I recently taught my class about the possibility of alien intelligence (which is different to the possibility of alien life). This featured the Drake Equation, which is
N = R* x fp x ne x fl x fi x fc x L
N is the number of civilizations in our galaxy, with which we might hope to be able to communicate;
R* is the rate of star formation in our galaxy.
fp is the fraction of those stars that have planets.
ne is the average number of planets that can potentially support life per star that has planets.
fl is the fraction of the above that actually go on to develop life at some point.
fi is the fraction of the above that actually go on to develop intelligent life.
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
L is the length of time such civilizations release detectable signals into space.
So, this gives us
N = 6 × 0.5 × 2 × 0.33 × 1×10-7 × 0.01 × 69 = 1.3 ×10-7 = 0.0000001
This is not a measure of the possibility of any type of life, just intelligent life.
Other assumptions give values of N that are (much) less than 1, but some observers believe this is still compatible with observations due to the anthropic principle: no matter how low the probability that any given galaxy will have intelligent life in it, the universe must have at least one intelligent species by definition otherwise the question would not arise.
Also, given that complex carbohydrates and amino acids have been discovered spectroscopically inside Giant Molecular Clouds and various nebulae (from which stars and planets are made), then the building blocks of organic chemistry already exist in the universe, ready to be deposited on planets which might well give rise to simple forms of life, such as bacteria and microbes.
In order (according to current theories) for life to arise, you would need a stable star (no blue giants, red giants or other variable or short lived stars), the planet needs to be inside the "goldilocks zone" (the place where liquid water can exist) and should be of sufficient age that life can arise.
Recently, Gliese 581 c, a possible terrestrial planet has been found around a stable red dwarf star (Gliese 581) at an orbit where liquid water could exist, so it is a possible candidate for life.