  During the formation of our solar system, a large number of planetesimals
were ejected into interstellar space by gravitational encounters with the
planets. Debris disks observations and numerical simulations indicate that many
other planetary systems, now known to be quite common, would have undergone a
similar dynamical clearing process. It is therefore expected that the galaxy
should be teeming with expelled planetesimals, largely unaltered since their
ejection. This is why astronomers were perplexed that none had been detected
passing through the solar system. Then, in 2017, the discovery of1I/'Oumuamua
transformed the situation from puzzlement to bewilderment. Its brief visit and
limited observations left important questions about its nature and origin
unanswered and raised the possibility that 1I/'Oumuamua could be a
never-seen-before intermediate product of planet formation. If so, this could
open a new observational window to study the primordial building blocks of
planets, setting unprecedented constraints on planet formation models. Two
years later 2I/Borisov was discovered, with an unquestionable cometary
composition, confirming that a population of icy interstellar planetesimals
exists. These objects have remained largely unchanged since their ejection,
like time capsules of their planetary system most distant past. Interstellar
planetesimals could potentially be trapped into star and planet formation
environments, acting as seeds for planet formation, helping overcome the
meter-size barrier that challenges the growth of cm-sized pebbles into km-sized
objects. Interstellar planetesimals play a pivotal role in our understanding of
planetary system formation and evolution and point to the possibility that one
day, we will be able to hold a fragment from another world in our hand.
