The habitable zone of a star smaller and dimmer than our sun is closer in to that sun. Even if the planet’s orbit is reasonably circular, the tidal effects may well be too much for life.
I’ve been listing reasons why a planet may be in the “Goldilocks” zone (not to hot; not too cold) and yet be suitable for life above the pond-slime level.
But what if the problem’s on a larger scale than the placement of planets? Some years ago some scientists speculated that major extinctions on Earth tended to come (more or less) every 30 million years. They hypothesized that our sun might be part of a two-star system (many stars are). And that, every 30 million years or so the companion, a small dark star, came close enough to disturb some of the billions of comets outside Pluto’s orbit. A bunch of comets would break free, and come dropping in onto Earth, doing evil things.
However, a space telescope designed to look for such a companion found nothing.
Recently, a similar idea has been revived in the form of a disk of “dark matter.” In this theory, the sun moves regularly up and down in the disk of our galaxy. As it passes through a sheet of dark matter, those comets get loose again. This one will be harder to prove, and it’s more or less certain now that most of the great extinctions of life on Earth are the result of volcanoes on the planet, not the result of impacts from space (unless impacts from space cause volcanoes, which is another point under debate).
Chapter 6: Galactic Problems
Then there are other problems, too, the main one being that a suitable planet may be harder to find than we think. Much of the galaxy is too dangerous for life. You can probably write off any of the stars near the center of the galaxy; there's too much radiation where stars cluster more closely, and maybe the only chance for life is out near the edges of a galaxy, where we are.
And a lot of stars are double stars (two stars rotating around each other). A habitable zone around such a system is possible, but rather unlikely.
While you're at it, write off those stars that, like Polaris, are much less stable than our sun (the brightness changes regularly). And stars that have a shorter lifespan than ours. You don't want solar flares frying the planet, or the whole thing going cold just as the first animal crawls out onto land.
A gamma-ray burst is probably a thin beam of radiation emitted when a star blows apart in a supernova, or when a couple of neutron stars merge. In a few seconds, there's as much energy put out as the sun will release in its entire 10-billion-year lifespan, so they say. There are a few in each galaxy per million years, which is lucky for us, but in the more than three billion years life's been on earth, there have been more than a few in our galaxy. If a gamma beam gets pointed at Earth (what are the chances?) whatever side of Earth is facing that way gets cooked clean of life. But life on the other side of the planet should survive.
Chapter 7: Maybe Life is Inevitable
At this point, let’s take a deep breath and just say, heck, maybe life is inevitable, darn it. Maybe, given the slightest chance at a planet with water, life will form out of the chemicals available on the planet or falling from space. And that it’ll hang around long enough to spread out and begin to evolve.
After all, the building blocks of life seem to be floating around the galaxy. And life on Earth seems to be found in places we once couldn’t have imagined. They’re bringing up life on drill bits from a kilometer below the bottom of the ocean. Tough stuff, life!
The Earth has been pummeled by asteroids and comets, has been froze over (probably totally) into a “snowball earth” form at least twice, and has had volcanoes change the atmosphere to one in which oxygen was rare and methane was the norm. (See my other booklet, Mother Earth, What a Life You’ve Had for a description of some of these disasters.)
Chapter 8: But Maybe There’s Nothing Higher than Single-Celled Creatures
Suppose life forms naturally, everywhere. Maybe evolving past that point is a freak event; after all, Earth spent a couple of billion years with nothing more than that.
We rather associate higher life forms with oxygen. It provides the energy to support us superior forms. Earth started out without oxygen, but about 3.5 billion years ago blue-green algae learned to use photosynthesis for power, giving off waste oxygen in the process. Of course it still took a while to get the oxygen level up to today’s twenty percent.
There is the possibility (some scientists consider it likely) that life pops up anywhere the habitat is suitable. It grows deep under water, then learns to modify the atmosphere of its planet to make the planetary surface suitable for life.
I've often told my daughter that the creation of the first cell was the hard part; all the rest was just inevitable.
But maybe life on most planets never gets past the algae-in-the-ocean stage. Currently, scientists put the evolution of the first life at about 3.5 billion years ago. These creatures, called “prokaryotes” are hard to find now (one type causes strep throat; another lives in acidic mud pots around volcanoes). But at one time they were, in their countless numbers, life on earth. All single-celled, and all seemingly happy for up to two billion years. Yes, folks, it’s probable that for most of Earth’s existence as a life-supporting rock, these things were it as far as life goes.
Sometime between 1.5 and 2 billion years ago, modern life came to be. That’s virtually all the life on the planet. These higher beings are called “eukaryotes” and they have one distinguishing feature; every cell in them is made up of two parts. If you look at one of your body cells, you’ll find it has a smaller cell, called a “mitochondrion” snuggled happily inside it. As a team they work wonderfully together. And without that partnership (or so it seems) multi-cellular being (such as you and broccoli) probably wouldn’t have evolved – after all the primitive prokaryotes spent a couple of billion years and didn’t evolve very much.
Now, to get a mitochondrion inside a bigger cell isn’t that big a deal. Either the bigger cell ate the little one or the little one tunneled into the bigger one and lived there as a parasite. Both happen all the time.
Nor is it all that unusual that the smaller and bigger one could develop a mutually beneficial (symbiotic) relationship.
The larger cell multiplied by splitting in half. First it divided its DNA ladder, created a new one, and split into two cells, each cell getting one of the two DNA ladders. What may be unusual is that the smaller cell, the mitochondrion, managed to do the same process within its cell walls at the same time, then ensured that one of each half of its new cells went with one of the new larger cells. Tricky.
How tricky. Well, a world of prokaryotes, doing their daily business, had taken two billion years to get that synchronized splitting working. They became eukaryotes. Furthermore, there’s a general scientific agreement that it happened only once. All of us are descended from one successful effort so long ago. [Note: A new History of Life – there’s a reference at the end of this book – claims it might have happened once more, somewhere in the Age of Dinosaurs]. That looks like long odds to me.
Chapter 9: Extinction Events – Rough Times on Planet Earth
Sooner or later we’re going to have to look at what scientists call “extinction events” on Earth.
We’ve all heard what a little asteroid did to the dinosaurs. And there are thousands of asteroids tootling around out there, so it’s a wonder we all don’t have that deer-in-the-headlight look in our eyes.
And there are comets. A decent size comet could have done damage much like that that wiped out the dinosaurs and about 99% of life on Earth. Earth recovered, of course, in a few million years, but what if a planet gets hit a lot by comets? There are billions of the suckers in a couple of clouds out past Neptune. Those that formed closer to us were taken care of by Jupiter (we thank you, great planet), but if we didn't have Jupiter in the right place, or if there were another star circling our solar system then we'd be in a rather constant rain of comets.
Other solar systems, most of which probably have billions of c
omets at their outer edges, may not be lucky enough to have a Jupiter in the right place, and there are probably limits to how many extinction events a planet can have before life keeps having to go back to square one each time.
And anyway, just yesterday I got an item in a science feed called Asteroid Impacts 3.3 Billion Years Ago Caused Earth's Oceans To Boil. It says that 3.3 billion years ago (when life was single-cell organisms) at least eight large asteroids hit the Earth, raising the temperature of the atmosphere to 932 degrees Fahrenheit for weeks at a time. That’ll boil the oceans, for a starter.
And life survived that. But we’re interested in higher forms of life, the ones that can call us and say, “ET would like to talk to you.” It’s likely any planet hit by a very large asteroid would continue on with nothing more than simple life forms, ready to start almost back at square one on the evolution chart.
Maybe it is a dangerous place in this universe, and we’ve been luckier than anywhere else.
Yet, in each case of disaster life seems to have come back. It may have taken ten million years (as in the Permian/Triassic extinction), but it came back, often came back in a considerably more complex format. So there are arguments that extinction events could have slowed down progress in evolution, but there are just as many arguments that extinction events were necessary to jump start evolution into a next phase.
On the other hand, extinction events often wipe out larger life forms. An impact such as the dinosaurs met would almost certainly reduce the existing mammal population back to species not any bigger than rats, and there’s no certainty that their descendants would have acquired more intelligence that, say, lemurs.
Even a smaller extinction event could wipe out higher life forms. If we were reduced to the hunter-gatherer stage, would we again produce complex civilizations with science? It would be more difficult; the easy ores and sources of fossil energy were used up by our ancestors and our descendants would be hard put to follow our path.
Chapter 10: Maybe There Are No Other Civilizations
Our planet is tilted on its axis; this gives us seasons. If it weren’t (according to one theory) we wouldn’t have an advanced civilization. In areas where the climate doesn’t change much, plants and animals, parasites and food sources balance themselves out after a while, and leave room for a marginal existence.
In places where there are strong seasons, there are abrupt abundances of food, in the form of migrating animals, and, more importantly, crops in abundance near the end of the season. Gathering and storing this abundance (the wheat or rice harvest, for example) required the organization that turned us from hunter-gatherers to builders of civilizations. Without cold-hot or dry-rainy seasons, advanced sciences might never have evolved.
Perhaps the slap of that planet that gave us our moon provided, many years later, the impetus to get humans out of the trees and, eventually, into the river valleys where building pyramids and walls seemed like good ideas.
Another happy science-fiction concept is the planet with no species above the grunt-and-stick level of intelligence.
At first, this seems ridiculous; things evolve, after all, and we're here, aren't we.
But intelligence could be a rather freakish happening. It requires a rather large brain in relation to body size, and a creature willing and able to put up with the cost of maintaining that brain. It doesn't come cheap - there's a lot of energy required, and at least one monkey is known to have evolved a smaller brain than before, just to survive better.
And raising kids to use a big brain means a vulnerable child for years. That's a problem in itself.
The dinosaurs managed to rule this planet for two hundred million years without seeing the need to produce a species with more intelligence than a dog. And dogs - well, they could have just gone on being dogs forever, without getting any smarter, especially since they didn't have hands.
Sure, we see the results of one ape turning into us. But there were apes on most continents with a warm climate and only in Africa did one of these get "smart". The orangutans of Asia and the monkeys of South America seem to have stopped evolving. The marsupials of Australia didn't evolve intelligence in 50 million years of isolation. There's little reason to suspect that, had humans not evolved, chimpanzees or gorillas or raccoons would have turned to raising wheat, building boats, and issuing junk bonds.
It could be a freak occurrence that one ape, conveniently endowed with fingers not needed for walking, found a protein source (a really great seafood location is suspected) that would support a larger brain. Even then, most of the varieties that this ape evolved into didn't last. There's some evidence that in the last million years that our own species came within a hair of extinction from climate catastrophes, and, if that had happened, maybe intelligence would not have happened again.
Yes, intelligence may be a freak on any planet, and a freak occurrence with any species. A recent article claims that humans weren’t all that bright, until an accidental folding in some DNA upped the IQ of stone-age people immensely.
Chapter 11: Maybe There Are No Other Scientific Civilizations
Starting with Edgar Rice Burroughs, many science fiction and fantasy writers like to have their heroes encounter or be part of sword-swinging civilizations.
Consider the possibility that a few of our own civilizations, seemed quite happy to maintain a culture that discouraged discoveries in science (other than those needed to fend off barbarians). The process of experiment, observation, theory – and sharing of the knowledge gained by these – simply wasn't important, nor were people encouraged to collect facts not immediately useful. Changes to traditional thought were discouraged. Or so it seems, on a surface glance, to a modern audience.
Maybe there are alien civilizations out there where a warrior class and long swords are maintained, er, forever. Where individuals who suggest their planet isn't the center of the universe are always burned.
It seems like a long shot, but I'm trying to look at all situations.
Chapter 12: Radio Waves? You've Got to Be Kidding
Perhaps alien civilizations out there don't care whether we exist or not. If so, we may have trouble knowing if they're out there. People started searching for aliens by listening to radio waves, because that's how people communicated when they started looking and listening to the universe to find aliens.
But, to an alien - and greater - science, radio waves may be as old hat as smoke signals and drum messaging are to us. If it turns out that nothing can go faster than light, aliens may not think it worthwhile contacting primitives, or spending dozens or hundreds of years getting to visit us. So they don't send us any signals, and consider us stupid if we're not using linked neutrinos for communications.
Even now, the planet is using the electromagnetic spectrum less and less. Most of my information no longer comes via antenna, but along cables instead. Broadcasting may be a phase other cultures have passed through long ago, and maybe we're looking for the wrong thing, like the primitive technological dimwits we are.
Chapter 13: They Don't Want Us to Find Them
Here's another possibility. Suppose there's a galactic civilization out there. Suppose it knows we're here. But all the members vote to keep us separate.
It can't be from fear of us taking over the universe; we're too far from that, and wiping us out is really easy, whenever they want to do that.
But if we ourselves discovered a previously unknown tribe somewhere in the Amazon, one that knew nothing about the rest of the world, anthropologists would go silly trying to figure out how to study them . And those anthropologists would immediately try to figure out how to study that tribe without tipping the tribe off to the benefits of Modern Civilization
Unless I'm mistaken, one of the premises of Lilo and Stitch was that Earth had been declared just such a preserve. Actually, it was declared a preserve for mosquitoes, and humans had to be left alone because they were part of the mosquito food system.
So there you go.
We're quaint and primitive, and being studied. UFOs are either anthropologists that got seen, or joy-riding teens sneaking through the fences out of curiosity.
This, of course, doesn't answer the question of why our listeners haven't heard any radio traffic coming from out there. So now I'll address that one.
Chapter 14: They're Out to Get Us!
Canadian author Farley Mowat wrote a couple of histories of the tugs that went into the north Atlantic to try to rescue boats in trouble. The Grey Seas Under was about the tug, Foundation Franklin, and gave me the bejeezly desire never, ever, to go near the ocean when there's a storm.
Anyway, at one point in the book, it's mid-war, and a freighter is in trouble off Newfoundland. It's broadcasting an SOS. The tug sends a couple of messages that they're coming, and please, please, stop the broadcasting. There are, after all, a number of Nazi U-boats in the area, and a salvage tug would be a real prize for a submarine.
The freighter keeps broadcasting right up to the moment the tug pulls alongside and someone from the tug runs for the radio shack on the freighter to get the idiot away from the Morse key.
Well, some science-fiction authors have hypothesized just such a scenario in the universe, with some alien civilization seeking out emerging technologies and squashing them like cockroaches. And a bunch of surviving civilizations saying to themselves, "The poor fools on Earth. If only we could warn them without endangering ourselves."
So there could be a Vegan Orbital Fort out there looking for emerging civilizations to eliminate. We've learned from our own history that it's a rough scramble to survive and the meek don't always inherit the planet they evolved on. In some scenarios mechanical or digital entities eliminate biological ones, and a planet terminator can be as simple as a large rock or a couple of baseball-size black holes dropped onto a blue globe.
Some people figure that it's inevitable that organic intelligences (like us) inevitably create superior digital intelligences that at best ignore us, and at worst, try to get rid of us. Wait - that's the premise of the Terminator movies. Sorry.