Usually, those who argue against it fail to grasp the sheer size of the Universe. To take our galaxy as an example, it contains at least 50 billion planets. Statistics would suggest that if only 0.1% of those planets have the right conditions for life, there are an awful lot of them numerically. Added to the fact that the Milky Way is only one of around 200 billion galaxies in the observable universe, the statistics become overwhelming. The problem with identifying them is also, unfortunately, down to that same size. They will be very difficult to spot, and virtually impossible to visit. For the second part of your questions, the requirements would be an atmosphere of nitrogen, with free oxygen like our own. The next ingredient is water, the universal solvent, and we know water molecules to be common throughout space. The last main ingredient for life is carbon, because carbon combines more readily with more substances that anything else.
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Very low.
Very, very few planets have suitable conditions for life at all. Those conditions include the following:
* Orbits an appropriate star (right output, reasonably stable, not too close to other stars or celestial objects).
* Orbits at the right distance.
* Has a strong magnetic field, or some other mechanism to screen out radiation.
* Has oxygen, water, and other chemical precursors for life.
Very few planets would satisfy those criteria. But for human life, even that isn't enough. Humans are adapted very closely to the conditions on Earth, and would be unlikely to survive on many other planets. In particular, the concentration of oxygen must be very close to 21% (the mechanism by which your lungs pick up oxygen relies on it being close to this value). If it changes by just a few percent, your lungs wouldn't work, even if another life-form could be adapted to a different concentration.
Very, very few planets have suitable conditions for life at all. Those conditions include the following:
* Orbits an appropriate star (right output, reasonably stable, not too close to other stars or celestial objects).
* Orbits at the right distance.
* Has a strong magnetic field, or some other mechanism to screen out radiation.
* Has oxygen, water, and other chemical precursors for life.
Very few planets would satisfy those criteria. But for human life, even that isn't enough. Humans are adapted very closely to the conditions on Earth, and would be unlikely to survive on many other planets. In particular, the concentration of oxygen must be very close to 21% (the mechanism by which your lungs pick up oxygen relies on it being close to this value). If it changes by just a few percent, your lungs wouldn't work, even if another life-form could be adapted to a different concentration.
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Yes, at this point of our technological evolution the chances of finding a planet capable is slim to none, as we do not have any spacecraft capable of exploring further than the solar system, and even that takes a long period of time to reach the plants in our system. Sometime, in the future, we may find another planet like Earth, but noone can really tell when that day will come.