Astronomy Now Home

The Drake Equation part 3: Life
Posted: 24 May 2010

Bookmark and Share

Life is a mystery. This isn’t just a philosophical statement about existence, but a biological one too. This May biologist J Craig Venter of the J Craig Institute in California, who was on the team that sequenced the human genome in 2000, announced that his group of crack biologists had created an entire bacterial genome containing 582,970 base pairs and inserted it into an inert, host bacterium, coaxing their new bacteria into life, which then began to reproduce. This isn’t quite the same as creating life from scratch – they were using existing materials extracted from other living cells and reassembled, a bit like Frankenstein’s Monster – but besides the technological ramifications of being able to custom build your own unique bacteria, it is a step in the right direction to understanding the origins of life. That is crucial to SETI, for if we can understand how and why life developed on Earth, we’ll be in a better position to appreciate whether it could happen somewhere else, too.

F(l), the fraction of habitable planets in the Galaxy that are called home to some kind of life-form, is the next factor in the Drake Equation, one that we currently have no answer to. Outside of actually detecting signals from extraterrestrials, one obvious way to determine f(l) is to search for the telltale traces of life – a disequilibrium of ozone or methane in a planetary atmosphere are two standout examples – but we’ve already seen that such measurements will be inordinately difficult.

Are there other worlds like Earth, covered in life and water, out there in the cosmos? Image: NASA/JHUAPL/Carnegie Institution of Washington.

If looking ‘out there’ is too difficult, perhaps we can bring the search back to Earth instead. In his book The Eerie Silence, Professor Paul Davies of the BEYOND Center at the University of Arizona (see our interview) writes extensively about the search for a shadow biosphere here on Earth. To understand what a shadow biosphere is, imagine that life didn’t just start once on Earth, but began several times, in different places and times, each moment of creation independent of the others. Human life is but the topmost branch in the tree of life, but what if there were other trees of life on Earth? These other trees would be filled with bacterial life (anything bigger and we would have surely noticed it by now), which could easily remain hidden given that over 90 percent of the bacterial world is unexplored and unknown. Shadow life forms may work off slightly different chemistry to life hanging out on our tree, and clever experimental tests could shed some light on them. Maybe their nucleotides in their DNA are different to ours, or they use different amino acids to what standard life uses, or their sugars and amino acids have an opposite ‘handedness’ to our own, which are described as left-handed (known as chirality, it describes the arrangement of atoms in molecules that are not symmetrical, and life prefers molecules that have ‘left-handed asymmetry, like a left handed glove as opposed to a right handed one). Instead of being carbon-based they could be silicon-based (though this does seem far-fetched, at least on a carbon-rich world, because carbon can form many more complex molecules essential to life’s process than silicon can). Their weird biochemistry could give them some ‘super-human’ abilities to survive in toxic conditions, or in arid deserts or at the bottom of the sea clustering around volcanic vents. All such ‘extremophile’ life found thus far has been shown to be part of our family tree of life, but who knows about those we’ve still to discover?

The point is, if it could be shown that life started more than once on Earth, it implies that life is no fluke, and could arise on other planets that are at least like Earth. Perhaps then we could assume that every Earth-like planet, rich in carbon and water, happily ensconced in its star’s Goldilocks Zone, will produce life. For the purposes of the Drake Equation, however, accurately predicting how many planets life arises on is impossible. Our best guess, based on what we know about our our own Solar System, is one planet out of four or five that are potentially habitable. Increased exploration of Mars and Europa in the future may raise this number, but we shouldn’t forget that our Solar System seems hardly typical, so base any assumptions about other planetary systems on our own at your peril.

Even trickier to estimate is the fraction of planets where life becomes intelligent, f(i). To begin with, ‘intelligence’ is a loaded word. Let’s give humans the benefit of the doubt and say we’re intelligent; can we also consider dolphins intelligent? What about chimpanzees? Even your pet dog shows a degree of intelligence. If we’re intelligent, should our cave-dwelling ancestors also be considered intelligent? What about neanderthals? It depends by what standard we are measuring intelligence by. Perhaps ‘complex’ life would be better than ‘intelligent’ life, because there is a big difference between large creatures and a planet ruled by bacteria. Plus, the next factor in the equation, f(c), is the fraction of planets on which technological civilisations develop, and it is a good bet that most people equate intelligence to technology.

Even if biologists can pin down the origin of life, explaining where intelligence comes from is just as difficult. Is it just an evolutionary accident that luckily happened to befall us, or does it fall in the class of ‘convergent evolution’ that describes characteristics of life that evolve independently in many species over and over again? Wings, legs and eyes are all examples of evolutionary convergence. On the other hand, elephant trunks, giraffe necks and bats’ sonar sense are examples of specialised, one-off evolutionary traits. Partly depending upon our definition of intelligence, we don’t know if intelligence is a convergent trait that happens to most complex creatures given long enough, or whether it is a one-off fluke that we humans benefitted from. Given that dinosaurs ruled the planet for many hundreds of millions of years without managing to invent the wheel, and that it has taken 4.6 billion years for us humans to arrive on the scene, depressingly it may be that intelligence could just be one of those flukes, unlikely to happen very often on other planets.

Assuming that evolution does take the road to intelligence many times, how long can that intelligence survive? This is the final factor of the Drake Equation, L. It is important to the equation because we want to know how long a beacon could be around for before something happens to it or the civilisation that built it. If L is just a few thousand years, what are the chances that we are going to exist and be listening at the same time as other civilisations given that the Galaxy is twelve billion years old and we will have all evolved at different rates? On the other hand, if species can survive at a technological level for millions of years, it improves out chances of meeting, or at least communicating.

What affect does the presence of Jupiter-like planets have on habitable worlds?

Homo sapiens have been around on Earth for approximately 50,000 years. We’ve been organising ourselves into large communities for several thousand years. We’ve been technological (broadly speaking, for you could argue that iron tools or the wheel are technology; perhaps ‘scientific’ would be a better word, since science leads to discovery and discovery leads to new technology) for a few hundred years. And we’ve had the ability to transmit radio signals into space for a few decades. We’re a very young species, too young to base the lifetime of alien civilisations on our own. Perhaps we will blow ourselves up next week, but perhaps we will progress through this dangerous bottleneck of unchecked population growth, climate change, war and runaway technology to go on to have a prosperous future extending many tens of thousands, maybe even hundreds of thousands or millions of years, into the future. Perhaps, once a species has left the nest and expanded out into space to colonise many worlds, it can conceivably last forever, even if segments of its empire dies out.

One possible explanation of the Fermi Paradox, which asks why, if aliens are out there, we don’t see evidence of them, is something called the Great Filter . It proposes that somewhere along the evolutionary path to intelligent life, is a filter that stops civilisations from progressing. The Great Filter explains the Fermi Paradox simply by saying something inevitably brings death to every civilisation in the Galaxy. Perhaps the Great Filter is in our past, something that makes it difficult for complex life to develop. That would mean on Earth we have managed to sneak past the filter, but on other planets they may not, and the only other life we would then find among the stars would be microscopic bacteria. Or perhaps the Great Filter is still in our future, blocking the path to a long life for humanity. It could be nuclear war, it could be runaway technology such as nano-machines running amok, it could be disease or environmental damage, an asteroid strike, a nearby supernova, or something we’ve not even thought of yet. It doesn’t have to wipe out humanity altogether, just keep knocking us down so we can never progress much further than where we are now.

Another way to look at L is as the lifetime of a beacon, rather than the civilisation that built it. An automatic beacon could continue long after the civilisation that built it went extinct. Such beacons would be relics, like the pyramids or Stonehenge, not sending us a message about what is, but about what once was. Beacons could far outlive their builders and so if the Great Filter comes after the period in which civilisations could build beacons, then we should still be able to see their signals. That so far we haven’t detected them should be a worry to us all, for what fate befalls them could befall us.

Can we improve on the Drake Equation? Some scientists say yes. We’ve already explored how it doesn’t cope very well with the different types of stars or planetary systems we are finding, and some tinkering to factor in these variables may be required. Peter Ward and Donald Brownlee, in their book Rare Earth, wanted to add the number of planets with a large moon for the reasons given in part 2, as well as the proportion of planetary systems that have a gas giant in a similar location as Jupiter to sweep up comets that could potentially be heading our way (however computer simulations modelled by Jonti Horner, now at the University of Durham, and Barrie Jones of the Open University indicate that Jupiter may fling just as many comets our way as it deflects safety away). Conversely, adding in an extra factor for planetary systems without a hot jupiter may be necessary if the Kozai mechanism proves to be dominant. We should also consider that an advanced civilisation could have spread to other planets, widening the number of worlds with intelligent life, even if that life is not native, so David Brin suggests adding several new factors to the Drake Equation to address this, namely the velocity at which a ‘galactic empire’ can expand into space, and the lifetime of the zone of colonisation into which a species has expanded. Meanwhile, Milan Cirkovic of the Belgrade Astronomical Observatory in Serbia contemplates super-civilisations, millions or billions of years in advance of us. To such beings radio may be as antiquated as smoke signals, so Cirkovic recommends adding another factor to the equation, which is the ratio between the duration of the communication window before a species qualifies for super-civilisation status and leaves old technology like radio behind, and the overall longevity of said species. Because these super-civilisations would likely be impossible to contact, says Cirkovic, there’s little point including them in the Drake Equation.

The Drake Equation has got a lot of stick over the years, perhaps some of which is deserved, but a lot of it isn’t. As an accurate measure of how many extraterrestrial civilisations are out there it fails miserably, but as a barometer of the kinds of things we need to know and come to terms with if we are to have any hope for our expectations of SETI being realistic, running through the equation is a useful exercise. Yes, we may not have any useful numbers for many of the equation’s factors, but it does tell us what things we are still ignorant about, and what we still have to learn, and that is a good start. Certainly, positive answers to the Drake Equation from the likes of Carl Sagan and Frank Drake himself have helped maintain enthusiasm for SETI over the last five decades. As they say, what is learned on the journey to discovery can be just as important as the discovery itself. Will the same be true for SETI? Only time will tell.

More SETI articles available here