**2. Volcanism present in the solar system and the extrasolar planetary system TRAPPIST-1**

The types of active and inactive volcanism in our own neighborhood are various. **Figure 1** gives an overview of the different types of volcanism found or strongly assumed on celestial objects in the solar system. To classify the different types of (cryo-)volcanism found on objects in the solar system, we distinguish between the case when the respective type of volcanism is active right now and verified (e.g., by measurements of space probes) or strongly assumed due to observations, measurements or theoretical models, and the case when signs of at least former volcanic activity were identified. We also include the (at least former) presence of a liquid subsurface ocean as part of cryovolcanism.

The melting up of a subsurface ocean requires a strong energy source. This is either powered from the interior of the body hinting at the presence of silicate volcanism in its core. Another or even simultaneously occurring energy source can be the deformation by tidal forces of nearby objects, which can liquify silicates or ice and heats up potentially present silicate magma and/or a (subsurface) ocean further. This might result in icy objects in cryovolcanic activity, for example, in the form of geysers penetrating through the ice crust of Saturn's ice-moon Enceladus [31]. By cracking up the ice crust a cryo-form of plate tectonics could be initiated, for example, on Jupiter's ice-moon Europa [33].

In addition, we identify several objects that should be considered as potential candidates for re-evaluation of the potential of tidal-based volcanism based on recent studies. For example, the presence of crystalline water ice and/or ammonia ice on the surface hints at the presence of a mechanism that actively redeposits new material, as crystalline water ice and/or ammonia ice is not stable in the long term in these environments due to destruction by energetic particles (see, e.g., [92–94]).

Domes, which are mountains and bulges in the crust of a celestial object, could be remnants of extinct eruptive volcanoes or could be plumes that do/did not penetrate fully through the crust. We see the identification of domes on the surface of a celestial object as an indicator for at least former eruptive volcanic activity.

Moreover, we included the objects resulting from our recent study [95], which we identified as new and (in the case of the solar system) not yet elsewhere considered candidates for tidal-based volcanism.
