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Life As We Know It

Entry 947, on 2009-02-17 at 20:51:06 (Rating 1, Science)

Yesterday I blogged about the new stuff which has appeared in the area of astronomy over the last 20 years. I realised today that there are several other significant changes which I didn't mention. These aren't just things which interest astronomy geeks (or at least they shouldn't be) because they should be really significant for anyone with any sort of intellectual interest in the world.

The most significant is maybe the discovery of exoplanets. Yesterday I talked about how complicated the Solar System had got with the new classification of dwarf planets and how many items are either in or are likely to be included in that group. But there is also the far more important discovery that planets are very likely to be ridiculously numerous elsewhere in the universe. In fact, we already know about 300 exoplanets (that is planets orbiting other stars).

Why is this a big deal? I mean, shouldn't we expect that planets are numerous? If the Sun has 8 planets plus numerous dwarf planets, plus millions of asteroids, hundreds of moons and millions of comets, why shouldn't other stars be the same? Good question. Logically other planets should exist and everyone expected they would, but proving they exist is actually really difficult.

Why is this so significant? As far as we can tell, planets are the most likely place for life to exist. This is because they have solid surfaces (at least Earth-like planets do), they usually have stable orbits and therefore stable temperatures and those temperatures can potentially be within the range that life can survive in. Also planets often have atmospheres and liquid water which we think are a likely requirement for life.

All of the above assumes I'm referring to "life as we know it" (that classic, and rather over-used phrase). OK, sure, that's true, but there are good reasons to believe that life as we know it is likely to be the most common form of life everywhere (assuming life does exist elsewhere which itself is currently a completely open question).

Life on Earth is based on carbon but we think that is likely on other planets as well because carbon is the only element which bonds in sufficiently complex ways to make the huge complicated molecules of life possible. Those bonds are only stable at a narrow temperature range (let's say 0 to 100 Celsius, just to make it simple) so we can also constrain the temperature life can survive at. Its beginning to look like "life as we know it" might have a similarity to just "life". Of course, its possible that synthetic life might be created using advanced technology, and that might not be based on carbon, but I'm sticking to "naturally" occurring forms here.

So the significance of these exoplanets is that they might be places where life exists. I mentioned that the planets themselves are hard to detect. How would we detect a small part of the planet, like its life forms. It seems impossible. Well, maybe not...

If we extend the "life as we know it" assumption above we can assume that oxygen might be involved in the metabolism of life in most cases. That's because oxygen is an abundant "heavy" element created in supernovae and is active enough to be a good source of energy production. It also has the advantage that it is so reactive that it tends to disappear quickly in a natural state where it isn't being replenished by a life process.

So if we see oxygen on a planet it might indicate there is life there. Once we know a planet exists, spectrographic analysis becomes a possibility. I'm not saying its easy - it isn't - but it is possible.

Planets are being discovered at such a rate that its only a matter of time before one very like the Earth is found. The current smallest planet is about twice as big as Earth because big planets are much easier to detect than small ones of course. The planet would need to be rocky and in a stable orbit around a main sequence dwarf star, like the Sun. It would need to be the right distance from the star so that it was the right temperature. And it would preferably need to have an atmosphere and/or ocean. In those circumstances I would be surprised if life didn't exist.

My prediction is that within 5 years we will have several candidate planets which are showing signs that life exists. We just won't be able to prove it beyond doubt which will, no doubt, be very frustrating!

Comment 1 (1890) by Anonymous on 2009-02-20 at 09:14:33:

There could be life based on elements like silicon in places like the center of our own Earth and other planets. You want to limit life to being based on carbon but is that really realistic? If life was based on another element it might be able to survive in more extreme conditions than you think.

Comment 2 (1892) by OJB on 2009-02-20 at 13:15:07:

Carbon bonds in ways ten times more complex than the next most complex element: silicon. If we assume life requires complex molecules then carbon would be, by far, the best candidate. Also, I think silicon chemistry requires more energy and higher temperatures than carbon so it might work in different conditions, assuming it could work at all because of the lack of bonding complexity.

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