2. Why can't sea stars live in terrestrial environments?

The sea stars are echinoderms and are fully dependent on a having a saline aquatic or marine environment, from shallow intertidal to the deepest trenches or sea bottom. No member of Phylum Echinodermata (sea stars, brittle stars, sea lilies, basket stars, sand dollars, sea urchins, sea cucumbers) have been known to survive in a non-marine or non-seawater environment. They might tolerate moderate changes in salinity such as in brackishwater (lower salinity) or even air exposure but they may not survive for long because they lack a capability for osmoregulation of salinity as well as full resistance to dessication. Nor will they be able to reproduce properly because their gametes and larvae will most likely die quickly when exposed for extended periods to freshwater or air. None of the echinoderms will survive for longer than 24 hours when exposed to the air environment despite their somewhat thicker epidermis and/or exoskeleton.

It's speed of rotation.