Final Prerequisites for Life Sustaining Planets
How big must the planet or moon be?
Maintaining an Atmosphere
Based on these two criteria, a minimum mass for a life sustaining
planet or moon: Mars Mass. This is 1/10th the mass of the Earth.
The moon, incidently, is 1/100th the mass of the Earth.
Stability.
Ultraviolet Light.
There is a minimum size for an acceptable host planet for life. This
is based on two concepts:
Geothermal Energy
If the planet is too small, it will cool too quickly! Recall,
radiative decay and solidifying of the core where two important
sources of geothermal heat still existing on Earth today (page 14
of notes). The moon cooled in just 1 billion years, and is no
longer geologically active. Mars has only recently cooled.
The moon does not have a great enough mass to be able to hold an
atmosphere. Again, Mars is barely massive enough, having a very
thin, but significant atmosphere, about 1/100th that of Earth.
Geologically, chemically, temperature, sunlight (stable, warm
star). must remain at least somewhat stable for fairly long
periods of time, allowing life to adjust if it can.
Major impacts must be deflected, or
minimal. The presence of Jupiter and
Saturn has decreased the rate of all impacts to Earth by a factor
of 10,000.
UV light
can dissociate molecules in the cells, killing organisms.
Before the atmosphere had Oxygen, there was no Ozone to
shield organisms from the harmful Sunlight, though
our much thicker atmosphere did shield some. More over,
The Sun produced considerably MORE UV light in the first billion
years than it does now (even though it was less bright.. this
is because it was smaller, but hotter right after it formed).
Organisms living deep in the oceans or underground would be
shielded. Such organisms might be hard to find on a distant
planet or moon.