# Life and Galactic Age: What galactic ages make a planet habitable?

How old does a galaxy need to be to harbor life for prolonged periods of time? Our galaxy was 8 or 9 billion years old by the time life emerged on Earth, so the lower boundary must be younger than that. However, young galaxies are not good places for life, as they contain many massive O and B type stars that explode violently in supernovae and can destroy a planet's atmosphere. Very old galaxies aren't good places for life to develop either, as they would have a higher percentage of red and brown dwarfs with masses less than 0.2 solar masses, and within orbit around those stars tidal heating would occour within the habitable zone of the star and squeeze any volatiles like water off the planet. But how old does a galaxy have to actually be to sustain life?

• To be fair, how would we know? We have just one data point to work with: Earth. For all we know life could have formed after the first 100-million-years and either died out, don't want to talk to us, or are so far away they think they're the only life in the galaxy. And for all we know life could easily be sustained right up until the end (we're not dead yet... but we're working on it...). So, to set your expectation - any answer you get is IMO a best-guess. (The last 10 years have had a lot of, "we believed it impossible... but we found one!" astronomical discoveries.) Sep 5, 2020 at 23:25
• Well we can use stellar ages, which we know, to help us. O stars die within a few million years and B stars within a few tens of millions of years, but B stars can reform from supernovas of O stars, so maybe 500 million years old for the younger end? Sep 6, 2020 at 0:01
• Also, life seems to require heavier elements that would require a certain amount of stellar cycling until there is enough carbon, oxygen, nitrogen, ect. But I'm not an expert on the timeline. Sep 6, 2020 at 0:17
• This sight suggests under perfect conditions in areas with high stellar turnover, there may have been the needed elements within a billion years, but that requires lots of supernovae, and that isn't good for life either. medium.com/starts-with-a-bang/… Sep 6, 2020 at 0:36
• Another limit is that the first supernovae had to spread heavier than hydrogen elements around for planets to form.
– Mary
Sep 6, 2020 at 1:05

I would challenge the premise that a young galaxy is a dangerous place for a habitable planet. Bear in mind that while higher star formation rates mean a greater quantity of hot O and B stars, and therefore supernovae, these objects are still quite rare. For instance, in the Milky Way, only $$10^{-5}$$% of all stars are O stars. Even in an epoch of high star formation - which is generally believed to have peaked in the universe at a redshift $$z\sim2$$, or just under 3 billion years after the Big Bang - it's unlikely that many would be nearby.

To plug in some numbers: the local mean stellar number density is $$n\sim0.1\;\text{pc}^{3}$$, implying a mean inter-star distance of about 7 light-years, or just under twice the distance to Proxima Centauri. If we increase the stellar number density by two orders of magnitude, we decrease the mean distance by a factor of 5. But even this means that there should only be one O star within about 100 parsecs of the any given star - likely too far to cause problems. If I remember correctly, 8 parsecs is the distance at which a supernova becomes dangerous, and it's highly unlikely that that O star will be that close.

The other consideration is whether the elements for life would exist at a given time. It seems like we only need a few billion years for the heavier elements to form in large enough quantities to become abundant in the nebulae that form Population II stars and their planets.

Putting this altogether: I think it's quite reasonable for habitable planets to begin to form around 3-4 billion years after the Big Bang, a little bit after the star formation peak. Certainly, the longer you wait, the better the odds; planets will become more and more enriched with carbon and oxygen, and star formation rates will decrease.

• Well look at the large magellanic cloud. It has an extreme amount of very massive O and Wolf-Rayet stars. Sep 7, 2020 at 1:01
• @NipDip Yeah, the LMC is definitely an interesting case study. I think it's forming stars at roughly the same rate as the Milky Way, but since it's about 100 (?) times less massive, it's effectively forming 100 times as many stars relative to the existing stellar population. Sep 7, 2020 at 3:33

Depends on the largest question of context or how you set your world.

If you see intelligent life which we can interact with and understand as the result of evolution and as things that evolved in their own unique environment then it won't matter.

Because it's basically life only happens as something something life starts happening and natural selection takes care of the rest.

Have a planet with no water or atmosphere or with acid rain on ammonia based air or even the sun.

Just time and the existence of a platform, planet or asteroid or something like that, will mean that life forms will emerge and with time will be intelligent.

The part about intelligent life here means basically human like or human comparable creatures. Like 99% of all human writing is. This is kind of a must because anything produced by a human is human produced so people who think this story is alien either wrong or just being metaphorical.

Thing point is important as we can only interact with stuff that have bodies and with a brain and thoughts and motives like us...etc.

Think of a pet. You can life with them. But can you keep a rock, tree, or a jar of air as a pet? At least it would be a "pet"

Now think of creatures made of thin air and operate on 1165 dimensions and my limited human brain can't handle that. I just can't continue.

Anyway this theory will claim that there are No requirements for life and it just happens with enough time.

On the other hand you can introduce a god or creator or old aliens or something which can mean a lot.

First maybe they did create the universe the way it is or set it up so that particular life forms can exist. Why? Who the heck knows. Earlier point about the minds of absolute alien things.

Here Lovecraft is not a bad influence. I mean on how alien they are and how utterly we are nothing compared to those old things.

Anyway creator can make it so that the universe was created in such a way as to accommodate the little ones, us and the other species, but here is a question: Why this way and not that? Why does the sun in our world appears yellow?

I think there is a degree of absolute arbitrariness in their. It just is.

Even with religions but lets not make it about that.

Anyway your point about older stars seems kind of not that much as like we said either creator creates species which can live under those stars, old or new, or that life evolved to live under those starts. Same end result really.

Like how the Turians got their biology or how the Quarians got their immune system messed up.

Just growing from the soil of the planet, metaphorically, and those who could survive did.

1 factor we have to consider is energy. I really don't think a universe without any energy, suns obviously but could be anything, can be comprehensible to us.

I am no astrophysicist though. So maybe you can have a cold dead universe. But I would love to hear on what you think life will be like in that place.

And lastly we only got like 1 planet with life on it. I mean it seems absurd to judge the entire biological world to a mere planet but also we can judge the entire universe to some of our knowledge. Otherwise there is no thinking.

So the take away from this is how much can we say for certain that A is right?

Big does not mean no laws and also does not mean our laws. So yes I would say in the realm of biology it is possible to develop and grow and have life of all forms in almost all places. But the actual hard data is messing because you know.

Does this help? Not sure man. I am just saying no body went beyond the universe and got a list of hard requirement of life or knows about the exact start age to have what life forms under it.

So if I can advise you then just introduce some side effects of older stars in your world and have the story go as usual.

• Introducing a creator and the Lovecraftian Mythos in the same paragraph is dangerous. I wonder, if a human asked the creator "What is the purpose of humanity?", would the answer be "They were created to be prey for my beloved first creations." Sep 6, 2020 at 2:08
• Yes, but planetary formation cannot occour in young galaxies, they're just too chaotic. Plus, life cannot arise on stars or gas giants. Sep 7, 2020 at 1:00
• @NipDip. We just don't know. Besides this is covered in the post. Can life do that? We have no date. Just because we did not see a black swan does not mean there are no black swans, get me? Sep 7, 2020 at 3:46