Try to apply a set of constraints to a problem to reduce the possible set of results is a good thing, provided the constraints are extensive. Two things are un

Try to apply a set of constraints to a problem to reduce the possible set of results is a good thing, provided the constraints are extensive. Two things are universal in the, erm, universe, physics, and by extension chemistry and I don't see any consideration of those. There's many reasons why the Earth is in the Goldilocks zone. It's just the right temperature for liquid water, we have oxygen in the atmosphere, and the planet doesn't weigh too much. Without oxygen, creating mechanical work from fuel by burning, is 'tricky' (though maybe not impossible), no mechanical work, no industrial revolution, no technology. There's an argument that cooking food (fire) leads to better diet, more calories, and the evolution of the brain. Whilst there are types of alternative biochemistry (https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi...) carbon is by far the most likely. And lastly, if the earth were slightly more massive, the mass of (chemical) fuel required to propel a rocket becomes too much. I'm doubtful that we'd see biological 'big aliens', or even 'small aliens', that significantly differ from our size, if they exist.