Thursday, 18 August 2016

Our precious planet. Part 1: a safe place in the cosmos (updated 2016)

[This is an updated version of the post initially published 3 years ago]
In the next four posts (including this one)  I hope to show the strange combination of circumstances which have led to our civilization. You could dismiss it as the god of chance, since chance can be used to explain anything or nothing. Or you could say it had to be like this or we would not be here to witness it. Fortunately, early man didn't take this defeatest attitude: if he had there would be no science.

As far as I know I’m not saying anything new or cranky here; only trying, as a layman who follows science, to list the factors that make our planet unusual and tailor-made to our requirements. If you can find anything in the following that needs challenging, correcting or clarifying please let me know. 

Part 1 shows how the Earth and Sun are placed in a benign part of a hostile universe and in a region where the 92 elements of the periodic table are available (e.g.carbon,  oxygen, silicon, iron, uranium).

Part 2 will show how the Earth and its companion Moon have provided the conditions needed for biological evolution.

Part 3 will show that our civilization’s development also depended on the properties of the Earth and the Moon.

Part 4 will show how life and civilization could only have emerged not just where they did, but when they did.

 Part 1. A safe place in the cosmos

The universe contains of the order of a hundred billion galaxies which basically fall into three different types: irregular, elliptical and spiral.  Each galaxy contains around a hundred billon stars but only spiral galaxies, like the one in which the Earth and Sun are situated, are likely to have safe zones.

About 25% of galaxies are irregular chaotic swarms of stars with abundant gas and dust. Any planetary system attached to one of these stars is likely to be bathed in radiation hostile to life and subject to a randomly changing interstellar environment in which ‘nearby’ supernovas explode frequently. This is unfortunate since they contain a lot of young stars, not too dissimilar to the Sun, which synthesise the elements (e.g. carbon) needed for life.

Some 65% are elliptical galaxies in which the stars rotate around a central black hole in an ellipse. Because of its oval shaped orbit a star within such a galaxy is periodically brought in close to the black hole and bathed in lethal radiation. In addition, the stars in elliptical galaxies are smaller than the Sun and unable to manufacture the elements needed for life.

The remaining 10% are spiral galaxies, which include our own Milky Way. Although largely hostile to life these have safe regions and the Sun with its companion planets is in one of them. The Sun orbits the galaxy in a rare circular path so that it is not transported too near the central black hole, which give off dangerous radiation.  Also, it is in a ‘young star zone’ where the 92 chemical elements found and needed on Earth have been produced by supernovas (exploding stars) and not in a globular cluster (see below) or other regions of our galaxy where these chemicals are not present.
Supermassive black holes seem to be a normal feature of spiral galaxies but the one at the centre of ours is unusually small (4 million solar masses) and appears to have optimized  the production and distribution of supernovae. Somehow a balance has been achieved between two conflicting requirements for life: the abundance of elements needed for life and a radiation environment which is, by cosmic standards, benign. The halo of dark matter around and at right angles to the Milky Way Galaxy is also appears to be playing a life crucial role

See also ET life should be local

These are densely packed masses of stars within galaxies and any star inside a cluster is unlikely to have life-bearing planets because they are made almost entirely of hydrogen and helium and because the orbit of a star within a cluster is  unstable. Globular clusters were made in the early life of the universe, before most chemical elements were manufactured in large stars and released into interstellar space to form the clouds of matter out of which new stars, planets and life itself are formed. 

Several things about the Sun are optimum for life:
  • It is a  rare G2 yellow dwarf which emits light suitable for photosynthesis (needed to power plant life and produce oxygen) and which is maximally transmitted through water (important for submarine life). 
  • The Sun is also the right size and mass (a large majority of stars are not). The overall stability of a star is closely related to these.
  • It is situated well away from damaging gamma rays and X-rays, i.e.  from gamma-ray bursts, black holes or supernovas.
  • Unlike many stars, it is not part of a multiple star system, which would give us chaotic conditions alien to life.
  • It does not behave violently (by cosmic standards!) in its present quiescent phase which began 50,000 years ago and will end in 50,000 years time. Outside this time window even the relatively stable sun flares up enough to preclude advanced life.
All this adds up to our galaxy, our position in it and the nature of the Sun, providing a rare cosmic opportunity for life to evolve. This combination of circumstances is on top of the extraordinary fine tuning of the physics of the universe to permit life to exist. (How it starts we don't know. We don't even know what it is, i.e what holds the multifarious multitude of diverse, integrated processes within a living organism in being and  what causes continual correction to the millions of errors each day, until death sets in.)
Author, 2077: Knights of Peace

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