Physicist: Yuppers. In as much as the probability that they don’t is effectively zero.
The statistics on this are a little weak, since we only have one real data point. If you define intelligent life as tool-using, then (based on the age of the oldest tools and the oldest fossils, and the progress of the Earth to date): Intelligent life has existed 0.06% of Earth’s history, and animal life has existed for about 16% of Earth’s history. Moreover, the vast majority of life on Earth (and the toughest) is microbial. So by “yuppers”, I mean that space bacteria almost certainly exists.
As far as the fancy aliens (with their lasers and tentacles) that I assume the question is really about: probably. The universe is crazy big. However, stars are far apart (especially around here), and the likelihood of finding intelligent life is really low.
In the last decade there have been some surprising results from the panspermia people. It seems to be entirely possible, even likely, for life to get kicked from planet to planet and even from star to star. The three difficulties are getting off a planet, surviving in space, and landing somewhere else. During a major impact the material immediately around the impact is vaporized. A little farther out and things are pulverized. Just beyond the “automatically dead zone” is a thin ring where material from the planet’s surface can be thrown into space smoothly (no more than a couple hundred G’s) and without excessive heating. Although no animals could survive the shock, massive G forces have very little impact on single celled life (too small to slosh).
There’s a wide variety of life from Earth that does fine in space. Things like Water Bears, and some bacteria can put up with the cold and radiation, and are more than happy to drop into a state of suspended animation for the trip (forever if they have to). The classic example is a few cells of Streptococcus that survived on the moon (on Surveyor 3′s camera) between 1967 to 1969.
Something you may notice, if you collect large meteorites, is that although the surface tends to be pretty messed up, the interior is frequently quite intact. Although the fall looks pretty impressive, the heat and fireball don’t have time to cook the meteors all the way. In fact the hottest parts of the meteor vaporize during the fall, which serves to keep it cool (like sweat, but like… a rock version). Although it’s unlikely for living things on any one rock to make it through all three stages intact, keep in mind that there are actually many rocks flying around that have been knocked off of planets in the past. There are so many, that one of the cheapest ways to collect samples from Mars or Venus is to go to Antarctica. (If you find a rock sitting on top of a 3 miles of ice, where do you think it came from?) One of the biggest “life is out there” stories came from exactly this source.
Here’s the point: If there’s life anywhere it’s likely to spread everywhere, like… well, like life. Panspermists think that life may have started on some other planet around some other star, and that this life then infected the Earth. This would help explain why the Earth was covered with sophisticated (microbial) life almost immediately after it was capable of supporting life at all. Or to spin it around, if there’s life here (check) it’s had over 3 billion years to get blasted out into the nearby universe.
Mathematician: There are compelling reasons to think that life exists on other planets (perhaps even on a huge number of other planets). If life spontaneously arose on earth from a soup of molecules through an evolutionary process, then all you need for life to be created is the right planetary conditions, the proper raw materials, and a sufficient amount of time. The right conditions may include things like being close enough to a sun that the planet is reasonably warm, but far enough from that sun so that it isn’t burnt to a crisp. The right materials probably include carbon and water among other things. In any event, once you get these things right, you just add time (a billion years probably would suffice) and viola, life is born. That means that for earth to house the only living organisms in the universe, these requirements would have to have been met one time and one time only in all the billions of galaxies that have formed during the 14 billion year history of our universe. That sure sounds pretty unlikely.
Here’s another way to think about it: there is some probability that a randomly selected planet will form life on it within a billion years. If is sufficiently small, then there would be almost no chance of any life forming, including our own, and hence we should not exist. If is sufficiently large, then life would exist almost everywhere in the universe. The only way that we should expect to be the one and only planet with life is if is just right to produce about one planet with life over all the years and on all the planets that have ever existed. But we have no evidence whatsoever indicating that should be perfectly balanced in this way, indicating that the chance of alien life is a good one.
But does technologically advanced alien life exist? Well, if life occurs on many other planets, then we should expect technologically advanced life to occur on at least some of them. Whatever caused natural or sexual selection to select for high levels of mammalian intelligence on earth could lead to intelligent aliens as well. On the other hand though, if technologically advanced civilizations tend to wipe themselves out fairly quickly (say, within a hundred thousand years) or if the process that creates highly intelligent life requires sufficiently rare conditions, then advanced aliens could certainly be the exception rather than the rule.