The Science of Why 2
Are we alone in the universe or are aliens out there?
YOU CAN’T EVEN START TO answer this question unless you believe that life—intelligent life at that—could have begun on planets other than Earth. You don’t have to believe that: it’s still quite possible that we are unique in the universe—that no matter how many billions of galaxies exist, containing billions of stars that have untold billions of planets orbiting them, we’re the only ones. But the attitude that we’re the center of everything has been eroding since the 1500s and has reduced us from being the one and only to being one of eight planets orbiting a humdrum star in one of an incalculable number of galaxies.
It’s challenging to figure out whether we’re alone in the universe when we don’t yet have evidence of life anywhere else. But there’s a way of approaching it, mostly thanks to astronomer Frank Drake, who, in 1961, invented something called the Drake equation.
The Drake Equation is a series of unknown quantities that give a sense of what we have to know before we can be confident that there are other intelligent civilizations out there. It’s written like this:
Translated into English, the equation says that N is the number of technologically advanced civilizations out there right now that we might be able to discover. Exciting stuff! N means extraterrestrials. N means aliens!
But N is dependent on everything to the right of the equals sign. As each term is taken into account, N shrinks. That means the chances of us finding another species in the universe, then, is based on:
Rx = the total number of stars • fP = the fraction of those stars with planets • ne = the number of planets that are the right distance from their star to allow the existence of life • fl = those planets that actually do support life • fi = those where intelligent life managed to evolve • fc = the ones that acquired advanced communications technology • and the last number, L = the number of technological civilizations that actually survive long enough for us to detect them.
When Drake came up with his equation, many of the numbers in it could only be guessed at. But since then we’ve managed to get a little more exact.
Science Fiction! We are most familiar with life-forms evolving on land or in water. But are there other possibilities? Two famous astronomers, Fred Hoyle and Carl Sagan, imagined weird and wild gaseous life-forms. In Hoyle’s late-1950s science fiction novel The Black Cloud, a giant cloud of dust and gas invades our solar system and blocks out the sun, threatening all life on earth. The cloud, more intelligent than us, lived off the energy of radiation from stars—what we call sunlight. Earth was saved when the cloud decided to move on. Carl Sagan, in a paper for NASA, put forth the idea of three kinds of giant balloon-like organisms existing in the atmosphere of Jupiter: floaters, sinkers and predators. Floaters would be kilometers in size and would survive by gathering sunlight or processing the chemicals in the atmosphere. Sinkers, like the ocean’s plankton, would slowly fall through the atmosphere but could absorb other things as they fell (such as floaters), the way raindrops grow as they fall. And hunters, of course, would target other organisms to absorb.
Planets orbit stars, so, to start, we need to know how many stars there are in the universe. Our galaxy, the Milky Way, has at least 100 billion stars, and that could be roughly the same number in any galaxy. There are somewhere between 10 billion and 10 trillion galaxies, so if you multiply those numbers (using the larger estimate of galaxies), you get an incomprehensible 1,000,000,000,000,000,000,000,000. Lots of stars.
Did You Know . . . There are different estimates, but there could be as many as 60 billion habitable planets in the Milky Way galaxy.
New technologies, like the Kepler space observatory, have given us a much better idea of how many stars actually have planets around them. There are many planets out there, but we expect the ones that might hold life are those that are roughly the same size as Earth and located in what’s called the “habitable” zone, where water can exist as a liquid. We earthlings assume that water, crucial to life on Earth, would be equally important elsewhere. That means that a planet can’t be too close to its sun (where the heat would evaporate the water) or too distant and cold (where the water would freeze.)
We have already discovered more than 4,000 planets orbiting other stars, and it’s likely that, on average, every star has at least one planet, and at least one star out of every five has an Earth-sized planet in its habitable zone. And that’s not including the claim that more than 90 percent of the galaxy’s planets have yet to be created. A planet’s size is important, too, as it’s harder for life to evolve on a giant gas planet like Saturn than it is on a rocky planet like ours.
Unfortunately, we have no idea how likely it is that a planet—even one in the habitable zone—can support life. So far we only have one example—us—in our solar system. That makes it hard to guess about elsewhere, but even if evidence of past microbial life were found on Mars, that would change the odds considerably. Scientists are hopeful that life might be common, because the chemical compounds crucial for life aren’t limited to Earth at all but are found scattered all over the galaxy.
As difficult as it is to estimate how widespread life might be, what about intelligent life? Here, although it’s really a guess, scientists seem comfortable with the idea that if you find ten planets with life on them, it’s likely that one will have intelligent life. What’s much more important is whether those intelligent species are able to become technologically adept, because only then will we be able to detect or even communicate with them.
Did You Know . . . Philosopher Nick Bostrom has argued that we don’t want to find other species in the universe. According to Bostrom, the rarity of intelligent life in the universe is proof that there is some event, a crucial barrier, that holds back all but a very few lucky technological civilizations, and that so far we’re the only example of that.
Why is this important? If this crucial step is in our past, we’re successfully through it, and the fact that we seem to be the only ones to have made it suggests that achieving technology is a very rare event. But if that barrier to becoming a fully technological, space-exploring civilization is still ahead of us—if many planets have already reached the stage we’re at now, and moved on, why do we see no evidence of them?
Funnily enough, what we find on Mars is important to Bostrom’s theory. He is hoping we won’t find a single trace of microbial life on Mars, because that would signal that life happens often on other planets. And if that’s true, it’s much more likely that intelligent life, like ours, has appeared elsewhere and has been wiped out. For Bostrom, if there’s no life on Mars, we can dream that we’re unique. But if there is, that might suggest a bad future for our species.
That brings us to the last two numbers in the Drake equation. Detecting technologically advanced species would be awesome, but communication with them is the real goal. We have been a technological species for at the very most a few million years. (Stone tools 3.3 million years old have been found in Kenya.) And technology allowing us to communicate with distant civilizations has been around for only about a hundred years. That’s not very long when you consider the lifetime of the planet—4.6 billion years—and it isn’t very much time for another civilization to find us. With that timetable, aliens could have been calling us for millennia and given up long ago because we didn’t answer!
Did You Know . . . We have been inadvertently sending signals to extraterrestrials for longer than we’ve been listening for them. Before cable, TV signals used to be literally broadcast through the air. Those signals may well have been traveling through space. Just think: programs like Rod Serling’s Twilight Zone have been traveling at near light speed since 1959, putting it somewhere between 50 and 60 light-years out there. (Would The Twilight Zone freak out aliens?) Unfortu-nately, most radio broadcasts never make it outside Earth’s atmosphere.
It’s clear that, despite all of our advancements, we still don’t have exact numbers for all of the terms in Drake’s equation, which makes it impossible to come to a conclusion about alien life. Solutions to the equation range from one civilization in our galaxy (us) possibly hosting technologically advanced life to hundreds if not thousands.
Scientists have now started varying the Drake equation to ask: How likely is it that an intelligent civilization has ever arisen in the universe? The conclusion was that unless the odds are worse than 1 in 10 billion trillion (1 in 10,000,000,000,000,000,000,000), intelligent life has to have happened. Surely the odds have to be better than that, right? Of course, we haven’t heard from any of these civilizations yet, but we keep hoping they’ll make contact. Maybe they’re just waiting for an invitation.