Photographs of the Earth from space are in themselves enough to make most people believe that our planet is unusual. But there is much more that science has revealed to show just how unusual it is, even leaving aside its placing in the galaxy and solar system (see Part 1).
So I’ve tried to list some ways in which the Earth, unlike any other planet known to date, is suitable for life.
Formation of the Earth-Moon system
While the Earth was still hot rock another planet is thought to have hit it and caused myriad fragments to fly off, eventually coalescing in orbit to form the Moon. To help in the evolution of life (see below) the Moon had to be just the right size, mass and distance, and this in turn depended on the colliding planet having the right size, mass, momentum and angle of collision.
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Temperature and Distance from Sun
The Earth’s size and mass permit it to orbit in the ‘Goldilocks zone’ where the temperature is OK for life and the tidal forces do not lock it so that one side is always dark and cold while the other bakes.
Circular orbit and tilt of axis
Unlike all the other planets in our solar system the Earth has a near circular orbit which means the temperature is stable apart from gradual seasonal variations due to the tilt of its axis.
The north-south axis, about which the Earth spins, is tilted at an angle that stops the winters getting too cold and the summers too hot. Again, this is unique to Earth. The moderate changes of spring, summer, autumn and winter in the northern and southern hemispheres are in fact conducive to the thriving of life. In fact the tilt of the axis appears to be optimal for this – not too great, not too small.
Unlike all the other planets in our solar system the Earth has a near circular orbit which means the temperature is stable apart from gradual seasonal variations due to the tilt of its axis.
The north-south axis, about which the Earth spins, is tilted at an angle that stops the winters getting too cold and the summers too hot. Again, this is unique to Earth. The moderate changes of spring, summer, autumn and winter in the northern and southern hemispheres are in fact conducive to the thriving of life. In fact the tilt of the axis appears to be optimal for this – not too great, not too small.
Rotational speed and stability
The laws of mechanics lead to the planet spinning on its axis once in 24 hours, a rate which keeps night and day at a fairly even temperature. (Mars has about the same length of day but its air is very thin and there is no liquid water, so that its temperature varies wildly between night and day.)
The Moon spin stabilizes the Earth’s rotation and stops the axis wobbling too much. Without this conditions would be too chaotic for life to evolve. Note that the Moon had to be the right size, mass and distance to perform this role. No other planet we know about has such a favourably placed satellite.
Tides
Tidal forces due to the Moon and Sun permit the sea to gather life-enriching minerals as it dissolves them from the land when the tide is in. Rivers also wash nutrients into the sea. The tides are likely to have played a role in the transition of life from sea to land. And certainly the lunar cycle both guides and influences many forms of life today.
Today the air (nitrogen, oxygen, carbon dioxide and other gases) is right for the life of today. But when the early forms of life were evolving the composition was very different because life influences the atmosphere, adjusting its composition to suit itself. The atmosphere, in conjunction with plate tectonics, is also important in smoothing out temperature fluctuations and transporting clouds, viruses, bacteria, spores, insects, birds and dust around the globe.
There is a layer of ozone (a special form of oxygen which has 3 atoms per molecule instead of 2) which protects life from harmful ultraviolet radiation from the Sun. It was significantly damaged by CFC chemicals but is responding to restorative measures.
Water
Early forms of life are thought to have evolved and diversified in the oceans which cover three quarters of the Earth’s surface. Comets, made largely of ice, hitting the Earth in its early stages appear to be the most likely source. Alternatively the water could have come from the Earth’s interior or the cooling of the primordial atmosphere.
Minerals washed into the sea, especially salt, have maintained its salinity at an optimum level for life throughout geological time.
Water has the unique property of expanding when cooled down close to freezing point: were it not for this water would freeze from the bottom upwards and our oceans would be solid ice. we reside in the only known place in the universe where large expanses of liquid water have existed for billions of years. Finally, water is optically clearest at the same wavelength that the Sun radiates most light - had it not been then photosynthesis would have been much less pronounced and insufficient to permit anything like the varied and abundant life with which this planet is blessed.
See also The deep mystery of existence.6. Water, the elixir of life.
Asteroid bombardment
Periodic hits by large asteroids (lumps of rock up to several miles in diameter) have been at a frequency and severity small enough to allow evolution to win over mass extinctions but big enough to spur the evolutionary process. Jupiter, Saturn and other celestial objects seem to have gravitationally tuned the arrival of asteroids to match this condition. There appears to be a remarkable degree of orchestration of these events. One current theory being investigated is that dark matter on the edge of the Milky Way Galaxy played a role.
Plate tectonics
Unlike any other planet we know of the Earth’s crust comprises large plates floating on a subterranean ocean of molten rock. The heat needed for this comes from the decay of radioactive elements made in supernovas (U-235, U-236 and Th-232) billions of years ago. When the plates collide they cause earthquakes and volcanoes as well as force up parts of the crust to form land from which tides and rivers transfer life-giving nutrients to the sea. Crustal movements also bring up carbon dioxide into the atmosphere, essential for life, and, by a finely regulated process, bring ores to the surface layers where they can be mined.
Magnetic field
Because of the molten iron circulating inside the Earth we have a magnetic field protecting us from lethal solar and cosmic radiation (the Van Allen Belts). Life today could not exist without it and no other planet we know of has anything like it, as far as we can tell.
I suspect there are hidden factors which make this an even more unlikely abode for carbon-based life. Carbon is necessary for any kind of life we can conceive of because C atoms are uniquely versatile in their capacity to combine with each other and with other atoms (valency) to form the structures and systems of the living world.
Reservoir of viruses and bacteria
Right from the earliest geological epochs, i.e. over billions of years, it appears that viral and bacterial life were present. Where and how these extremely complex and sophisticated organisms originated remains a mystery. The numbers are staggering. Placed end to end they would stretch millions of light years into space and they exist at least miles down into the Earth's crust. There is a strong symbiosis between them and the rest of life. A human being has ten times as many bacteriums resident in his body as there are body cells (100 trillion vs 10 trillion, although the bacteriums are smaller). The evolution through geological time of the biosphere as a whole could well have been governed by or through them.
In Part 3 we see how the Earth is uniquely fitted to allow a high tech civilization to evolve.
Our Precious Planet. Part 1: A unique place in the cosmos
Links to all 4 parts of Our Precious Planet and related links can be found here.
John Sears
Author 2077:Knights of Peace
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cosmik.jo@gmail.com
