A couple of things to consider:
- As various works of fiction, such as Star Wars - Return Of
The Jedi, have portrayed, we should also look at moons
of decidedly non Earth-like planets. And if we do so things
come up like the radiation environment around the planet,
and heavy tidal heating of the moon by the planet's gravity.
- If you want an environment for possible life, "Earth-like" isn't
the only possible criterion. Europa, currently one of the
best hypothetical bets in our system, offers a deep ocean
isolated from space by heavy ice, for instance.
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