How would we recognize truly alien life on a planet that is probably like a half molten Venus, and why we don't even bother trying. How could we detect any kind life on any extrasolar planet at all?
Given what we know about chemistry, astronomy, and physics we can say plenty about extrasolar planets. We can even have informed scientific opinions or the presence or absence of life as we know it on such planets. Why can't we say anything about life as we don't know it? To understand this we need to understand how life as we know it could be detected by astronomers.
The first thing to understand about discoveries like the planet found in orbit of Alpha Centauri B is that we generally don't get any kind of a picture. As planets orbit a star they exert a force on the star. This force causes the star to wobble back and forth. By measuring these wobbles astronomers are able to detect planets. The basic laws of planetary physics are so well understood that we can determine the mass and density of a planet. From the mass and density of the planet we can model the chemistry of the planet. From modeling the mass, density, and chemistry of a planet we can then compare to planets in our own solar system. The models and theories used to draw inferences about the nature of extra solar planets are well understood and observationally tested theories.
With current technology an optical image of something as faint as an earth size planet around an alien star isn't possible. With technology from the near future perhaps we will get a picture of a brownish greyish or at best bluish dot. That technology was/has been developed for a mission known as the terrestrial planet finder. A mission which was cancelled, but will probably be resurrected due to this discovery. (With the discovery of planets so nearby a resurrected terrestrial planet finder mission will be better funded and result in a better instrument in the end. ) The Terrestrial Planet Finder (TPF) would have been able to cancel out the light from the star and make it possible to see the spectrum of light reflected off each planet. By analyzing the colors reflected or not reflected in the planet's spectrum we can determine it's chemical composition exactly. The TPF would be able to determine the distance of a Earth size planet from it's star as well.
If we find a planet of about Earth's mass, orbiting a sun like star, at the same distance as the Earth, with a similar density and chemistry as the Earth's it would be a safe bet we have found life as we know it.
What about life as we don't know it?
Life as we don't know it would be based on a different chemistry, or be so strange we can't even imagine it. If there is a radically different life form that we could have a prayer of recognizing as being life it would be based on Silicon.
We say this is because silicon is like carbon in terms of it's abundance on the surface of Earth like planets (though not in the universe as a whole). Silicon can bond to up to four other atoms in the same way that carbon can. Hydrocarbon like molecules are possible with silicon substituted for carbon. Silicon can also form long chain molecules with an oxygen molecule between each silicon atom.
In any case these kinds of silicon molecules would form far less readily than their carbon based counterparts. The chemical bonds in them would be stronger and stiffer. This is key for the question of life as we don't know it on a planet like the one nearest Alpha Centauri B. It is so hot and close to it's parent star that one side of it would always face the star and be molten. The other side would be incredibly hot. This is especially true if it has somehow held on to a Venus like atmosphere. This planet would be a half melted Venus. A planet with acidic rain, a scorching hot surface temperature, and crushing pressures. Life as we know it would be destroyed by such an environment. Life as we don't know it, based on stiff un reactive silicon/silicon-oxygen bonds, might just be able to survive in such an environment.
On a slightly more Earth like planet the silicon based life forms would metabolize so slowly that we might not recognize them as living. Even with a really close look recognizing such life forms would be difficult. We would just see the natural growth and replication of crystals or geological weathering as they break apart to form new crystalline life forms. This video illustrates what that would look like.
A life form like that could exist right here on Earth and I don't think we would recognize it as life. A few serious scientist have considered the possibility that our form of life started from organic molecules deposited on a substrate of silicate crystals, or clays, which do self replicate but are not considered living. The theory was that the self replicating property of these simple silicate crystals was picked up by the organic molecules assembled on them. Some work is still being done on to test this theory.
So why don't astrobiologist take looking for this alternative biology seriously? It is because of the information we are able to get from astronomical observations. We are able to know the chemical compositions, masses, densities, and distances from the parent stars of these planets (from which we get a general idea of the climate on an extrasolar planet). That's all the information we have to work with. From this information we would need to reach a conclusion on the presence or absence of life.
Our solar system contains two Earth mass planets, Earth and Venus. As far as we know and can agree on there is no silicon based life on either planet. This is in spite of the fact that the crust of either planet is composed of more silicon than carbon. Only Earth is known to have life and it is carbon based. What's more, if their is silicon based life existing in the crushing pressures, acidic "rains", metal "snows", and hellish temperatures of Venus we cannot recognize it. Even at a Venus like temperature silicon based life would look much like a crystal (see video). If we could not really tell if a silicon based biosphere exist on a Venus like planet we could never recognize it on an extra solar planet.
On a planet as hot as the one near Alpha Centauri B silicon based life could inhale carbon dioxide, eat silicates, drink sulphuric acid dissolved in liquid mercury, and excrete compounds of carbon and sulphur. What spectral signature would that have in the light reflected from a planet? How could we distinguish that from a hellishly inhospitable planet? We would not know what to look for in our astronomical data.
We don't bother looking for silicon based life because we would not recognize the chemistry of it's metabolism on a planet wide basis from astronomical data. That is the best reason for the so called carbon-water chauvinism of astrobiology. That is very different from concluding that such life cannot exist. If such life exist it would not be the first time biology has been surprised to find life. Just 40 years ago the idea that life could exist without sunlight, in the deepest oceans, would have been heresy. Now we know different, imagine what a TPF or improved TPF could show us in another 40 years?
If we can identify carbon-life-product molecules in the atmosphere of an exo-planet, then I think we can probably also identify silicon-life-product molecules. I'd expect silicon life would have to produce large molecules the same way as carbon life does.
Silicon life already on Earth? We wouldn't have know how to look for silicon life to find it. If it's near the surface, we'd probably find it without looking. Humanity's performed quadrillions of informal experiments - entering caves, throwing rocks, baying at the moon, rubbing sticks together... If there is silicon life on Earth, the material of it would be useful for -something- even if we didn't know it was alive. Neolithic man would have used it to sharpen sticks or insulate their homes. We'd have words for the stuff. If it was near the surface, and not microscopic, I think we'd know.