Why Earth Supports Complex Life
In order to fully understand why this collection of essays exists, it is necessary to first learn about Astrobiology. Click on the link below before reading the essays if you want to learn what astrobiology is.
A Brief Introduction to Astrobiology.
How can we be the only ones?
The
Right Kind of Galaxy
After the Big Bang, there was matter and anti-matter.
The two annihilated each other and canceled out, yet we are here.
So there must have been slightly more matter than anti-matter, and this
leftover matter is what out universe is made up of today.
After the Big Bang, matter started to cluster and form into the simple
elements of hydrogen and helium. With
this clustering gravity was produced, and still more matter was pulled in.
This matter began to form nebulas; clouds of dust and gas, and galaxies
and stars began to form at the same time from these nebulas.
Galaxies have a highly concentrated center with many stars.
Stars that are too close together can do a lot of damage to neighboring
planets if they went super nova. "Life
is a very complex and delicate phenomenon that is easily destroyed by too much
heat or cold and by too many gamma rays, X-rays, or other types of ionizing
radiation. The centers of many
galaxies produce all of these." (Ward & Brownlee 28)
With this in mind, it seems highly unlikely that any life could exist
close to the center of any galaxy.
But there is such a thing as too far.
Life cannot develop too far out from the center of a galaxy.
Outer areas of our galaxy don't have enough heavy elements to support
life. Farther away from the center
of the galaxy "the relative abundance of elements heavier than helium
declines. The abundance of heavy
elements is probably too low to form terrestrial planets as large as
Earth."(29)
"A key to habitability in various galaxies may be their abundance of heavy elements. Planets that form around metal-poor stars may be too small to retain oceans, atmosphere and plate tectonics. Metal-poor planets may not be able to support or maintain animal life. It is known that entire galaxies are metal-poor and hence likely devoid of animal life.” (31)
Spiral galaxies seem to be the best suited to support life.
The oldest galaxies in the universe appear to be irregularly shaped, and
"it is unlikely that any of the stars in these galaxies could have
Earth-like planets because the heavy elements to build them were not yet
abundantly available." (31) The Universe was not old enough to have started
making the heavier elements required for life.
Elliptical galaxies are just as fruitless.
"Elliptical galaxies are regions with little dust which exhibit[s]
little new star formation. The
majority of stars in elliptical galaxies are nearly as old as the
universe." (29) With a low abundance of heavy elements, oxygen to breathe,
carbon for cells, iron for blood, cannot form.
With this information we can see that life has its best chance of
survival in a spiral galaxy. Elliptical
galaxies don’t form very many stars, and when they do, there are very little
heavy elements to create life. Irregular
galaxies don’t have enough heavy elements to start or support life either.
Life on Earth is a lucky little fluke of nature.
-K. Marks
Galaxy
Location as Related to Complex Life
Although our galaxy alone is approximately 85,000 light years in diameter, it is
still a very delicate process in determining where a planet containing life
could succeed. Our galaxy is spiral in form as oppose to elliptical or other
irregular shapes, meaning it is shaped like arms revolving around a center
point. Since it is more compact in the center, where everything revolves around
the core of our galaxy, the planets there are very dense. There could be 100,000
stars packed into an area 10-100 light years across.
These areas are known as global clusters. Global clusters are spherical shaped
clusters that could contain over a million stars, most of which are very old.
Within that cluster, the stars around each other send of various forms of
radiation, via gamma rays, UV rays and X-rays, into space. Also, the frequency
radiation waves increases the closer to the center of the galaxy you travel.
Depending on the size and components of these stars, when they die,
supernovas, which are explosions of a star, is a possibility. They can excrete
debris and even sterilize life as far as 30 light years away. Since our nearest
star is 4.2 light years away, and there are 23 know stars total within 13 light
years of our sun, the chance we have of being affected by these catastrophes is
significantly lowered.
Therefore, we are fortunate enough to have our planet closer to the margin of the galaxy, yet not too far out. We are able to avoid threat such as the preceding, and other threats, such as neutron stars that emanate radiation. Due to the distance of the nearest neutron star, the energy dissipates before it can reach us. "Our sun and planets are protected simply by the scarcity of other stars around us" (Ward and Brownlee 28).
Instead, we are in a position that allows our planet to be rich in necessary elements. We are surrounded by life and growth of newly forming bodies. Life is on here because Earth is in the happy middle ground of the galaxy. "Our star - by chance - is located in the 'habitable zone' of the galaxy."(28)-S. Mangus