^ If we're going by personal experience, then the probably is 1, or, 100%, because the only star system we know contains at least one planet that is capable of supporting life. Even discounting Earth, some natural satellites may support life (or have supported it in the past), and there is eevidenc ethat Mars either supports or supported bacterial life; and, before its major greenhouse effect (500+ million years ago), Venus may have supported life. As for other solar systems, it's a best-guess scenario, but the probability of planets or moons supporting life isn't tiny. One must consider that stars generally produce the same stuff; we have gaseous and rocky planets, for the most part. These planets have roughly similar atmospheres and surfaces, and even cores. Of course there will be water cores, iron cores, sulphur rain, and heavy surface carbon dioxide on various planets, but there are only so many elements that can dominate a solar system. That's a limitation of physics and chemistry. Planets generally don't orbit too far away from suns, because after a while the sun's gravity doesn't have enough pull, and so the planet escapes the solar system. This means that in a lot of solar systems, planets will be relatively close to their stars. In the case of very, very large stars, planets could be very far away from their star(s), but there is no reaosn to believe that this is the norm. In fact, the Sun is an average star; some consider it to be rather large, but not in comparison to the true giants. And our solar system is rather typical. And every solar system will have what is called a "habitable zone" (not too hot, not too cold) where life may be supported. And the chances of planets existing in such zones is very high. Our own solar system contains 3 such planets, all of which either support life or may have supported life in the past. Venus is extremely hot right now, but in the past it was much cooler. It's entire surface has been re-made after a brutal greenhouse effect. For all we know, human-like creatures inhabited the planet and then ruined it through their own insidious industry. Maybe (in desperate times) they even sent probes to Earth to plant bacteria on our planet. And that could explain life as we know it today. It's far-fetched, but it's possible. In any case, it is suspectyed that Venus was much less inhospitable a long time ago.
Mars was once better off as well. There is some evidence that water ice remains on Mars, and there is even conjecture that water exists in its liquid form on Mars. NASA is uncertain whether it has evidence of bacterial life from Mars. Even outside of the habitable zone, some moons may contain water or support primitive life. Alas, given that there are billions of galaxies and that each galaxy supports billions of stars, the chances of life existing "out there" are roughly set at 100%. The only catch is that the immense distances between life-supporting habitats may prevent different forms of life from visiting each other, except in rare cases of nearby planets that support advanced life. And even such distances may yet be traversed by technologies we haven't acquired.