1.2.5 Why is the earth inhabited?
As far as we know today, life exists only on earth. Asking why no other planet or star is suitable for life -as we know it- to take place is only natural. We do not have all the answers yet. All we can ay today is:
- The earth is at the right place in relation to the sun. If it was farther away the temperature would be too low for human beings, and the oceans would freeze. Closer to the sun, it would be too warm, and the oceans would boil off.
- The moon is just the right size to stabilize the earth's orbit. With a smaller moon, mall perturbations would accumulate changing the earth's orbit, and this would change the earth's climate in the long run making life impossible.
- The large sized Jupiter is also necessary. Its strong gravity throws most steroids in outer space preventing them hitting the earth.
- If the earth had a smaller mass, its gravity would not be able to keep its oxygen. And if its mass was greater, it would retain many poisonous gases.
- Earth and the solar system are at the right place in the Milky Way galaxy. If it was too close to the galactic centre where there is a big black hole, the radiation field would kill us.

To be able to create life earth must have been quite stable for hundred of millions of years, and this is not easy.
- A proton is lighter than a neutron. So, eventually, all neutrons will decay into protons. If the protons were 1% heavier they would decay into neutrons. As a result all nuclei would be unstable and disintegrate. Lie would be impossible.
- Protons are stable and do not decay into anti-electrons. To create stable DNA, protons must be stable or hundred of millions of years.
- If the strong nuclear force had been a little weaker, nuclei like deuterium would disintegrate and no heavier elements would have been built by nucleo-synthesis in the first years.
- If the strong nuclear force had been a little stronger, stars would have burn too quickly and life could not have involved.
- Any variation of the value of the weak nuclear force would also have made life impossible.

In conclusion, if any of the constants of the universe had been slightly different, we would not be here. These arguments are part of the Anthropic Principle.

According to Sir Martin Rees the universe depends on six numbers:
1- Epsilon, the relative amount of hydrogen fused into helium at the time of the big bang, must be 0.007. If it had been 0.006, the strong nuclear force would have been too weak to bind the protons and neutrons together. Heavier elements could not have been formed in the stars. If Epsilon had been 0.008, fusion would have been too fast and no hydrogen would have survived the big bang. All the stars would be cold today.
2- N, the strength of the electric force divided by the strength of gravity, is 10E36. If gravity had been smaller, stars would not have condensed, fusion heating them would not have taken place, and the universe including the planets, would be frozen. If gravity had been stronger, the fusion in the stars would have used the fuel too fast and life would not have started.
3- -mega is the relative density of the universe. If omega had been smaller, the universe would have expanded and cooled too fast. If it had been too large, the universe would have collapse before life could have started.
4- Lambda, the Cosmological Constant, determines the acceleration of the universe. If it had been a little bigger, the anti-gravity force would have blown the universe apart, cooling it, and making life impossible. If lambda had been smaller or negative, the universe would have collapsed in a big crunch.
5- Q, the amplitude of the irregularities in the cosmic microwave background, is equal to 10E-5. If it had been smaller, the universe would be totally uniform, a lifeless mass of gas and dust (instead of stars and galaxies). If Q had been larger, the matter would have condensed earlier in big structures that by now would have collapsed in black holes.
6- D is the number of spatial dimensions. In one or two dimensions life would be impossible. With more than 3 dimensions the planets' orbits would not have been stable and, again, life would have been impossible.