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=Primary Conditions Necessary For Life As We Know It=

By: Jonathan Brown Gilbert
There are many different things that contribute to our impeccable situation here on earth. Life as we know it (carbon based lifeforms) require a couple of conditions in order to develop and prosper. First on the list, they need enough hydrocarbon compounds to form amino acids, proteins, DNA and RNA One popular point is the importance of water. It would appear that for life on Earth, water has always been of the utmost importance. Earth is distinctly wet juxtaposed with other (terrestrial) planets and indeed signs point to the oceans as the origin for life on our bountiful planet. Water is one thing but an important point to make is the necessity of **l** **iquid **  water not simply H2O. For water to remain liquid it must stay within the perameters of 0°C and 100°. T he state inwhich the water resides is determined by the overall planetary temperature, this is dictated by a couple of different things. As the heat emitted by a star dissipates into the space around it, the further an object is from the source, the less heat an object would receive. Therefore the distance at which a given planet orbits a main sequence star has great effect on the amount of heat that reaches said planet. Also, the size of the planet as well as the size of star are quantifiable qualities that have bearing on the planets temperature.

=Habitable Zone=  T he Habitable Zone (HZ) is the region in which planetary temperature are neither too low nor too high to sustain life as we know it. As expressed above, a planets temperature is determined primarily by the size of the planet, the size of the star it orbits and the distance between the two. To calculate the approximate average temperature of a planet we follow this sequence of events: The star the planet orbits has a radius represented by Rs and a temperature represented by Ts. The planet also has what's called an albedo (level of reflectance) represented by A of its surface. Its planetary temperature will be represented by Tp. The distance between the star and the planet we call a.

The following is an equation demonstrating how to calculate the equilibrium (planetary) termperature of a planet given its Ts, Rs and A.

Tp = { (1-A) / sqrt2}power 1/4 • (Rs / a)power 1/2 • Ts (1) (J. Schneider, 1995).

We fill in the values known for Earth and we find A=0.39, Tsun =5770 K¹, so Tp=280 K (which is quite close to the actual figure of 287K)

So from equation (1) we have a planet having a temperature of approximately 300 ± 20 K to allow for liquid water must be located at a distance from the star given by a = Rs ( Ts / 300)² (2)

Where the albedo is A=1.

It ranges from approximately 2 Au² for hotter stars where Ts = 6500 K. to about 0.1 Au for cooler stars where Ts = 3000 K



There is also the idea of **Galactic Habitable Zones** (GHZ). This concept implies that as well as there being a select area surrounding each star that expresses conditions preferable for life as we know, the same zoneing tecnique applies to each galaxy as a whole. The GHZ hypothesis dictates that the width of the GHZ is determined by two factors. The first is a line illustrating approximately the closest a star could be located to the galactic center (the center of a its galaxy) without threats to complex life as the stars closer to the center of their galaxies show increase in such threats as comet impacts and sources of ionizing radiation. This is called the inner limit. The other is determining factor is (shokingly) refered to as the outer limit. Conditions beyond the outer limit pose a threat because of the abundance of heavier elements, namely carbon.

→References Page
¹Kelvin (K) is a measurement of temperature similar to Celcius (C) only 0 K is defined as absolute zero where as 0 C is equal to the freezing point of water

²1Au = the distance between the Earth and the Sun