  With the discovery of TRAPPIST-1 and its seven planets within 0.06 au, the
correct treatment of tidal interactions is becoming necessary. The
eccentricity, rotation, and obliquity of the planets of TRAPPIST-1 are indeed
the result of tidal evolution over the lifetime of the system. Tidal
interactions can also lead to tidal heating in the interior of the planets,
which can then be responsible for volcanism and/or surface deformation. In the
majority of studies to estimate the rotation of close-in planets or their tidal
heating, the planets are considered as homogeneous bodies and their rheology is
often taken to be a Maxwell rheology.
  We investigate here the impact of considering a multi-layer structure and an
Andrade rheology on the way planets dissipate tidal energy as a function of the
excitation frequency. We use an internal structure model, which provides the
radial profile of structural and rheological quantities to compute the tidal
response of multi-layer bodies. We then compare the outcome to the dissipation
of a homogeneous planet.
  We find that for purely rocky bodies, it is possible to approximate the
response of a multi-layer planet by that of a homogeneous planet. However,
using average profiles of shear modulus and viscosity to compute the
homogeneous planet response leads to an overestimation of the averaged
dissipation. We provide fitted values of shear modulus and viscosity to be able
to reproduce the response of various types of rocky planets. However, we find
that if the planet has an icy layer, its tidal response can no longer be
approximated by a homogeneous body because of the very different properties of
the icy layers, which lead to a second dissipation peak at higher frequencies.
We also compute the tidal heating profiles for the outer TRAPPIST-1 planets (e
to h).
