Friday, November 6, 2009

Trope-ing the light fantastic (Earths)

That's Earths, plural. Obviously Earth itself exists and can hardly be a trope.

But what about the many Earthlike planets in SF?  (How often does the starship Enterprise encounter a solar system without an "M class" planet or moon?) Are Earthlike worlds realistic or a trope?

Our native solar system has but one Earth, of course. Real-life searches for extrasolar planets best spot large, massive, and close-to-their-primary objects. The observational methods are not yet sensitive enough to spot Earthlike planets (see current list of extrasolar planets here).  IIRC, the smallest extrasolar planet yet found is about five Earth masses. There may be -- and presumably are -- other Earthlike planets, but searches to date say little yet about the prevalence of such planets.

Why "little" rather than nothing"? Because the prevalence of what we do see -- Jovian planets near suns [aka "hot Jupiters"] implies that in many systems rocky planets like Earth, that form close in, would be eaten by their suns or ejected by the process(es) that caused the Jovians to move inward.

Another data point: Rare Earth: Why Complex Life Is Uncommon in the Universe, by Ward and Brownlee. I don't entirely agree with the book's arguments -- IMO, it concludes too hastily that complex (aka, multi-cellular) life must be very much like terrestrial life -- but it does discuss lots of interesting and seemingly low-probability reasons why Earth and Earth life turned out as they did. (Our planet has had an interesting history, folks.) 

Even a world that starts out like Earth -- rocky, with water, orbiting within its star's habitable zone -- need not develop into anything like today's Earth. As Mars and Venus ended up quite different from our home world ...

Example: Unless and until a planet develops photosynthesizing life, it won't have oxygen in its atmosphere. Oxygen is very reactive stuff -- it doesn't hang around for geological time unless replenished.

Example: carbon dioxide is a trace element in the atmosphere. Carbon tends to end up in marine carbonates on the ocean floor. Unless those carbonates get recycled into the mantle via plate subduction -- requiring radioactive materials in the planetary core to drive plate tectonics -- and then spewed by vulcanism back into the atmosphere as carbon dioxide, there's no greenhouse effect. Without greenhouse effect, Earth would be very cold.

 Earthlike planets, like human-like aliens, offer plot advantages. Description of settlements on such planets can skip environmental difficulties to concentrate on more personified challenges like alien microbes and scary predators. And Earthlike planets make possible more interesting real-estate conflicts with aliens.

So are Earthlike planets a trope? It depends. Scattering Earthlike worlds across the galaxy is permissible within our current state of ignorance. Positing Earthlike worlds in most solar systems sure seems like a trope.

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