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If I understand correctly, the Earth lies in an area of the galaxy - the "local bubble" - that is relatively free of dust and gases and where no new stars are being born.

What is the current theory on the relation between life on Earth and this location? Or in other words: Would we expect life to exist anywhere in the galaxy, e.g. also inside some nebula, or only in specific areas, and how are they characterised?

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3 Answers 3

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There's a short answer here and a long answer.

Short Answer: It isn't special.

Long Answer: Yes and No.

The Local Bubble really isn't so special with regards to where it's placed in the galaxy. Location-wise, it's perched on the inner edge of the Orion Arm, one of the Milky Way's spiral arms. The Orion Arm, too, isn't incredibly special. It's relatively small - it's often referred to as the "Orion Spur" - and near 1/4 of the way out from the galactic center. Not too far away, but not too near. The donut-shaped area 4,000-10,000 parsecs out from the galactic center is, however, sometimes called the Galactic Habitable Zone. In theory. life as we know it could exist quite nicely any where in this area (I should note that the scientific community is divided on whether the Zone is important, with some saying that it is arbitrary).

But let's dive in deeper. Actually, we're in a decent spot in the "Arm." We're on the edge of it - not too near large groups of stars that could effect our system via gravity, or near any supernova candidates. And the "Arm" itself isn't really a spiral arm. It is, as I said earlier, more of a spur, or bulge. There aren't as many stars to watch out for. Also, we're near the corotation circle - i.e., we orbit the galaxy at such a distance that we move at a similar speed to the arms, reducing further the likelihood of crossing them. Is this necessary for life? Nope. But does it help? Oh, yes.

There are, of course, other factors that make this area a nice spot: No black holes nearby, gamma-ray bursts, etc. But could life survive elsewhere? Yep. These same characteristics can be found in many other places in the galaxy - and life can probably survive in even more dangerous spots.

By the way, I'd avoid using the term "nebula". Technically, we're not in one, or anywhere near one. The term can refer to a multitude of objects - planet-forming areas, "clouds" surrounding white dwarfs, and other rather photogenic places - none of which we inhabit.

I hope this helps.

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It only seems reasonable you could restate "there's nothing around us that kills life" to "the life that spawned around here isn't killed by the various things around here", no? –  corsiKa Aug 12 at 7:09
    
I found your mention of the Galactic Habitable Zone (GHZ), the corotation and being outside the galactic arms most helpful hints in your answer. Googling around and reading some scientific articles on the GHZ, those that favour such a theory put forth similar arguments. So in sum it seems that maybe life in our galaxy is most probable in (prerequisite 1) a circular area not too far and not too close to the galactic centre and (prerequisite 2) between the galactic arms (or, more specifically, outside dense and high radiation areas), [contd.] –  what Aug 12 at 7:36
    
[contd.] resulting in a space very roughly akin to the three green sections in this image from the final episode of the Fox tv series Cosmos: evolutionnews.org/ghab.png –  what Aug 12 at 7:37
    
@CorsiKa, where did I use that phrase? –  HDE 226868 Aug 12 at 16:42
    
You didn't use the phrase specifically. But you implied it by saying we're in a nice spot followed by a small list of things that would potentially kill us. –  corsiKa Aug 12 at 20:02

As my username indicates, I'm eager to point out that we are currently inside the "local fluff", which is "stuff" which has been ejected long ago by some disturbingly active young stars in that evil Scorpius-Centaurus association through the local bubble. The waves of their stellar winds just happen to pass us by in our current million year era.

However, our Sun does circle the entire Milky Way every 250 000 000 years. Dinosaurs appeared about one galactic orbit ago, but died out a quarter of an orbit ago. Earth has had quite advanced life during at least one galactic orbit. That's why I argue that there is little dependence of a solar systems habitability on its orbital position in a galactic scale. (Or that life on Earth has been incredibly lucky, and finally someone will, right? So what do we know from observing no one but ourself?)

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Thanks. No relation to distance from the galactic core either? I understand: the farther from the core the older. And since life takes some time, maybe it cannot exist in "young areas"? Or on stars outside of galaxies? And "finally someone will" what? –  what Aug 11 at 19:54
    
There's no relationship between distance from the core and age. –  HDE 226868 Aug 11 at 20:01
    
@what I don't think the Sun has changed its distance to the galactic core by more than a few or ten percentages per orbit. But even half way towards the center, no hostile environment has been observed. Mostly just empty space there too. "We've" survived several orbits around the Milky Way! That's a huge trip! --- By "Someone will eventually get lucky", I mean just the anthropocentric idea that given whatever anyone can observe, they can only exist in conditions which made them able to observe that. If a thousand men run across a minefield, someone will survive. We might be him. –  LocalFluff Aug 11 at 20:11

This may not be a very scientific answer. I think it gives some incredible insight, so I thought I'd share it...

What "Vlad the Astrophysicist" said:

  • If we condense the entire time that has passed since the Big Bang into 5 min.
  • And we consider the vastness of space and the time each civilization lasts.
  • Then life and civilizations that develop from it rise and fall in less than a split second. Likely never coming into contact with any others.

We can only assume life is quite possible anywhere where the conditions happen to right. But it's very hard to actually quantitatively put a number to this since the universe is so HUGE. All we can say is basically, life (as we define it) is more likely develop in the "Goldilock" zones found in the galaxy, which are bound to exist. Notice this may vary depending our definition or understanding of "life".

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