Are there trees on other planets? For some of us, this question is equivalent to asking how at-home in the universe we are. If trees were only found on earth, then earth is the only acceptable home for we arboreal types, and we'd have little use for interplanetary travel. If there are trees on other planets, then maybe it would be worth looking around a bit& For the sake of this argument, when I say "tree" I mean "tall branchy organism that harvests light for its living". So the question is, given what we know, is it possible or even likely that tall branchy organisms that harvest light are found on other planets? The line of reasoning follows the logic of the Drake equation (http://en.wikipedia.org/wiki/Drake_equation ), which attempts to show how one might estimate the number of other planets with "intelligent life" on them (leaving aside the question about whether there is intelligent life on earth).
The first piece of this concerns the probability of life on other planets. The current observations from astronomy give cause for optimism on this point (assuming that one likes the idea of life on other planets). We're pretty sure there are millions or billions of galaxies (one estimate based on Hubble Space telescope observations estimates 125 billion). Each of these 100+ billion galaxies may have millions or billions of stars. What about planets? Based on a variety of observations, many of which are interpreted as direct observations of planets, astronomers are increasingly certain that a pretty high proportion of stars have planets. Some models of star formation even seem to make the production of earth-like planets more likely than previously thought - although the observational data( http://en.wikipedia.org/wiki/Methods_of_detecting_extrasolar_planets) on this is still lacking. Not that the latter point is crucial; we don't know enough about gas-giant planets yet to really say whether or not something like life is possible on them. Theories about how life might originate on a non-living planet are still in their infancy. However, so far the main problem with the theories is that there are too many of them and they are incomplete, not that any theory has insurmountable problems. To summarize, we have much better evidence for large numbers of planets than we did even a decade ago, and the pieces seem to be falling into place for a theory of abiotic origins of life ( http://en.wikipedia.org/wiki/Abiogenesis)that doesn't make it all that unlikely on planets with the right kind of chemistry. Planets with life on them could easily number in the millions, although there's plenty of uncertainty to go around, with estimates ranging from one (earth) to billions.
Now from here the really IMPORTANT question is not whether life on these planets develops along the lines that we call "intelligent", but whether or not TREES are likely to evolve. Light harvesting organisms are probably nearly universal; life needs energy to run, and the nearest source of handy-dandy free energy to a typical planet is the nearby star. Light competition is probably nearly universal; the light comes from the nearby star - that is, above the surface of the planet. That means that any organism that positions its solar collectors above those of its neighbor "wins" (I suppose something along the lines of "ankle biting" might be a way for a short light gatherer to win, but I'm not aware of any strategy like that among our plants). That means that successful competitors for light are likely to be tall in any place that's good enough to support them. So now we have tall light-harvesting organisms; we're almost there.
What about "branchy"? This one's a bit more subtle, but you actually see branches all over the place in nature once you look for them. Trees, river systems, tracheae in lungs, blood vessels, nervous systems all have a branchy structure. It turns out that branching is in some sense the most efficient solution to the problem of making a bunch of outlying areas feed into a single point. For mechanical strength reasons, having a single sturdy column supporting a bunch of solar panels is more efficient than having a long skinny column supporting each panel, so tall light harvesting organisms have to solve this problem of feeding a bunch of outlying points into one point. You can solve this problem with an explosion pattern, where every outlying point has its own path to the center, but this takes a tremendous amount of material or energy to make each individual path. You can solve it with a spiral or meandering pattern, but then the path between any outlying point and the center is very long and roundabout. Branching is both pretty direct and pretty efficient to construct, so nature seems to use it all over the place.(See the sketch at top of article.)
This argument comes from a book called Patterns in Nature by Peter Stevens, which is, alas, out of print. Branches work best, and nature seems to "find" them as a design solution pretty easily.
So, there are probably a lot of planets, many of which probably have organisms, and tall branchy organisms that harvest light seem to be a pretty likely type - at least we don't know anything that precludes their having originated elsewhere. When the space explorers report back about the planets with trees, I'll have my rope and harness ready.
Now, suppose we find a planet with fairly low gravity and lots of water, so that the trees could get, say, a few miles tall and&