Some recent research on the long-term future of the Milky Way prompted me to dig out and re-polish this post from the Life, Unbounded archives of 2010. Turns out it's more relevant than ever...
The galactic theme in the context of planets and life is an interesting one. Take our own particular circumstances. As unappealingly non-Copernican as it is, there is no doubt that the Milky Way galaxy today is 'special'. This should not be confused with any notion that special galaxy=special humans, since it's really not clear yet that the astrophysical specialness of the galaxy has significant bearing on the likelihood of us sitting here picking our teeth. Nonetheless, the scientific method being what it is, we need to pay attention to any and all observations with as little bias as possible - so asking the question of what a 'special' galaxy might mean for life is OK, just don't get too carried away.
First of all the Milky Way galaxy is big. As spiral galaxies go it's in the upper echelons of diameter and mass. In the relatively nearby universe, it and our nearest large galaxy, Andromeda, are the sumo's in the room. This immediately makes it somewhat unusual, the great majority of galaxies in the observable universe are smaller. The relationship to Andromeda is also very particular. In effect the Milky Way and Andromeda are a binary pair, our mutual distortion of spacetime is resulting in us barreling together at about 80 miles a second. In about 4 billion years these two galaxies will begin a ponderous collision lasting for perhaps 100 million years or so. It will be a soft type of collision - individual stars are so tiny compared to the distances between them that they themselves are unlikely to collide, but the great masses of gas and dust in the two galaxies will smack together - triggering the formation of new stars and planetary systems.
Some dynamical models (including those in the most recent work based on Hubble telescope measurements) suggest that our solar system could be flung further away from the center of the merging galaxies, others indicate it could end up thrown towards the newly forming stellar core of a future Goliath galaxy (Milkomeda?). Does any of this matter for life? For us the answer may be moot. In about only 1 billion years the Sun will have grown luminous enough that the temperate climate we enjoy on the Earth may be long gone. In 3-4 billion years it may be luminous enough that Mars, if not utterly dried out and devoid of atmosphere by then, could sustain 'habitable' temperatures. Depending on where the vagaries of gravitational dynamics take the solar system as Andromeda comes lumbering through, we might end up surrounded by the pop and crackle of supernova as the collision-induced formation of new massive stars gets underway. All in all it doesn't look too good. But for other places, other solar systems that we see forming today, it could be a very different story.
Imagine a terrestrial world just beginning to form right now. By the time Andromeda is ploughing through our galaxy in 4 billion years, merging and settling, there could be a species sitting there writing a blog about what a 'special' time and place they exist in. They might marvel at how fortuitous it was for their sun to be thrown further out from the new galactic center - away from all those nasty supernova. They might stare in awe at the myriad new planetary systems being formed as the gas from these cousin galaxies collides, a golden era for new worlds. They would doubtless write papers on how galactic mergers might be signposts to highly habitable regions of the universe - unlike all those boring isolated galaxies out there. It sounds like they're getting carried away, but really they're not doing anything we don't already. Getting perspective is one of the most difficult things to do, especially from our tiny mote of existence.