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The Hunt for Habitable Planets


The question is whether we can find different biosignatures in other planets

What we can do right now is to probe planets which are not necessarily very similar to the Earth, usually most of the time are hotter or also slightly bigger.  But this, in any case, is extremely interesting information because we need to understand the big picture of how the plant forms, how it evolved and why is its chemistry as it is before we get into the question of habitability.  Today, we don’t know whether what we define as habitable conditions is something very similar to what we find on Earth or we should expect something different and so we need a bit of statistics in order to make a right comparison to what we find on Earth, a planet in our own solar system compared to what is happening elsewhere and it is through that statistics that we can make then a further step which is what, or how, we can define whether a planet is habitable or not.  Today we just define habitability in terms of just overall temperature of the planet and so that is calculated, roughly, looking at the distance between the planet and the star and the type of star so you can know roughly what kind of temperature you should expect.  But this is very imprecise, as you can imagine.  What you really want to do is be able to sound remotely what is the condition of that planet.  And the way you do so is to look at the atmosphere, both the chemistry but also the, ah, weather somehow, in that planet.  And we can start to do so today for planets which are not necessarily close to habitable conditions but this is certainly something we might be able to do in the next decade or so. 

What we can do remotely is to sound the overall condition of a specific planetary environment, so again the kind of thermal conditions and also the kind of molecule present in that atmosphere then try to understand whether these are real signs of the planet being inhabited or not; that’s a different question because it means that we need to be able to distinguish what kind of chemistry can be caused some sort of inorganic reaction compared to what it would be in atmospheres of an inhabited planet.  

In the Earth’s atmosphere the presence of ozone and oxygen are…oxygen and ozone are so abundant that they cannot be explained with just inorganic chemistry and we need to rely on the fact there is constant source for this molecule otherwise there would be, ah, we would expect it not to be present.  In fact, ozone and oxygen…oxygen in particular is a very reactive sort of molecule so without the constant presence of life and, in particular, vegetation that is producing oxygen at the levels of 21% in the abundance in our atmosphere, oxygen should not really be present and detectable as it is for our planet.

The question is whether we can find different biosignatures in other planets and use those biosignatures to probe the presence of life or not.  And this is a difficult question because in theory we should be able, or would like to be able, to distinguish what can be caused by inorganic chemistry, so just simple reactions that, chemically speaking, you can explain without the presence of some biomass or biogenic origin on the planet.  In practice, probably more a subtle question and if we are to, for instance, detect such a micro-organism like we have on Earth that are producing bases like methane or other type of molecules it would be extremely hard to detect their presence just through the chemistry of the atmosphere because it will be very hard to distinguish this particular signal from the chemistry produced by other processes.