Just The Caffeine Talking

Great Filters, Part 3: Planetary Filters

I've discussed galactic and stellar-scale Great Filters. Now it's time to look at them on a planetary level.

The first is the question of how stable planetary systems are. This is a problem which goes back to Isaac Newton. When Sir Isaac compiled his great work the Principia, he was bothered by one implication of his discoveries. There didn't seem to be any general solution to the "Three-Body Problem" — that is, how the interactions of multiple bodies in a planetary system would work. A general solution would show obvious regimes of stability; otherwise you'd just have a constant iterative process of constant perturbations with planets shuffling around and occasionally getting flung out into interstellar space.

The Italo-French mathematician Joseph-Louis Lagrange discovered one set of special conditions for the three-body problem, the famous "Lagrange Points" of stability around a moon or planet orbiting a larger body.

Before the discovery of exoplanets, the question of stable planetary systems was very much up in the air (beyond it, really). With only one sample, our own Solar System, we really had no way of knowing if we lived in a freak example of a stable planetary system, or if such things were commonplace.

The answer seems to lie between those two poles. There are lots of planetary systems, so many that it appears to be the norm. Only stars too young to have formed planets at all may lack them.

But the study of the orbits of those exoplanets has revealed a surprising amount of instability. Apparently planets migrate all over the place in stellar systems, shifting orbits, switching places, and in all likelihood getting kicked out into interstellar space. The stable systems that remain are simply the aftermath of chaos, smoothed down by billions of years.

So how is this a filter?

Well, we still need to figure out what proportion of planetary systems are unstable in the long term. Even if a potentially lifebearing world doesn't get tossed into space, jostling around its home system can't be a good thing for struggling life forms on its surface. We need to know how long planets are likely to remain in the habitable zones of their parent stars, where temperatures are warm enough for liquid water but not hot enough to boil away all the lighter elements.

This isn't an easy number to pick. Astronomers are still figuring out how planets form and how they interact over billions of years. Since we see lots of planetary systems, with plenty of worlds, instability can't be too great a problem. And many of our best methods for detecting exoplanets are biased toward weird systems — it's easier to detect a large world orbiting close to its parent star, and it's precisely those systems which are the product of lots of planetary migration and interaction.

I'm going to make a completely wild-assed guess and assume ten percent of star systems have chaotic interactions among their planets which make it unlikely for any of those worlds to develop life. In last week's post I winnowed the number of star systems which might be home to lifebearing worlds to about 10 billion in the Milky Way. This lops a billion off that figure.

Next comes the big one: how many Earthlike planets are there? By "Earthlike" I mean a rocky planet with liquid water and an atmosphere density somewhere between 1 percent and 200 percent of Earth's, orbiting in the habitable zone (sometimes called the "Goldilocks Zone" of its star system.

At present we know of some 4,000 exoplanets. Of those between 20 and 50 are in their star's habitable zone, depending on whether you take the optimistic or pessimistic view of limits on that zone. For simplicity's sake we'll say 1 percent of planets are in the habitable zone. For the Milky Way, that means 90 million worlds.

Now, some of those bodies are not exactly what you'd call "Earthlike," with masses more comparable to Neptune or even Saturn. It's hard to see how a giant planet like that, even with a rocky core, could avoid a lethal greenhouse effect. Roughly half the worlds on the list of exoplanets in the habitable zone of their parent stars have masses greater than 10 times that of Earth, which I consider beyond the pale. So our 90 million becomes 45 million.

After that comes a grab-bag of filters based on what we know of Earth's history, with absolutely no hard numbers to base our estimates on. These potential planetary filters include:

Giant Impacts: We believe the Moon was formed when Earth collided with a Mars-sized body early in its history. This may have affected Earth's chances of developing life, by blowing off the planet's original atmosphere and preventing Earth from winding up like Venus. That's highly speculative, and it's also unclear how common such impacts might be. A lot of airless bodies with ancient surface features show really big impact basins, so it's quite possible that almost planet gets walloped by something big in its early years. For lack of any hard numbers I'll call that one a coin toss: 50 percent.

Moons: It's been suggested that the Moon makes Earth a home for life by stabilizing the planet's rotation and axial tilt. I'm not sure I buy it — it sounds a lot like correlation (Earth has a big moon, Earth has life) being turned into causation (Earth's big moon is responsible for Earth having life). Since moon formation appears to be tied to impacts, I'm just going to fold this one into the previous filter.

Snowball Earth: When plant life evolved on Earth and began transforming the planet via toxic waste products, one side effect was a drastic drop in temperature as the plants sucked up all of the atmospheric carbon dioxide, reducing the greenhouse effect keeping Earth warm. Earth's surface was covered by ice, with life surviving only in the oceans under the icecap. The "Cryogenian" era may have lasted up to 200 million years, ending only when the Sun's gradual increase in temperature overcame the Earth's cooling.

But what if Earth had been at the outer edge of the habitable zone, or the Sun's output wasn't increasing as rapidly? Snowball Earth could have lasted hundreds of millions of years longer, or been permanent. To me, this one seems like an important filter, one likely to repeat elsewhere. How to estimate the outcome of two independent processes is very hard, so I'm just going to make this one another coin-flip: 50 percent.

Waterworlds: Earth's surface is 75 percent ocean. Raise that sea level by just a couple of miles and the figure would be close to 100 percent. If we assume that more massive worlds retain more light elements, then any world more than, say, two or three times Earth's mass might be entirely covered with water. This wouldn't prevent life from evolving, but might well prevent technological civilizations from arising. While it is possible that an aquatic alien civilization might come up with ways to communicate across interstellar distances before they discover fire or electricity, I'd hardly call it likely. That one's another coin-flip.

Summing up these guesswork filters gives us 1 in 8, which I'm going to adjust to 1 in 10 for simplicity, and to cover any other planetary filters I didn't think of. Applying that to our existing figure we get 4.5 million worlds in the Milky Way which might be home to life and civilization. That's still a lot, but it's certainly less than the 10 billion figure we had at the end of my last post.

It's also cause for some limited optimism. Our world is already a .0045 percent long shot. But with literally millions of possible inhabited worlds, we're certainly not out of the (dark) woods yet. Where is everybody?

Next time: Life Filters.

Great Filters, Part 2: Galactic and Stellar Filters

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. 

Simply put, there are parts of any galaxy you just don't want to be in: the galactic core is bathed in radiation from the central black hole, and is rich in bright, short-lived stars which end their lives in massive supernova explosions. Such explosions bombard nearby systems with blasts of high-energy particles, making them unfriendly places for living things. Gas and dust lanes along the spiral arms also have too many supernovas for comfort. According to some estimates, about half of the galaxy might be outside the Habitable Zone.

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

 

Great Filters, Part 1

Two weeks ago, during the world's first CYBER WorldCon, I participated in a panel discussion called "COVID-19: A Great Filter?" The panelists were myself, writer and Coronavirus expert Vylar Kaftan, and astrophysicist Valentin Ivanov. Our topic was the concept of the "Great Filter" and whether a plague like the current Coronavirus outbreak could qualify.

         The more I thought about Great Filters, the more I realized there was no way to fit the topic into one 50-minute Zoom panel, or even a single 'blog post. So I'm going to write a series of posts about potential Great Filters, working my way through different types and examining what we know and how plausible they are.

         But first: what the heck is a Great Filter?

         The term comes from an essay by economist and futurist Robin Hanson, which you can read here. To summarize, we know that technological civilization capable of communicating across interstellar distances is possible, because we exist and could do it ourselves. But if that's the case, as Enrico Fermi famously asked, "Where is everybody?"

         Science doesn't like unique phenomena. There should be either multiple technological civilizations, or none. We know it's not impossible because we exist, so . . . where is everybody?

         Just to complicate matters, it's unlikely that most civilizations out there would be about the same age as our own. It took billions of years for Earth to produce hominids, and hundreds of thousands of years for humans to develop radio telescopes and space travel. Even a variation of just one percent would mean other civilizations would be millennia, if not eons older than our own. Even at modest rates of growth they should be detectable across interstellar distances even if they aren't deliberately signaling to us.

         Frank Drake tried to quantify the probabilities of such civilizations arising in his famous Drake Equation, which you can read about here. He broke apart the likelihood of civilizations existing into a number of sub-probabilities. How likely are stars to have planets? How likely is it for a planet to support life? How likely is it for intelligence to arise? And so forth.

         Hanson took that another step. Obviously something keeps the number of technological civilizations very low. How do we know that? Again, because we're here. If civilizations were common, either we'd have heard from them, or seen signs of their existence . . . or we wouldn't exist because a bunch of settlers from the Perseus Arm of the Galaxy colonized Earth right after dropping an asteroid on Yucatan to clear away those pesky dinosaurs 65 million years ago.

         The big question is whether the Great Filter — the real bottleneck keeping the number of civilizations vanishingly small — is behind us or ahead of us. In other words, is the origin of life or the origin of intelligence the Great Filter, in which case we've passed through; or is there something about to happen to us which we can't predict or prevent, and which has hit every other nascent civilization in the Milky Way?

         As I said, this is a subject too big for a single post, so I'm breaking down the potential filters into categories. First come Stellar Filters: things related to the Galaxy or to stars which make life or intelligence unlikely. Next is Planetary Filters, reasons to think Earth might be extremely rare in some way. Then Life Filters, looking at just how likely it is for life to arise and evolve into something which could become intelligent. After that comes Intelligence Filters, also including issues of language and tool use. Then Civilization Filters, which might make large-scale social systems uncommon in the Galaxy.

         That brings us up to the present, and then I'll start looking at Filters we still may have to get through. Disaster Filters are things which might bring down or destroy our civilization. Space Filters are things which might confine us to a single planet or a single star system. Perception Filters are reasons for optimism: maybe we just haven't seen everybody yet. And finally there are Meta Filters, which are essentially ways to reject the entire question.

         And so, having finished that long-winded introduction, I will stop here and leave everyone hanging. Next week: Stellar Filters!