"Extremophiles" is a bit of a parochial term - this is the name for organisms that live happily in environments that we consider extreme. Too cold, too hot, too acid, too something to support life, in our terms. Studying the lifeforms that disagree with us on what is a good place to live has started a new field of astrobiology and a new appreciation of the possibility of life existing in the wider universe. Discussing this on In Our Time were Monica Grady (Open University), Ian Crawford (Birkbeck University of London) and Nick Lane (University College London).
The study of extremophiles started with the discovery of a rich ecosystem based on extremophiles living at hydrothermal vents in the sea floor near the Galapagos Islands (an amusing coincidence that it's these islands in particular). The discovery was made by the scientific crew of a submersible called Alvin in 1977, and was a revelation as although extremophiles were known to exist this was the first evidence that there were more than a few outlier species. Previous assumptions about the requirements for life were shaken up by this discovery. The experts emphasised that we (and organisms like us) live in the "extreme" environments when compared to the universe as a whole - we require conditions that generally don't exist. So the discovery that life could exist in more "usual" conditions meant that it's more plausible that life might exist somewhere other than on Earth.
The science of astrobiology was started by these discoveries - this is a multidisciplinary field, which the experts positioned as being part of a trend in modern science. The 20th Century was in many ways about increasing specialisation in the sciences, but now there is a move towards seeing the bigger picture with more collaborations between groups with different specialities. Astrobiology is not exobiology - that would be the study of alien lifeforms and we haven't found any (yet). Instead astrobiology is the search for life elsewhere.
One of the assumptions that was overturned by the extremophiles found by Alvin was that sunlight was critical for life. Knowing that it's possible for life to cope with no sun* opens up the possibility that life might exist on Jupiter's moon Europa, for instance. Europa has a hot core (due to the friction generated by the various gravitational forces exerted on it) and an icy shell, with liquid in between. It also probably has hydrothermal vents. It just wouldn't have sunlight under the shell, but that might not matter after all.
Another foundational insight for the field of astrobiology was the work of Carl Woese in the 1970s on developing a Tree of Life based on genetic data. The traditional view of the high level groupings of organisms is five kingdoms: animals, plants, fungi, protists, bacteria. But Woese's work showed that the real high level division is into 3 kingdoms: bacteria, archaebacteria and eukaryotes. Eukaryotes include all multicellular organisms (plus some single celled ones). Archaebacteria include the extremophiles and were once thought to be just a subset of bacteria - but the genetic data shows that they are as unrelated to bacteria as we are. They also arose first - bacteria and eukaryotes diverged from them later.
Astrobiology is not the same as SETI - the latter is searching for signs of intelligent life elsewhere in the universe, but astrobiologists will be overjoyed to discover a single celled organism existing somewhere other than Earth. The experts spent a bit of time discussing the Drake Equation and how astrobiology fits within that framework. The Drake Equation is an answer (of sorts) to the question of how many extraterrestrial civilisations we might be able to communicate with. I say "of sorts" because, as Bragg pointed out on the programme, the terms of the equation started out as all unknowns. What the equation is useful for is breaking down the question into manageable chunks that can then be investigated. So one term is "how many stars have planets", and since Drake formulated his equation it's been found that pretty much all stars have planets - so clearly that's not a limiting factor. The question that astrobiologists are working on is "how common is life of any sort?" - which is a couple of the terms in the Drake Equation: the average number of planets that are capable of supporting life per star that has planets and the number of these capable planets that actually develop life.
There's still only one example of a life-bearing planet, so it's hard to extrapolate much about the origins of life and how common an occurrence it might be. One thing that might have bearing on the problem is that life only arose once on Earth - all organisms share a common ancestor. I did wonder, although they didn't discuss it, if we can be sure it only arose once - is it possible to disambiguate that from multiple origins only one of which survived? But even if we are sure that it was a one-off event on Earth this may not be because it's hard to do per se. It might be that once life gets going once it uses up the raw materials that it arose from, preventing subsequent developments of life. This is an idea that goes back even to Darwin although other parts of his "small warm pond" concept of the origin of life are no longer thought plausible.
The origins of life aren't the only thing that we only have one example of on Earth (with relevance to the Drake Equation). The jump from the simpler cells of archaebacteria and bacteria to the more complex cells of eukaryotes has only occurred once. Multicellular organisms have also only evolved once, ditto intelligence capable of building a technological civilisation. So even if it turns out that there are many planets supporting life of a sort out there in the universe, intelligence may still be very rare or even unique.
Panspermia is another hypothesis about how life got to Earth - or conversely how it may have got/will get to other places. This is the idea that life is spread through the universe via meteors etc, and so life may not've originated on Earth. There are several things that suggest that this is possible, even if we don't know if it actually happened. For instance we do find bits of rocks on Earth that originated on other planets (the Manchester Museum has a small piece of the Moon and a small piece of Mars that got to Earth as meteorites). There are also micro-organisms on Earth that can live within rocks. And we know from experiments done on space missions that some micro-organisms can live through the heat of entry into the Earth's atmosphere. At this point in the discussion Bragg mentioned Fred Hoyle had been laughed out of the scientific community for proposing something similar many decades ago. Grady pointed out that one reason this sort of theory is looked down is that all it does is shift the question up one level: What's the origin of life on Earth? Space! What's the origin of life in space? Dunno. The modern concept of panspermia is also not the same as Hoyle's - which involved free-floating life seeds travelling over large distances, rather than accidental transfer between planets via meteorite. (This whole section of the discussion made me think of the start of the film Prometheus, which of course is another reason people raise their eyebrows at panspermia - all too often it comes with a side order of "and that's how the aliens made us".)
Finding life on other planets is made more difficult because we don't entirely know what we're looking for. There was a meteorite discovered in Australia that was thought might have fossil micro-organisms in it that hadn't originated on Earth. Eventually it was decided that these weren't the first signs of extraterrestrial life, but it was controversial for a long time. Grady noted that it was easier to figure out in that particular case because it was a rock that had landed on Earth - the task gets much more difficult when another sample means another round trip to Mars. However the only way we're likely to find out if there's life elsewhere is by going and looking - whether that's with robotic explorers or human explorers.
As the Australian meteorite case shows there is a high level of proof required before astrobiologists will be willing to agree that they have found signs of life that are definitely of non-Earth origin. However the experts felt that they (as a field) are getting better at figuring out what to look for. The essential requirements for life are now thought to be water and carbon, but even with those requirements in common with Earth life extraterrestrial life might look very different. The experts emphasised how much chance is involved in evolution - even if you could re-run the history of the Earth it would look completely different despite starting with the same conditions.
This programme felt oddly mis-named - not often the case for In Our Time episodes which generally stay on topic rather well. But this wasn't really about extremophiles, it was about the search for non-Earth life.