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Get a life II

© Rodolfo Astrada

In the first part, we described some constraints as relates to galactic location and host star characteristics for life the way we know on Earth to be viable.
These constraints ranged from metallicity (relative abundance of heavy elements created and dispersed by supernovas) to crowding, to avoidance of starbirth regions and so forth.

We'll now explore the more "local" constraints, those related to placement within a planetary system, and a life bearing candidate planet composition and immediate surroundings.

Choose a block to live in

Remember one of the last observations in the previous part was that - at least for organic chemistry - the existence of some solvent in liquid form was both unlikely and at the same time almost mandatory.
Water is not the only candidate, but the same in terms of restricted regions of viability hold for other solvents. Titan, for example, has the right conditions for methane to be in liquid form, and there are great expectations with regards to what may be brewing down there.

But for water at least, being the liquid phase ranges from 273 deg. K (0 dec. C) to 373 deg. K (100 deg. C.), the orbital distance to the Sun for oceans to exist is very narrow. Were the Earth less than 20% farther or less than 10% nearer, water should either freeze solid or vaporize. The Earth is right where it had to be.

Orbit geometry is equally important for the same reason; were it not just barely elliptical but more elongated, then the Sun's radiation should fluctuate widely from perigee (closest approach) to apogee (maximum distance) with the corresponding impact on global temperatures.

The backyard

The Earth - Moon system is remarkable as an improbable outcome, or at least that was thought up to very recently. After centuries of hypothesizing about the Moon's origins, it is now widely believed the collision of the very young Earth-to-be (smaller than today's) with something about the size of Mars almost annihilated both entities. By a very narrow margin, the massive cores of both bodies - mostly iron - merged in the new proto-Earth, attracting a sizable part of the just dispersed cloud of lighter debris.
An important amount was anyway left over and quickly settled into a new aggregate, and the Moon was born.

Planetary satellites, as we know from other examples in the Solar System, are much smaller in relation to their hosts, so the Moon's size is remarkable as compared with the Earth.