The first set of potential "Great Filters" I'm going to discuss are those which operate at the level of stars and galaxies — things which might make most galaxies, or most star systems, unsuitable for the development of life.
But first, a digression. In science, especially in cosmology, the number 1 is a very funny number. Scientists don't like unique phenomena. They like to put objects or processes into a class, and study the characteristics of that class. So in the Solar System, planetary scientists study the four rocky worlds, the two gas giants, the two ice giants, and the low-mass dwarf planets. They look at similarities and differences.
Unfortunately, Earth is unique in many ways, the most obvious being that it's covered with life. A single lifebearing planet in the Milky Way — or in the entire observable universe — is just weird. There are lots of stars, lots of planets, lots of galaxies. But only one Earth. If there is some factor which prevents lifebearing worlds, but somehow skips Earth, that's weird, too. On the cosmic scale, 0 is a much more plausible number than 1. There should be no lifebearing worlds, or many.
Digression over. Let's look at some Great Filters.
The first is Active Galaxies, or Active Galactic Nuclei. One thing which radio astronomers began to discover in the 1960s was that some galaxies emit a heck of a lot of energy. Here's a NASA Web page explaining it in more detail.
For a long time the mechanism wasn't well understood, but astrophysicists have figured out that supermassive black holes at the core of certain galaxies are the likely culprits. As matter falls into a black hole, it gives off a huge amount of energy, nearly the equivalent of converting its mass into energy. Active galactic nuclei have black holes which are taking in a lot of mass, releasing a lot of energy, and — via magnetic or gravitational effects —blasting out that energy in huge beams or jets. Obviously, galaxies which are pumping out enormous amounts of X-ray and gamma radiation are going to be a pretty deadly places. Most kinds of life we can imagine would have a very hard time evolving and surviving in an environment like that.
But active galaxies aren't that common. There may be tens of thousands of active galaxies we can observe, but there are hundreds of billions of galaxies in the sky. So as a filter, active galactic nuclei aren't very important. There's also the issue of time: the closest active galaxy is hundreds of millions of light-years away, which means it's hundreds of millions of years in the past. It's entirely possible for a galaxy to go through an active phase and then settle down, perhaps eventually becoming an environment where life could evolve.
The notion of a lethal galaxy is a chilling one if you think about it: a hundred billion stars, many with planets, and all utterly dead. You could stand on a world (wearing a thick lead suit), look up at the sky, and know that you were utterly alone. Brr.
Of course, even a "safe" galaxy can still have regions which are unsuitable for life. This leads to the next large-scale Great Filter, the Galactic Habitable Zone. The concept of the "Galactic Habitable Zone" is analogous to the region with in a star system where lifebearing worlds may exist.
A third galaxy-scale filter relates to the formation of necessary elements, which astronomers charmingly simplify as "metalicity" (to an astronomer, the universe consists of hydrogen, helium, and metals). Hydrogen and helium are primordial elements, but everything else is made in the cores of giant stars, then flung out into space when those stars explode. Obviously, regions without a lot of star formation don't have many supernovas and thus are low in metals. Current theories suggest that the outer reaches of the galaxy are too metal-poor to have many planets at all, let along life-bearing worlds like Earth.
So you can't be in a part of the galaxy with too many supernovae, or you'll get cooked. But if you're in a region with too few exploding stars, you won't have the metals to build planets or living things. Combined with the Habitable Zone limits, this suggests that only about a third of the galaxy is both habitable and likely to have planets. Of course that still represents tens of billions of stars.
Once again, there's also a time component. As generations of giant stars form, make heavy elements, and explode, the average metalicity of a galaxy gradually increases. This, plus the notion that active cores are more common in young galaxies, does at least give us a rough upper bound for how old any civilization could be. Our own solar system may be part of the first cohort of star systems with enough heavy elements to form planets and potentially have life.
There is a lot of wiggle room in those estimates, though. While there may be no planets ten billion years old in our galaxy, it's possible some stars got a head start on our sun by a couple of billion years. So one might have lifebearing worlds that are six or seven billion years old, instead of Earth's four to five. They'd be less common, but not impossible.
Now, on the cosmic scale this makes a HUGE difference. We can more or less rule out looking for signs of life or civilization in any galaxies more than a couple of billion years younger than our own. Which means any galaxy more than, say, two billion light-years away is underage and off-limits. At a stroke we've filtered out three-quarters of the Universe.
Our fourth and final stellar-scale Great Filter is the fraction of stars that are likely to have lifebearing planets. A star needs to have a lifespan long enough for planets to form, and its energy output needs to be steady enough for life to evolve. This rules out nearly all the brighter classes of stars, because they just don't last long enough. Most of the visible stars in the sky, especially the low-magnitude ones with names you'd recognize, are too big and short-lived to have planets with life.
However, because they don't last long, those big bright stars represent only a tiny fraction of the galaxy's stellar population. All the type O, B, and A stars combined make up only one percent of stars.
But at the low end of the stellar scale, small dim red dwarf stars make up the majority of all stars, by a veto-proof margin, and they may also be unsuitable for lifebearing worlds.
There are several reasons to reject red dwarf stars. A lot of them are very old, and thus formed before the heavier elements were abundant enough for solid planets. Their dimness means any planets warm enough for liquid water would have to orbit closer than Mercury circles the Sun, and that puts those worlds at risk from flares — something red dwarfs are very prone to. It's also likely that planets orbiting close to a red dwarf would be tidally locked to their parent stars, and it's very hard to see how a habitable environment could exist on such a world.
There are some dodges to get around these problems. A large moon circling a gas giant in close orbit around a red dwarf would avoid tidal locking (to the star, anyway) and would gain the protection of the giant planet's magnetic fields. There might be red dwarf stars which are stable and don't emit deadly flares. A double-planet system could also avoid tidal locking. These are all unlikely, but not impossible.
Red dwarfs make up three-quarters of all stars. The objections already noted may eliminate about ninety percent of them. (This is a bit of a wild-ass guess, but because there are so many unknowns I'm leaving some room in this filter.) Combined with the other filters resulting from the Galactic Habitable Zone and the metalicity issue, we've eliminated a good 90 percent of the stars in the galaxy from consideration.
That still leaves something like 10 billion stars in our galaxy — and equivalent numbers in most other galaxies — which might have lifebearing worlds, but star type and location in the galaxy are definitely a Great Filter, one which we have obviously passed through.
Ten billion is still a big number, and leaves the question open: Where Are They?
Next time: Planetary Filters
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