The last few weeks I haven’t had a chance to write anything on the website, answer any of the questions or make a youtube video. The reason is because every year the Institute of Physics gets me to do a Christmas lecture on “big” Science topics (action shot from this year's event pictured above). It’s great fun, but most of my recent spare time has been spent researching, planning and organising that. But now I’m done for another year I can continue writing, so I thought I’d do a couple of pieces summarising the talk for those who weren’t there. Enjoy…
The man who invented Star Wars
The first person to suggest the idea of alien life (and therefore a huge amount of Science fiction) was Giordarno Bruno in the 16th Century. He reasoning was that the Universe is so vast, it's ridiculous to suggest our planet is the only one with life. Unfortunately, this didn’t go over too well with the Roman Catholic church because it presents a few sticky issues for crucifixion theology.
For example, in Romans 6:10 and Hebrews 7:27 we are told Jesus’ death happened once and “for all”. If there are aliens on other worlds we have to wonder: are they included in the “all”? If not, God doesn’t value and forgive every conscious being, if they are included, then how do they learn the Christian message? Jesus only dies once, so he can’t visit lots of alien worlds and preach to every being out there. These are fascinating questions, but they were uncomfortable in the 1590s.
So, naturally, the church had strong words with Bruno. Those words were: we’re going to burn you alive. Giordarno Bruno was, in a sense, the first martyr for Science because he was put to death for his commitment to astronomy. Today it’s perfectly legal to talk about the possibility of aliens, so let’s do that.
What is life?
Defining life is tricky. In the Star Trek episode The Tholian Web, the enterprise crew meet beings made entirely of crystal. In the episode Charlie X, they meet creatures who exist as disembodied faces. They meet sentient clouds of light in Metamorphosis and so on. Life is such a varied thing on Earth, how can we possibly define it for the rest of the Universe?
Perhaps there could be creatures made of plasma living in the hearts of stars. Perhaps there are beings living in extra dimensions of spacetime who inhabit the cores of black holes. Perhaps we’re being visited by alien life all the time but it’s so different we don’t even recognise it.
If we say “life comes in an infinite variety of forms” then by definition, the Universe is full of creatures. We could argue that stars are living, as are nebulae and pulsars. That answer isn’t useful. If we want to discuss the question practically we need to limit our scope.
For my money, NASA has come up with the best definition of life so far: “a self-sustaining chemical system capable of Darwinian evolution”. That’s a mouthful but it simplifies to: “very, very complicated chemistry.”
On Earth and other planets, we notice there are lots of simple chemical substances lying around. Crystals, rocks, gases, liquids etc. In fact, there’s more of these simple chemicals to be found than complex ones. Most of planet Earth is made up of things like molten iron, nickel, rocks and discarded Coldplay albums. Interesting chemically, but too structured to be considered "living".
Thing is, simple chemical substances are very regular. Rows and columns of atoms stack up to form crystals, metals and rocks, or they float about freely to make liquids and gases. Simple structures aren’t capable of doing anything interesting because they’re so organised. Chaotic structures like gases have the opposite problem – things float around so easily it becomes a chemical free-for all, nothing is stable enough to interact and evolve. So we have to rule out simple Chemistry and simple chemical reactions…they aren’t living in a meaningful sense.
All life on Earth (over 1.7 million different species discovered so far) is complicated. Atoms arrange in chains, spirals, sheets, clumps and clouds. Molecules are specifically shaped to fit through certain gaps and not others. Particles carry each other from place to place like nanoscopic machines, structures are pulled apart, rearranged and even self-constructed. The Chemistry of life is like an insane factory and this is what we need…intricate, complicated and varied chemical reactions.
Chemistry is a bit like Lego. Atoms and molecules come in lots of different shapes and sizes, some more complex than others. Suppose you had one of those simple pieces which was just a single bump and hole. If your planet is supplied with these pieces only, all we get are chains varying in length. There is nothing more complicated or interesting possible.
Well, most atoms are like that. The majority are shaped to accommodate one or two bonds and that’s it. If all your planet has to play with are simple atoms, life isn’t going to arrange itself into existence. All you can hope for are crystals, rocks, gases and liquids.
Some atoms, on the other hand, are capable of forming several bonds. Carbon and Silicon can form four, Phosphorus can form five and some of the metals go even higher under the right conditions. Out of all these multiple-bond atoms Carbon is the smallest, making its bonds the tightest and strongest. This means Carbon atoms are really the most versatile. Although it has been suggested that life could arise from Silicon.
You do need lots of other elements besides carbon of course; Carbon atoms on their own tend to form simple crystal structures but if you start throwing in other elements, you start creating complexity. There’s all sorts of analogies for this, here are two I’ve thought up.
When you’re making a soup, Carbon is like the water. On its own, pretty boring, but if you chuck in bits of vegetable, spices, stock etc. (other elements) you get something much more interesting. Or, if you prefer a more Christmassy analogy, Carbon atoms are like the branches of a Christmas tree and the other elements are the decorations. On their own, nothing happens, but put them together and pow!
One of the really exciting things we’ve discovered in recent years is that the chemicals found on Earth are not unique. In fact, the elements needed to form complex structures are found throughout the galaxy. The reason we know this is that different elements give off specific frequencies of light. If you look with the right kind of telescope you can analyse the light signatures of different parts of space and find out what chemicals they’ve got. And it turns out that Earth is not rare at all.
Actually, the chemical building blocks used for Earth life are abundant wherever we look. Not only that, but the more advanced chemical structures (things like amino acids, which the elements combine to form) are also found elsewhere. The whole galaxy has the ingredients for life readily available.
If your complex chemical building blocks are all packed together in solid form, life doesn’t arise because they can’t move around and exchange information. Likewise, if your building blocks are floating freely and not coming into contact for any length of time, reactions are rare. Chemical processes, particularly complicated ones, really need to be sitting in a liquid.
Ideally this liquid should be unreactive so it doesn’t interfere with the chemicals floating in it, but it should be good at dissolving the ingredients. There are only a handful of such liquids known, the most common being water. We don’t know for definite that life needs liquid water but it’s a good candidate. Every time we find water on Earth, we find life, so it’s definitely favourable. Life might be able to form in other liquids like ammonia or hydrogen sulphide, but seeing as all those chemicals are pretty common to the Universe it’s immaterial which one we pick.
Most planets probably have the right starting point for life but we need to address the other crucial factor: temperature. If your planet is too close to the sun it orbits, the liquids boil away and all those complicated chemicals burn up. If your planet is too far in the other direction things get cold. The liquids freeze solid and chemicals don’t have enough energy to react successfully.
In order for a planet to have liquids on its surface and keep the chemical ingredients from breaking apart, it has to be just the right distance from its star, orbiting in what’s called the Circumstellar Habitable Zone or…Goldilocks zone. Not too hot, not too cold. In our solar system, Earth is the only planet in the Goldilocks zone for definite (Mars might be, there’s a debate over how wide a goldilocks zone should be). And lo and behold, life has arisen.
To be clear, we don’t know how life actually got started and what else might be necessary (although the field of abiogenesis is yielding some intriguing clues). Perhaps you need a planet with a moon to set up stable tides for rock-pool formation, perhaps you need a magnetic field to protect you from cosmic rays, perhaps you don’t need either and life just “finds a way”. What we know with confidence is that life needs the right ingredients…which are found everywhere…and the right temperature to put them together.
Crunching the Numbers
Now we’re at the stage where we can do a back-of-the-envelope calculation on how likely life might be in our galaxy. First, take the number of Suns. It’s estimated to be between 200 and 400 billion. Let’s take the lower number to please the cynics. How many of those 200 billion Suns are like ours?
This is importanht because some Suns are less genteel than ours (particularly those near the core of the galaxy) and they’re so “bright” they destroy complex chemicals. But, fortunately, those evil Suns make up about a quarter of the galactic composition. Three quarters of the Suns in our galaxy are exactly like ours. So that’s 150 billion Suns. Right, how many Suns have planets going round them?
Well, the answer seems to be all of them. Planets always tend to form as a byproduct of star-formation (the dust which doesn’t get pulled into the Sun ends up orbiting in a disk until it starts clumping together to form planets). In fact, we’ve discovered over 3544 planets orbiting other suns – what are called exoplanets. So if we want to be realistic, we’re probably talking about 150 billion planets as well. Let’s be pessimistic though and assume only 90% of all suns have planets. Let’s also assume only one planet per Sun. That gives us 135 billion planets in our galaxy…minimum.
And how many of them are in Goldilocks zones? Well, of the 3544 we’ve discovered, about 7 of them are Goldilocks (that’s being really stingy with how narrow we make our goldilocks zone though). So that gives us 0.3% of planets forming inside Goldilocks zones. And what is 0.3% of 135 billion? 405 million.
There you have it. By limiting ourselves to carbon-chemical-based lifeforms only, the minimum amount of Suns, planets etc. we end up estimating that our galaxy should contain around 405 million possible worlds. Even if life is a tricky process to get going, those numbers are good. Life could be a 1 in a million shot and we’d still get 405 civilizations arising. And that’s just in our galaxy – which is but one of millions of billions.
Giordarno Bruno was right. It is statistically bizarre to suggest Earth is the only planet with life. There's a possibility it might be, we don't know for definite yet, but the odds are very much in our favour. So, is there anybody out there? According to Science the answer is probably!
George Lucas: amazon
Ian Malcolm: carboncostume
I love science, let me tell you why.