All life known to science on this planet is organic - it has carbon-based chemistry. Most of what it's made of is water, but leaving that aside, most of the compounds making it up, the "nuts and bolts" of biochemistry as it were, are organic. They are generally large molecules containing chains or rings of mainly carbon atoms. This is because a large number of different kinds of compounds are needed to make even the simplest single-celled living thing on this planet, because in turn biochemistry is very complex, and the element with the most complex chemistry is carbon. However, it's often suggested that silicon-based life is also possible.
Silicon, like carbon, can produce chains and rings and complex compounds with large molecules can be made of it, such as silicone rubber, silly putty and silicone oil. It's also much more common than carbon on this planet. In theory, any organic chemical has a silicon-based equivalent and in some cases the compounds are actually known to exist, such as silane, disilane and trisilane, analogous to methane, ethane and propane respectively. It does look at first glance that silicon-based life is possible.
However, looking at it more closely, as usual, makes things more complicated. The Miller-Urey experiment in 1952 involved passing an electrical spark through a mixture of chemicals thought to be found in Earth's early atmosphere, specifically methane, carbon monoxide, hydrogen, water vapour and ammonia, and it was found that in fact, given this mixture, many chemicals from which living things are made were formed, such as glycine, several other amino acids and sugars, along with bitumen (tar). It would be interesting to repeat this experiment with silane instead of methane, and in fact i've written this about it elsewhere:
However, it can't be guaranteed that a primitive alien atmosphere would be high in silane or other silicon-based gases, and although the known molecules in clouds of gas in space are of various kinds, but all the large ones are carbon-based and there are about 120 of them known compared to about ten known chemicals which contain silicon. Therefore it seems unlikely that a planet would have the right kind of silicon compounds on it for silicon-based life to arise naturally.
Having said that, even life on this planet does use silicon-based compounds such as silica glass, opal and silicic acid, often for structural purposes. The Venus Flower Basket sponge, diatoms and radiolaria all have glass skeletons and there are also examples of animals which have used transparent minerals as lenses for their eyes such as trilobites, which however had calcite (calcium carbonate) eyes. It's conceivable that life elsewhere is keener on using silicon-based compounds in other ways than life here, for instance as bones, teeth, eye lenses, stings and horns, or whatever their equivalents are on those planets. Such structures can be made by living things from glass, examples being nettle stings and the "scouring powder" ash derived from horsetails. The difficulty, i think, would be the production of substances like silicones and siloxanes from raw materials, although for all i know enzymes making those are possible.
Although this seems to rule out the idea of natural silicon-based life, i wouldn't go so far as to bin the idea. There is a project to make a minimal cell, which is the simplest possible cell which could function at all. Just because our biochemistry is complex and needs thousands of different chemicals to make it work doesn't mean that's the only way life can exist. I think it's conceivable that a new, silicon-based form of biotechnology would be able to produce a living organism based on silicon, and that this might even have replaced organic life in some places. I can see this going two ways: the "Grey Goo Scenario", where silicon-based microbes or devices turn much of the surface of a planet into copies of themselves and a silicon-based ecosystem evolves, similar to the oxygen catastrophe on this planet, or perhaps the opposite situation where a silicon-based life form has such special requirements that it can only exist within a specialised environment and is actually specifically designed as silicon-based to avoid the Grey Goo Scenario.
So i do think silicon-based life is possible, though not "natural" life (whatever that means), and that there could also be biospheres where life uses more silicon than it does on Earth.
Thumbnail based on an image by Mateuszica at the English language Wikipedia. All other images public domain, from the NOAA (Venus Flower Basket) and University of Tasmania.
You might remember this:
However, my main motive for making the first video is not so much the question of extraterrestrial life so much as the question of which species wrote the GCSE syllabus, because it seems not to have been one which had instinctive curiosity, was self-motivated or had much individual variation. I imagine it was rather termite-like, perhaps the Horlanians. Here's a painting of a Horlanian sky city:
The Horlanians live quite close to us. It's hard to work out exactly how far but it's more distant than Asterion, which is twenty-seven light years away, but closer than Ras Algethi, which is three hundred and sixty, so that's probably about the roughest estimate i've ever made! They're hexapodal asexual termite-like life forms which are presumably not silicon-based but are sentient, who have got really unlucky with their planet some time towards the start of our last ice age. The orbits of Horlan and Taban were quite close to each other, to the extent that they came close to colliding and Horlan was devastated by tidal forces, leading the Horlanians to leave the planet. However, they were only a Phase II civilisation, so they were not able to leave the system and instead colonised Taban itself. Their method of reproduction requires a substance carried in the atmosphere and they fertilise each other using aerial gametes. Because their original planet is the source of the substance, they were driven to the brink of extinction, but discovered that it was being carried into space by volcanic eruptions on Horlan, then captured by Taban, where it was richest in the upper atmosphere. They therefore built sky cities in which they now live.
All that besides, i think these are the people who run the AQA.
You see, the thing about madness is that it can be a form of escapism and when it is, that escapism can be marketable. I really must get round to talking about marketable madness soon, because it's worth it.
I'm not expecting this video to perform particularly well, but it was on the list and fairly easy to do. It's also too long of course, but what can you do?