This is a look at the Astrobiological occurance of life in the Universe, primarily derived through temperature and density.
The report takes a look at possibility of life on other planets from the stand point of size and nature of planets or stars, from which living organisms may arise.
It is written that there is no definite information regarding the origin of life even on Earth, authorities generally agree that coacervates in an aqueous medium, represent the earliest stage in the evolution of organic substances into living organisms.
That an atmosphere over the solid crust of the planet is important from allowing water to evaporate rapidly during the first stages of development. Living organisms without the aid of some liquid medium would be virtually inconceivable. Stars and planets are conceived from interstellar medium composed primarily of hydrogen. They theorize that in the early evolution of a star or planet the atmosphere is primarily hydrogen especially in its outer envelope.Thus the hydrogen atmosphere then must be at least partially dissipated. The present atmosphere of the Earth must be a secondary one formed as results of the process's of chemical reaction before the earth took formation as a planet.
Dissipation of hydrogen should not be so large a loss by a plant or protoplanet so as to change drastically its chemical content. On the other hand hydrogen must be a dominant ingredient in the upper layers of in the early stages because of its high cosmic abundance and its lightness.
The conclusion is that the most likely radius of life sustaining planets would be between 10/8thcm and 2x10/9thcm
Conditions such as density and also bear upon the occurance of life thus

Tenatively they conclude that a habitable planet has most likely a radius between 10^8 and 2x10^9, admittedly a generous estimate since that range includes the Moon and Mercury, both which are considered unfavorable for indegenious life due to their position in the solar system and their density.

This brief report on the limiting size of planets that would be conducive to life forming is based on the planet's ability to retain an atmosphere. This is based on the assumption that the planet is within the temperate orbital zone of a main sequence star, which is a star that is of a medium size and long life. The estimate is that hydrogen being the predominant element would be retained as an atmosphere if the planet was large enough and dense enough to have enough gravity to keep the gas from escaping out into space. Then later the planet might produce oxygen that would be suitable for life to form.

Being written in 1960, this was probably the best estimate of the time, but we now know that a planet also need to be geologically active with a molten core, which gives it the magnetosphere that protects the planet from high energy particles and keeps an atmosphere from being blown away by the solar wind. This is what probably happened on Mars, although it still has some atmosphere, the planet was too small and too far from the Sun to remain geologically active. Over time the core cooled off and the magnetosphere weakened and Mars lost most of it's atmosphere.

This document brings up a whole host of questions as to why this report was requested and by whom. Granted this time frame was the beginning of the space race and the government was interested in everything about space, but it still makes me wonder if there was some other reason.

Well the dinasaurs did pretty well when the earth was 1/4 the size it is now, so that should give you a clue.

Two words "Presumptuous Popycock!"

This assumes that life as we know it on earth is the only possible form of life. Without a complete understanding of microphysics we do not even have a sufficient understanding of life on earth to define life as we know it.

We are carbon based, how about basing life on an atomic structure which cannot even exist in our hot/cold high pressure/low pressure environment.

And there is the good old scifi favorite, energy based life forms.

Agreed, however my personal thoughts on the matter lead me to the question as to weather or not they are doing this to find other possible habitable planets for a 'backup' in the event of say for instance nuclear disaster to forward the continuation of Government and human life. At least that was what was running through my mind during the analysis.