeter-scale at elevated temperatures might therefore be
a key factor for terrestrial planet formation.
