Thursday, March 06, 2008

Life as we don't know it

In one of those curious coincidences that life occasionally throws at us in order to make us question our senses of logic and reason, this week's BBC2 Horizon programme talked about the Drake Equation ... the day after my post on the very same subject. Weird. The show was actually about the search for exoplanets; planets circling stars other than our own Sun. And it actually made me a tiny, tiny bit cross because it took such a limited and old-fashioned view of the possibilities for alien life. Let me explain ...

The programme talked about how scientists - well, astronomers anyway - are using gravity to find planets. A star's mass creates a gravity well; a dent in space-time that pulls objects in towards it. At the same time, planets circling the star have gravity wells of their own and exert a smaller though noticeable force on their parent star. A particularly large planet will exert a force strong enough to cause the star to 'wobble' and it is this wobble that astronomers look for in order to find exoplanets. They have now found lots of them and, just recently, they discovered Gliese 581c, a planet that exists within its star's 'habitable zone'. Gliese 581c may just have liquid water on its surface and, as the various astronomers and astrobiologists were delighted to explain, this means the potential for life.

Now this is where I started to take umbrage. You see, I think that it's a staggering, swaggering, swollen-headed liberty to assume that life has to have followed the same evolutionary path as ourselves. The term 'habitable zone' is a dead giveaway.

I’ve been re-reading Jack Cohen’s and Ian Stewart’s book Evolving the Alien (also reprinted as What does a Martian look like?) for the umpteenth time recently and something I hadn’t really taken on board before suddenly jumped out at me. In the early chapters, the authors spend some time discussing the term Astrobiology and why it’s the wrong term to use when discussing alien life. Firstly, it’s incorrect; ‘astro’ means ‘star’ and it's highly unlikely (although not necessarily impossible) that aliens would live on a star. Secondly, it’s a term coined by astronomers rather than biologists (if it were the other way round, the term would be ‘Bioastronomy’) and astronomers, gifted individuals that they are, are not experts on the life-sciences. Jack and Ian prefer the term Xenobiology, which means ‘strange biology’, or the broader Xenoscience, which embraces all of the scientific disciplines.

But why should we be bothered? Why is it so important to choose the ‘right’ name? It’s because, if we don’t, we may find ourselves listening to the wrong experts. For example, I’ve just trawled back through just a few of the many press clippings and downloaded stories and features I've accumulated, over the years. Among them I found these various snippets, all less than a year old:

'Mapping what gases comprised Earth's atmosphere during its history, scientists propose that by looking for similar atmospheric compositions on other worlds, they will be able to determine if that planet has life on it ...’

'Although water is an essential ingredient to life as we know it, wet hot Jupiters are not likely to harbour any creatures. Previous measurements from Spitzer indicate that HD 189733b is a fiery 1,000 Kelvin (1,340 degrees Fahrenheit) on average.’

' ... life can only possibly exist where there is liquid water, an oxygen-rich atmosphere and a planetary temperature that is in the 'Goldilocks' zone; not too hot, not too cold - just right.’

'According to statistical analysis based on how quickly life got going on Earth, life will start on at least a third of Earth-like planets within a billion years of them developing suitable conditions ...’

See what’s happening there? ‘Earth-like planet’, ‘essential for life’ and the ubiquitous ‘Goldilocks Zone’ are terms that steer us towards the idea that life can only exist on a world that is similar to Earth. Therefore life would most likely evolve in a similar way to life on Earth. That’s the astrobiologist speaking. The xenoscientist doesn’t think that way. As Cohen and Stewart say so clearly:

‘Instead of looking for carbon copies of Earth, then, we ought to be theorising about and looking for the different kinds of planets, and other potential habitats for life, that exist out there in the wide universe. ‘Exotic’ habitats should be seen not as obstacles, but as opportunities; instead of dismissing them with an airy wave of the hand and saying, ‘Obviously life couldn’t exist there’, we ought to be asking, ‘What would it have to be like if it did?’’'

Now that’s the xenoscientist speaking. And that’s the view that I subscribe to. As I've discussed in previous posts on subjects as apparently unconnected as Pac-Man and silicon-based life, other forms of life could be possible.

Life doesn't have to be life as we know it. It's far more likely to be life as we don't know it.

Life is a powerful force and doesn't tend to let things like an unsuitable environment get in the way. As an example, a colony of Streptococcus bacteria that was living inside the camera of the US Surveyor 3 unmanned spacecraft that landed on the Moon in 1967 survived there. The crew of Apollo 12 visited the craft in 1969 and brought the camera back to Earth inside a sterile container. What scientists found amazed them; these hardy bacteria had survived against all the odds and could still reproduce and carry on as normal once they'd been returned home. And all this despite suffering the rigours of launch, space travel and touch-down on an alien world, and also living in a vacuum, 3 years of radiation exposure, no nutrient, water or energy source, and deep-freeze at an average temperature of only 20 degrees above absolute zero (The temperature at which all movement - even of atoms - stops. Considered to be precisely Zero Kelvin, or –273.15° Celsius, or –459.67° Fahrenheit).

Bacteria have been found to exist in the most extreme environments imaginable:

  • Vacuum - bacteria can survive in near zero pressure and temperature, provided suitable care is exercised in the experimental conditions.
  • Pressure - bacteria have survived exposure to pressures as high as 10 tonnes per square centimeter (71 tons/sq-in). Colonies of anaerobic bacteria have recently been recovered from depths of 7 km (4.2 mi) or more in the Earth's crust.
  • Heat - Archaebacteria can withstand extreme heat and have been found thriving in deep-sea hydrothermal vents and in oil reservoirs a mile underground.
  • Radiation - viable bacteria have been recovered from the interior of an operating nuclear reactor. In comparison to space, each square metre on Earth is protected by about 10 tons of shielding atmosphere.
  • Long preservation - bacteria have been revived and cultured after some 25 million years of encapsulation in the guts of a resin-trapped bee.

Extraterrestrial life may well be so weird we would not immediately recognise it, and scientists looking for alien life should be seeking the unfamiliar as well as the familiar. Xenoscientists and xenobiologists warn that NASA's current approach to 'follow the water' works well if the assumption is that life everywhere is just like life is on Earth — based on water, carbon and DNA. But the 'life as we know it' approach could easily miss something exotic.

The US National Academy of Sciences Panel issued a report in 2007 that states just this. The US space agency commissioned the report from the National Research Council, one of the independent National Academies set up to advise the federal government on scientific issues. The panel of biochemists, planetary scientists, geneticists and other experts considered all the possible ways that life can arise and exist. Recent discoveries of extremophiles — organisms living in conditions of heat, cold and dark and using chemicals once thought incompatible with life — have changed ideas of where life can survive.

"The purpose [...] is to be able to look for life on other planets and moons with an open mind ... and not maybe miss some other life form because we looking for some obvious life form," explained John Baross, professor of oceanography at the University of Washington in Seattle, who chaired the committee. As a biochemist, Baross said lab experiments also show water does not necessarily have to be the basis for life. It might be possible for a living organism to use methane, ethane, ammonia or even more bizarre chemicals. But how weird can it get?

"We had some discussion about how weird to make this because there are so many concepts out here. There are so many theories about what life is and what could be a living system," explains Baross. "NASA and other groups are looking hard for extraterrestrial life. Telescopes search for spectral signatures from other planets that might suggest water is on the surface. Robots on Mars are seeking evidence of water, past or present. We wanted to actually think outside of that box a little bit and at least try to articulate some of the other possibilities besides water-carbon life."

All life on Earth uses some form of DNA or RNA to encode the basic information for replicating and changing, but perhaps other life forms exist that use a different method to do this, the report suggests. NASA might also think about returning to some of the more promising places in our own solar system to look for evidence of life, the committee said. They include Saturn's moons Titan and Enceladus and even steamy Venus.

"If you are a biochemist, Titan is of enormous interest, because it's a carbon moon. It does have clearly some liquid methane or liquid ethane lakes or pools. There could be chemical reactions going on that could be favourable for producing complex biochemicals," says Baross. "The exploration that could lead to a novel life form ... would be the most profound discovery ever made. Stumbling past it or worse, destroying it because it did not look like life, would be an equally profound tragedy."

If life forms on our own planet can exist without water, in conditions of intense heat and cold, in places where sunlight never reaches and inside the very rocks themselves ... isn't it likely that life elsewhere will have evolved to live in the same, and even more, extreme environments?

It's not the first time that a programme like Horizon has raised the hackles on the back of my neck. And not just me ... on this week's edition of Radio 4's Museum of Curiosity, particle physicist Professor Frank Close of Oxford University had a good moan about a 'scurrilous' episode of the programme that was so ill-informed that it intimated that the particle accelerator at CERN in Switzerland is creating anti-matter that could destroy the world. He blamed trash fiction of the sort put about by Dan Brown and the like and explained that, 'The most powerful things we can do here on Earth are puny in comparison with what Nature does all the time. We're being blasted by cosmic rays right now; they're passing through us in this room all the time. The energies of these cosmic rays are vast compared with the things we're doing at CERN and the Earth hasn't been destroyed by them. The experiments have been done before ... it's just the first time that we're going to be doing them.'

Yes, I'm possibly over-reacting but this is an important issue. The BBC has a duty to inform as well as entertain and popular science shows have a duty to present as accurate a picture as possible. Astrobiologists and xenobiologists have differing viewpoints. I don't know which is right - although I have my own ideas on that - but a programme that only presents one viewpoint creates a lopsided and innaccurate view of current scientific research.

It's a trend I wouldn't want to see get any worse.

Images:
'Low gravity alien' by Don Davis for the Cosmic Safari TV series.
Other images from various sources unknown (possibly models developed by Jack Cohen and/or Dougal Dixon).

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