  Three of the seven rocky planets (e, f, and g) in TRAPPIST-1 system orbit in
the habitable zone of the host star. Therefore, water can be in liquid state at
their surface being essential for life. Recent studies suggest that these
planets formed beyond the snow line in a water-rich region. The initial water
reservoir can be lost during the planet formation due to the stellar activity
of the infant low-mass star. However, a potential subsequent water delivery
event, like the late heavy bombardment (LHB) in the Solar System, can replenish
planetary water reservoirs. To study this water delivery process, we set up a
simple model in which an additional 5 M_Earth--50 M_Earth planet is embedded in
a water-rich asteroid belt beyond the snow line of TRAPPIST-1. Asteroids
perturbed out from the chaotic zone of the putative planet can enter into the
inner system and accreted by the known planets. Our main finding is that the
larger is the orbital distance of planet, the higher is the amount of water
delivered to the planet by an LHB-like event.
