Life in the universe
Last week, I went to a colloquium (a general-interest talk) by Dr. Charley Lineweaver from Australia National University on the likelyhood of finding Earth-like planets elsewhere in our Universe. It wasn’t that long ago that the first extrasolar planet (i.e., a planet that orbits another star somewhere in our galaxy!) was discovered, and many more have followed. But all of them are big, Jupiter sized planets, in close to the sun, and pretty much inhospitable to life (although, see below). What we’d really like to know is whether there are other planets like Earth out there. Aside from philosophical questions, it’s mainly for the prospect of interstellar colonisation somewhere down the track, but we’d also like to know the chances are of finding life on them.
Interestingly, a large part of his talk was spent on specifying exactly what we mean by “Earth-like planets”. Are we talking about size? Age? Distance from the sun? Atmosphere? There’s so many variables, it’s hard to know which are the most important.
An important question: If there are other Earth-like planets out there, are they older or younger than our planet? Based on models of planet formation around stars, and our obervations of the Universe so far, the current thinking is that most planets matching Earth are in fact older. By about 2 billion years. That means they’ve had that much longer to evolve (or wipe themselves out, if you’re pessimistic. But in that time, the cockroach equivalents will probably survive and evolve again 
Another interesting point he made was that we might expect a requirement for life to be liquid water. This puts limits on how far the planet can be from the sun, based on its surface pressure, atmosphere, etc. For example, greenhouse gases might let you have a warmer planet further away from the sun, while intense pressures (deep oceans?) might give you liquid water even closer to the sun. (High pressures will keep water a liquid even above 100 degrees celcius, whereas at low pressures water boils at lower temperatures (70 degrees at the top of Mt Everest!)
At this point, one of the audience members commented that according to his graphs, he was suggesting life could exist at temperatures of 300-400 degrees celcius (at high pressures), and that was completely unrealistic, since DNA breaks down above about 150. But, as the rest of the audience pointed out, this is making strong assumptions about what life on other planets would be like - does it have to be DNA based? Almost certainly not! It might have a completely different chemistry, evolved to survive its local conditions. It might be based on silicon instead of carbon, or who knows what. (Of course, allowing for that, life might exist on any planet, not just Earth-like ones, but that wasn’t the topic of the talk.)
There was a bit of heated debate - she was quite firm that life shouldn’t be able to exist at those temperatures. Then one of the professors, Michael Nielsen put up his hand, and simply said (I’m paraphrasing, since my memory isn’t so good!) “Well, we can build machines that work at those temperatures. So why not life?” I thought this was just a brilliant answer - it proves by example that you can have complex structures at high temperatures, and makes it very conceivable that the incredible complexity of life could find a way to work around it.
I have to say - Michael Nielsen is a brilliant scientist, and I think that this is a big part of it - he’s consistently able to see to the heart of a problem, and summarise the question (or answer!) in a simple, concise way. I’m always amazed when we’re having a research group meeting on a topic, and we’ve been puzzling (or arguing!) over something, and he’s able to clarify the problem in such a way that it’s clear what we have to prove or show to settle the argument. This is one of the most valuable skills a physicist can have, and is a big part of our courses - it’s also what makes us so valuable to a wide range of companies! Check out Michael’s blog - it’s got a broad range of general audience and real-world researcher posts. Good reading!
Anyway, the upshot of this colloqium was that we can probably expect the first Earth-like planet to be discovered in the near future. Perhaps we won’t be able to visit just yet, but it goes further to confirm we’re just a small fish in a big ocean!