Is the Milky Way in a habitable zone?

Question

The Earth is said to be in a habitable zone in the Milky Way where there isn't too much radiation or too many supernovae or nearby massive objects that could disrupt the orbits of comets . The sun unusually follows a near circular orbit around the galactic centre and rotates at the same speed as the orion arm which keeps it out of trouble. But is the Milky Way as a whole also in a blessed situation where it moves relative to other galaxies in such a way that it too protects the sun and solar system from harm?

Answer 1

To the best of my knowledge, this isn't something that's been investigated; the closest one gets are studies that try to determine what types of galaxies might be more "habitable".

Habitability in the sense of "galactic habitable zones" depends on a combination of stellar metallicity -- more metal-rich stars are more likely to have planets e.g., Zhu 2019, and are thus more likely to host life -- and the local density of recent star formation, since star formation produces the short-lived, massive stars that explode as supernovae,[*] which could be deleterious to (nearby) inhabited worlds.

So there's a sort-of trade-off: you want enough star formation to a) produce stars! and b) build up the metallicity of subsequent generations of stars. But you also want to (eventually) reduce star formation so that you get fewer SNe over the long term. To reduce the dangers of SNe, you also want a lower density of stars, so that if a SN goes off, there are fewer nearby habitable systems that might be affected. (This is why "habitable zone" studies of Milky-Way-like galaxies postulate that the dense inner regions of the galaxy would be bad for habitability.)

The best scenario might be to have enough star formation early on that you make a lot of higher-metallicity stars -- but then shut down star formation, so that the most recent generations of stars (and their planets) can evolve without the danger of SNe continually going off.

Here's a paper from 2015 that suggests that massive elliptical galaxies might be the "most habitable" types of galaxies, because they have lots of stars, many of which are metal-rich, and at the same time have low or non-existent star-formation rates. (Here's a counterargument of sorts, which suggests elliptical galaxies might be worse because interstellar comets would encounter other solar systems within the galaxy with higher velocities, making their impacts on planets more devastating.)

One "environmental" way to do this would be to remove the gas that enables new star formation after a few generations of stars have formed, so that the existing population of (already-formed) stars can age, along with any locally formed life, without being subject to lots of SNe over time. There's very good evidence that massive clusters of galaxies can do this, via such processes as ram-pressure stripping and strangulation.

So one could possibly argue that massive clusters provide a "higher habitability zone" for galaxies, since they are better at removing the galaxies' internal gas and shutting down ongoing star formation. (Massive elliptical galaxies and lenticular galaxies -- disk galaxies with little or no current star formation -- are in fact preferentially found in clusters.)

[*] I've ignored the role of white-dwarf-based Type Ia supernovae, which can occur in older populations, including those in elliptical galaxies.

[Edited to add:] In summary, I don't think it's very meaningful at this point to talk about "habitable zones" for galaxies; if one tries anyway, then perhaps the Milky Way, not being in a cluster, is in a slightly less habitable zone.

Answer 2

The ApJ paper Action Dynamics of the Local Supercluster meticulously calculates the paths of the Local Group and other clusters and groups of the Local (Laniakea) Supercluster. Figure 13 shows that the Local Group has not traversed harsh regions and is not headed towards any in the next Hubble time.

All galaxies grow by merging with other galaxies, so it is difficult to find a galaxy that is safe from mergers. A small galaxy may not have experienced one recently, but galaxies are almost always in groups, and merging will surely happen in the future. An isolated galaxy may very well have suffered several recent mergers, which may be why it is isolated. However, galaxy mergers are not much of a danger to life within the galaxies.

Stellar collisions or stellar flybys that might disrupt the Oort Belt comets arising from mergers would be extremely rare, given the vast expanse of space between the stars.

The additional star formation occurring for a period after a merger increases the number of Type II supernovae, a type of supernova that results from the rapid collapse and violent explosion of a massive star, with radiation that is deadly to life on nearby planetary surfaces. However, it is probably not the case that merging causes more supernovae over the long time scale since any stars formed following a merger would have formed eventually. In fact, after the burst ends, the star formation rate of merged galaxies drops below normal for a billion years or so. All the gas present in galaxies eventually turns into stars, with or without mergers.

However, the concept of the "habitable zone" must be completely revised in light of recent evidence suggesting that planets, moons, asteroids, and comets are likely to have substantial layers of liquid water in their interiors. Mars, for instance, was recently discovered to possess such features. Objects larger than about 200 km generally have sufficient radiogenic heating to maintain liquid water in their interiors, potentially making them habitable for billions of years. These oceans, typically located 10 to 100 kilometers below the surface, would be shielded from gamma rays, x-rays, UV radiation, and energetic particle bursts. Life in these environments wouldn't even require a nearby star to thrive. Given this, life on the surface of planets may be a tiny minority, as there are likely billions more subsurface oceans in the galaxy than terrestrial surfaces in the traditional star-heated habitable zone. This potential for life in unexpected places should inspire us about the possibilities in the universe.

This perspective is brilliantly explored (if I do say so myself) in the book Oceans Above: The Habitable Zone is Everywhere by Edward J. Shaya, which provides detailed lectures on how life can thrive completely safely anywhere around nearly any star or around no star at all, and discusses the implications for the future of humanity.