Searching for signs of life
Studying exoplanets and their transits has yeilded lots of interesting information on what their atmosphere is made of. But they're so far away, so what's the point? Why do we care what's circulating in their atmospheres? Graihagh Jackson put this to Patrick Short...
Patrick - It's important because these gases, and elements, and these sorts of things give us an idea of whether a planet might support life to begin with
Graihagh - That's Patrick Short who is a PhD student based at the Sanger Institute. I met him because he competed in a science communication competition called FameLab where applicants have to get up on stage and give a talk about something they feel passionate about. Patrick's topic? The quest to find ET.
Patrick - And then also it could provide clues of whether life actually exists. So methane, for instance, is produced by most living things here on earth so if we see a planet with a huge methane signature then it's probably a pretty good place to search.
Graihagh - What are the chances there is actually life out there? I know thousands of exoplanets have been discovered but really, is it likely that there's life out there in the first place?
Patrick - Yes, it's an important question. Actually, one that has vexed a lot of people for a long time so there have been some attempts to quantify this. One of the big distractions or criticisms is, I guess, that there are just so many unknowns that it's really hard to put a firm yes or no or even a probability of life in our galaxy or in other galaxies somewhere else in the universe but there have been quite a few pretty valiant stabs at it. So the most famous one was by Dr Frank Drake and he actually didn't intend it to really be his legacy or to be the thing that he was remembered for. It was basically intended to drum up conversation at a conference and the idea was basically to put a rough, back of the envelope calculation on what is the probability that we find life somewhere in our galaxy.
Graihagh - At this stage I think it would be a really good idea to go through Drake's equation to understand a bit more about what these limitation are. So what is the first item in the equation?
Patrick - So the first item is the rate of star formation in the galaxy. So they express this in terms of number of stars per year. So the Milky Way is about 13 billion years old and Drake puts his first estimate at 1 star per year. So that's a first step.
Graihagh - So 13 billion - let's write that down.
Patrick - So this would be n and star I guess...
Graihagh - OK. What next?
Patrick - So next is what proportion of these stars have planets around them. So not all stars have planets and Drake put this at somewhere between ⅕ and ½ . And then we've also got the number of planets per star. So given that it's got a planet, how many does it have in general and he puts this at between one and five. Given that you've got a star that's got planets we'll assume conservatively that it's got one planet and we'll assume aggressively that it's got five habitable planets per star.
Graihagh - And when we're talking habitable we mean the right sort of temperature, that it's got an atmosphere, and it doesn't spin too fast, it doesn't spin too slowly. All these types of things that are needed for life.
Patrick - Yes, exactly.
Graihagh - So we've got number and stars and then how many habitable planets around those stars. So what's next after that?
Patrick - Then we've got the probability that a habitable planet will result in life. So, I guess, you can consider it the probability that given all the right conditions to find that habitable planet that life will arise on that planet and Drake puts that at one. So he's quite an optimist - he says if we've got a star with a habitable planet then life will arise on that planet. And then this follows quite closely by the second term which is the probability that that life will turn into intelligent life, which Drake also puts at one. So he thinks that it's inevitable that if life is created it follows to intelligent life.
Graihagh - After this, there's still more is there?
Patrick - Yes. So then there's the probability that they don't develop communication skills so Drake was specifically interested in looking for civilisations that would have made contact with us or that we could make contact with. And he puts their probability of developing communication at between 1/10 and ⅕, so 0.1 and 0.2.
Graihagh - OK - communication. Surely that's the final one?
Patrick - No - we've actually got one more. So Drake has factored in exactly how long these communications will last. We we've been doing our search for extraterrestrial intelligence listening not broadcasting but only for a few decades so he places his lower bound and upper bound between somewhere between 1,000 and 100 million years.
Graihagh - OK. So if we take this equation and we do the conservative estimate, what do we get?
Patrick - So the conservative estimate which is: 13 billion stars, ⅕ of which planets, one of which is habitable and which we develop life 100% of the time, intelligent life another 100% of the time, 10% of the time they develop communication tools and they only use these communication tools for 1,000 years. Then Drake estimates that we would have actually only have about 20 habitable planets in our galaxy, which is the Milky Way.
Graihagh - That doesn't seem like very many. What about on the other end of the spectrum?
Patrick - On the other end of the spectrum if you want to go with Drake's most optimistic estimates, then Drake arrives at a maximum of 50 million intelligent species within our galaxy and again, that's just within the Milky Way. So we've got another hundred billion of these galaxies out there so, even if we pick Drake's lowest estimate which is 20, then we've got in the order of 2 trillion intelligent species that we could get in contact with.
Graihagh - I mean that sounds incredibly high so why the heck have we not been able to make contact with any of these civilisations or, indeed, they make contact with us?
Patrick - Yes, so that opens up a whole other famous chapter of physics history. So this question was asked by Enrico Fermi, he's a physicist and, I think the exact terminology he used is "if this is the case then where is everybody." And his point being that if we accept that somewhere between this conservative and aggressive estimate of the amount of life out there that surely either we should have heard somebody, detected something or had somebody get in contact with us. And especially the extra layer of evidence is that we're actually somewhat young in terms of the history of the universe. So, if we can imagine our society fast forwarding just a few hundred million years, then we should most certainly be able to colonise the galaxy and do all these intergalactic travels and certainly communication but, the fact of the matter is, we haven't heard from anybody. So the paradox here is if we can accept that the universe is teeming with life and one of the possible explanations for why we haven't heard from anybody and there's quite a few interesting ones, I think.
Graihagh - Like what?
Patrick - One of my favourites is this idea that if we consider ourselves young in terms of the intelligent life spectrum, then any other intelligent life out there that was older than us may be so incredibly advanced and more developed than us that they don't actually consider us to be intelligent life. Think of our relationship with ants; they communicate with scent and we really don't spend a lot of time trying to device scent systems to try and communicate with the ants. And then there's other more practical considerations that it may be that some of these estimates are of,f and there is life out there but it's so far spread out that the physical limitations of the universe make it impossible for us to communicate with one another in practical amount of time. And maybe they've tried to communicate with us but it's still on it's way so it's just being sent across the universe at light speed and maybe some day we'll hear it.
Graihagh - We could speculate all day on the possibilities but Fermi's problem still stands. Where is everybody? And then that sorta makes me ask, well why are we bothering anyway... Sat in the sun with Patrick in a park, with kids mucking about on the swings, I realised that one of the things we enjoy the most (and I suppose makes us fundamentally human) is that interaction with others. And that combine with our curiosity, well, then the search for life elsewhere in the universe, despite all it improbabilities, is still worth it.
Patrick - Yes, I'm very optimistic. I would be thrilled if we found life somewhere else. I think it would be super exciting. I don't know, I guess we'll just have to wait and see but... who knows?