Is there life on the exoplanets?

A hundred billion stars, a hundred billion galaxies ^? … just one Earth?

There are a hundred billion stars in our Galaxy ^?  and the accessible Universe contains at least a hundred billion galaxies ^? . We also know that at least three-quarters of the stars are accompanied by planets, a large number of which are in the habitable zone (HZ). It is therefore hard to imagine that the conditions and processes which led to the appearance of life on Earth more than three and a half billion years ago do not also exist on other planets outside the Solar System.

• It’s true that these conditions are relatively strict and, for example, the essential presence of liquid water means that the planet’s temperature must be neither too high nor too low.

• Similarly, if the planet is too large, it will become a gas planet with an atmosphere consisting essentially of hydrogen and helium, in which the chemical reactions of life are in principle impossible, even if certain studies are beginning to explore the possibility of micro-organisms developing in the upper reaches of planetary atmospheres.

• Other conditions are needed to protect the development of life process. Earth was thus protected by its magnetic field and by Jupiter, which deflected the very large number of meteorites at the beginning of the formation of the Solar System. It was also stabilised by the Moon, which enabled the creation of stable climates.

What is the habitable zone around a star?

The habitable zone"(HZ) is the region around a star in which liquid water could be found, that is where the expected temperature on the surface of a planet (where the pressure would be one atmosphere) is between 0 and 100°C. The HZ depends on the mass of the star and is to be found further away from more massive stars (with higher radiation).

What life?

So how do we make a general characterisation of life, without anthropomorphism, in other words without being blinkered by the only case of which we are aware, that of life on Earth?

A first and very general remark is that, among other things, life consists of complex coded information: that contained in the genes which is capable of reproducing itself identically (or almost), as the role of errors is essential for evolution to take place.

This information must be carried by an alphabet able to create words to describe it. In the physical world, there is no more effective alphabet than the carbon chains of organic chemistry: they are capable of describing a very large number of different words in a linear fashion (therefore legible according to a set order). The carbon atom is indeed unique in being able to achieve complex bonding with itself and with other atoms to form long molecular chains. One precondition is the presence of water to promote bonding.
This type of complex carbon chain has actually been detected away from Earth, in our Galaxy ^? and in others.

It is hard to imagine what other type of alphabet extra-terrestrial life could use. The vast majority of the complex molecules detected in the Universe are carbon-based.

How to detect life? 

The only means of extracting carbon from a planet’s primordial atmosphere is to break the highly abundant CO₂ molecules. In so doing, we then necessarily release oxygen. This is what happens on Earth, where – with the help of sunlight – the chlorophyll in plants transforms CO₂ into carbon-based biomass and releases oxygen, which the advanced lifeforms so dearly need. In addition, if the production of oxygen by living beings was not continuous, the oxygen would rapidly disappear in the form of rust, by oxidising the rock.
Finally, we are aware of no natural mechanism capable of abiotic production of oxygen on a planet, that is without the intermediary of life.

Astrobiologists today believe that if oxygen and water is detected on a planet, then life is highly probable.