We're looking at earth like planets because the only conditions we know of that support life is this one. Sciencey logic right there. See above. But to go further, the instances that we know that supports life as we know it involve water, adequate heat, a magnetosphere to protect from radiation, stable orbits, stable atmosphere, and probably a bunch of other stuff that someone like Dala or francopoli could pull out of their butts cause this is their area of interest and not mine but I don't have a lot going on this morning and I'm trying to draw a hut (feel like breaking down and just ripping off this guy but I'm actually not a fan of drawing stuff that's already existing) and it's not going well, so here I am, looking for a distraction. SO MANY FUCKING WAYS IT'S MIND BLOWING!!! Off the top of my head, people figure out orbits of planets through the flickering of stars, bending of light, and voodoo science. If a planet is too close to a star, they can probably surmise that solar wind is ripping away the atmosphere and I might be wrong, but I think they can actually take infrared pictures or something to see if it's actually happening. They can also surmise that it might be too hot, too radiated, or in some other way fucked up. As for determining a planet's atmosphere, they have these special spectrometers that look at chemical makeup of shit. I might be wrong, but I think it only works on stars at the moment cause maybe planets are too small or too far or something or maybe we actually do spectrometers on planets, I dunno. That said, if we can see the gas makeup of atmosphere on planets, there might be some tells. For example, the presence of oxygen means it might support life and Dala says that methane is a byproduct of biological chemical reactions so if there's methane there's a chance there's a herd of alien cows farting away to their hearts content. And, as usual, in trying to look something up I find a Wikipedia Article that basically makes an feeble attempt of mine to figure something out, moot.I'm not even sure where, how and why we are looking right now.
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That's a great point. Of course our "habitable zone" might not be so habitable to another lifeform born of an entirely different environment than Earth's. And think of how weird things might get when you consider regimes far from standard pressure... The recent arsenic-based DNA sham comes to mind (edit: not 'cuz it required non-STP conditions), but hopefully we don't use that to reinforce the notion that our exact organic makeup is the only possibility.