Gaian worlds are, of course, difficult to miss. Marked by several key characteristics, they stand out as swirling blue and white marbles against the backdrop of space, and represent a near universal sign of life. Foremost present of these characteristics is the presence of large quantities of surface water. Great oceans often cover these planets, to varying degrees, and their presence aids in maintaining another key ingredient, the tectonic cycle.
Without an active geological cycle of crust consumption and creation, the planet would quickly suffocate on its own internal fires. Carbon dioxide is locked up in rocks through continental subduction, and without this the presence of this and other greenhouse gases would, geologically, quickly escalate until the planet became a greenhouse world, much like Venus. Thus, the complimentary presence of geology and oceans help to maintain the planet's delicate equilibrium. But there is another component that helps to maintain the planet's habitability, and that is oxygen. And oxygen is almost always indicative of life.
Life on Gaian worlds varies widely, from geologic age to geologic age, from planet to planet. young Gaian worlds may have barren continents, but their seas may team with life. Ancient Gaian worlds may be the same, or all regions may be filled with a prolific array of biomes. Indeed, a single planet, over its lifetime, can vary widely in the degree of over all habitability, due to a variety of reasons, astronomical or planet-based. Earth herself has known times when the continents have been bare of life for hundreds of thousands of years, only to have it explode outward once more in a startling array of forms, and last for hundreds of millions of years.
The surface conditions of Gaian planets can have a wide range. The key elements to making a world Gaian are: oceans, continents or other numerous regions of land, and the presence of life. There are some Gaian worlds where less than 10% of the surface is covered by oceans, while others have oceanic coverage of up to 95%. Some worlds may have vast, nearly lifeless deserts, while others might be locked within ice ages for millions of years. Whatever the case, life almost always finds a way to adapt. Having been habitable for hundreds of millions of years, native forms can almost always adapt to changing conditions, even if they are on a geologically short time scale. Advanced biomes may go extinct, but there are always smaller forms of life that will survive and eventually radiate out, to take advantage of the new and open ecological niches present.
And, of course, ever Gaian world has the potential for evolving intelligent life forms, possibly multiple times over its billions of years of habitable history. On Earth, Humans can trace their immediate evolutionary history back several million years. Other planets have shown evidence of past cultures, now long extinct. Still other worlds have life forms living on them that are potentially future masters of their world. But finding a sapient species, in the here and now, one that is ready to take to the stars, or to even wonder at their existence, that remains quite rare.
Gaian planets are indeed the jewels of the Galaxy. Each world is an oasis for Humans, although by no means are they all fully habitable by them. Life evolves differently from world to world, and some planets may develop biochemistries that are highly toxic to Humans. Other worlds may possess trace gases in their atmospheres, gases that native forms have evolved to deal with, and may even require, but which Humans would still find dangerous to encounter. No world is a perfect match for our Earth, and even the most inviting blue and white marble, flecked with the greens and browns of life, warrants close examination.
Earth, the original blue marble (image courtesy of Don Edwards).
This is a typical view of the surface of Earth. Note the varied indications of geology and life (image copyright Margo Dollan).
Some Gaian worlds have a much larger land area than ocean area; as a consequence, the inland regions are largely uninhabitable deserts, and the small seas are quite saline. however, life can still be quite prolific.
From the surface, even the barren inland deserts of a largely dry Gaian world are spotted with life.
Some Gaian worlds are nearly landless.
These water-logged Gaian worlds lack continents, but are often dotted with volcanic islands.
The Frequency of Gaian Worlds: Fully mature Gaian worlds are quite uncommon, but their various incarnations make up for this lack with sheer diversity. Still, even a relatively hostile Gaian planet is a rare gem in the Galaxy.
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