Earthlike Planets

One of my pet peeves about most science fiction movies, TV series, and books is the prevalence of very Earth-like planets. It seems that every Sol-type star has at least one. It is obviously easier and much cheaper to film movies and TV series on Earth without using special effects, which might also explain the overwhelming prevalence of bipedal humanoid aliens. But science fiction books have no such artificial constraints. So why do so many SF writers ignore reality?

We live on an incredibly unusual planet. Let's take a look at our nearest neighbors.

Mars is 53% as wide as the Earth, has 28% of the area, 15% of the volume, and barely 10% of the mass. The length of its day is very close to ours, at 24.6 hours. Gravity is 0.367 G. It is less dense than the Earth, likely due to a smaller iron core. Mars has less than 1% of our atmospheric pressure, and what air there is consists of 95% CO2. Mars is frozen and dry; CO2 freezes out at the poles.

Venus is very much a terrestrial planet in size. It's radius is 95% of the Earth's, area 90%, volume 86%, and mass 81.5%. Gravity is 90% of a G (still different enough to be noticeable while walking, running, jumping). However, its day is 243 of our days long. It has 93 times as much atmospheric pressure, composed of 96% CO2. While Venus's atmosphere is only 3.5% nitrogen, that is still 4 times as much nitrogen by weight as in Earth's atmosphere. All that CO2 has created a runaway greenhouse effect that heats the surface to a higher temperature than Mercury--metals like lead or zinc would melt. The high temperatures have boiled away any trace of water, leaving a dry world with sulfuric acid clouds.

Why is the Earth so different? It is larger, and likely had even more atmosphere to start than Venus. Current models suggest that a Mars-size planetoid struck a glancing blow which created our moon and simultaneously blasted away all of the early atmosphere and melted the crust and upper mantle. All "air" since then is from secondary outgassing and the occasional comet impact. All that melting had a second effect: we have an active plate tectonic system that continuously churns out new crust and buries old. Our CO2 was absorbed by the ocean, precipitated as carbonates such as limestone, and buried. The bulk of the Earth's CO2 is tied up as calcium carbonate. Note that the early nearby large moon also stripped excess air. Consequently, the Earth has a tiny fraction of the atmosphere that we deserve, based upon our size. Thank God.

What fraction of worlds will follow a similar path? Will have that large moon? Remember, no other known planet has a moon as large in comparison. I'm guessing much less than one in a thousand.

Most worlds will be smaller or larger, with similar differences in gravity. Even if some principle leads to roughly Earth-sized rocky planets, they are bound to vary in size by an order of magnitude.

Most worlds will be warmer or cooler. This involves a complex interplay of atmosphere, size, rotation, period, solar flux, etc. Note that the Earth itself has experienced extremes of much higher average temperatures. Even the poles had tropical climates during parts of the reign of the dinosaurs.  There has also been snowball Earth conditions where the entire surface was frozen.

Most worlds will have much more or much less air. The odds that the surface pressure of another world would be 15 psi seems incredibly remote. Shouldn't there always be a puff of air from pressure differences when a transfer booth pops you out on the surface of another planet?

Why 20% oxygen? Even the Earth has varied somewhat, from a low of zero to a high of around 35%.

Why so little CO2? Venus has 600 times what we have. Hell of a greenhouse effect. Or, why do those other worlds NOT have active plate tectonics?

The sun is 25% brighter than it was early in Earth's history. Shouldn't the typical planet be noticeably brighter or darker than Earth? Yet all SF creatures seem to share our visible spectrum and tolerances for brightness (excluding horror movies and CSI TV shows where darkness is a given).

The oceans hold most of our water, covering 70% of the surface. The other worlds we know of (including large moons of Jupiter and Saturn) either have no water, or water (and/or ice) scores to hundreds of kilometers deep. Plate tectonics continuously rebuilds mountains. Without it erosion would grind down all land on Earth, washing it into the seas. Note that erosion will make Earth into a 100% water world eventually, when the mantle solidifies. We have enough water for a global ocean over 2.5 kilometers deep as it is.

Water worlds may be common; life may be common. Civilized intelligent creatures may be common (mostly living on water worlds). Technological civilizations should be extremely rare. It takes enough water for life to thrive, and little enough for land to poke through. It may be nearly impossible to create a technological civilization on a world without dry land. Note that there is no reason, in principle, that a large high-gravity water world could not support life. I suspect that's where we'll find most of it.

The only think likely to be rare (possibly even unique in our Galaxy) is a world with 1.0 Earth gravity, 15psi of surface pressure with 3psi being oxygen, 12psi of nitrogen and just traces of CO2 and H2O, with a crust 70% covered with oceans under partly cloudy blue skies and an average temperature a bit above freezing, with 24 hour days and an Earth-normal brightness. And lets not forget, with 6 foot tall bipedal humanoids with most sensory and communication organs resting precariously (along with a brain) on a protrusion above the torso. How rare is that on earth?

Don't get me started on interspecies sex.