Degrees of similarity:

1. DNA/proteins

2. Some other organic molecules, but still carbon/water

3. Different basic elements and/or solvents, but still chemistry

4. Not chemistry-based at all -- magnetic structures inside stars, or something like that (if they exist in or even on Sun right now, we could be looking right at one and never guess it is alive)

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Fiction has to be plausible. Reality is under no such constraint.