Phobis Terra

(This world idea is for a sci-fi mod for some sort of civilization type game, hopefully one with multiple map abilities.  I'm not sure if this will happen, but have come up with some general ideas for the mechanics and worlds for it.)

General Description

Phobis Terra is an Earth-like planet, with a broadly similar size, surface, orbit, and other such characteristics.  On smaller scales, though, the planet has a number of unexpected features, some of which may come from previous settlements.

The name of the planet was originally a joke amongst human colonists.  Humans had expanded to other places within the solar system by this point.  Compared to solar system colony worlds, Cryolith and Cauldron would be relatively simple to build colonies on, and some colonists thought that the risk of running into a civilization, infections, poisons, or other dangerous life was not worth the benefit of a more earth like planet.  Others made fun of this idea, joking about the "terraphobes", scared of "phobes terra".  When these colonists landed, the fears of others turned out to not be far off the mark, with very aggressive predators and strong diseases greeting the first colonists, and crime and piracy being harder to control as well as the colonies developed.  Phobis Terra still proved a good place to settle, but the name proved appropriate, and has continued to this day.

Large Scale Geology

Phobis Terra has a mix of water oceans and continents, formed by plate tectonic features, similar to Earth.  Other surfaces processes (glaciers, soil formation, rivers) are also immediately recognizable as well.  Soil and rock chemistry is slightly different, due to the different direction of life forms.

Atmosphere

The weather and atmospheric composition of Phobis Terra are largely similar to Earth.  The atmosphere is slightly more dense than Earth's, with a slightly higher oxygen concentration, allowing for more aggressive creatures, and also helps with human colonization.  Trace gases are also found in different concentrations than Earth, due to different life forms, but carbon dioxide and water vapor fractions are both similar enough toallow for the weather and safe colonization.

Oceans, Rivers, and Lakes

While bodies of water on Phobis Terra largely act similarly to those on earth, bodies of water from a distance often have a slightly darker and redder color, due to different photosynthetic chemicals being common on the planet.

Many rivers and lakes are also slightly more acidic, with higher metal concentrations, due to old mines and buildings leaching into the water.   Due to either the amount of time passed since the ruins have developed, and possible pollution control measures, this effect is only detectable by instruments, but does not appear to interfere with life or colonization on the planet.

Life

Phobis Terra's life comes in a surprising variety, with an much larger array of general body types than expected from Earth.  More surprisingly for scientists, while most life shares a similar type of genetic/information storage method, large subgroups of apparently native life use different systems.  Microbes in particular seem to be quite diverse, with the largest cluster of similar forms of life only making up about 1/3 of the total amount of microbes found on the planet.  While larger life is more consistent, large fractions of some types of organisms are still structured differently than the most common life form on Phobis Terra.

Even within the largest bloc of life found on the planet, many organisms are more dangerous and aggressive than found on other planets.  Predatory organisms give hunters, and even small organized military and hunting groups, great difficulty, prey organisms are themselves dangerous combatants, and even most stationary organisms have developed some sort of poison or spikes to protect themselves.   Diseases and microbes, however, have proven the most dangerous, due to the wide variety that exist on the planet.  Gangrene like sickness is a great concern, unlucky or lax colonists can attract microbes that eat into almost any material, and a few more standard infectious diseases and fast adapters exist that little immunity exists for.

Despite these differences, life on Phobis Terra has largely similar effects as life on Earth does.  It creates a similar atmospheric composition, influences erosion, and moves materials in a largely similar way to Earth's life.

Ruins and Previous Settlement

While no large civilizations were found on Phobis Terra, strange formations were seem throughout the surface of the planet that appeared to be artificially created.  Stubs, tall columns, geometric shapes, made of materials such as plastic, unusual rocks (similar to concrete), brick-like shapes, and corroded metal are found all throughout the planet.  In addition, unusual chemicals, such as mine drainages, and leaks of organic halides are found in other locations, which point more towards settlement than toward natrual causes.

After examining the different types of ruins, occasional symbols on ruins, and examining the different varieties of genetic bloc of life on Phobis Terra, scientists and archeologists proposed that this world had been settled at least 11 times, by beings from outside the planet, who either left or died off.  These beings would have brought different types of microbes, and other creatures, accidentally, and likely would have brought domesticated organisms as well.   These organisms would have gone feral as the settlers died or left, eventually integrating completely into the ecosystem of Phobis Terra.  Some scientists theorize that genetic engineering may have taken place as well, to explain the aggressiveness and danger of the organisms on the planet.

While the origin of these possible previous settlers is unknown, they did not attempt to settle Cryolith or Cauldron to the same degree, or were not as successful.  Those two planets have almost no ruins, and life on both Cryolith and Cauldron has the same genetic structure and organization all across the planet.

Explanation

(Disclaimer:  I am not in fact any sort of geologist, atmospheric scientist, planetary scientist, or anything related.  While I do occasionally attempt to read some papers, and more complex books, about planets for fun, I'm nowhere near an expert on any of this, and am making up these planets to have interesting characteristics, whether or not they would be be possible, and the resulting attempts at planets will read appropriately.  If any one of the above types of scientists is reading this and pulling their hair out, I understand completely.  :)  )

This world is meant to be the "Earth-like" starting world, as in a lot of space colonization based computer games.  In the game, I plan to have Cryolith be the most straightforward to settle, but also the most resource poor, Phobis Terra be resource rich bit requiring more defenses against wild animals, diseases, and various pirate, rebel, criminal, etc. groups.  Cauldron would also be risky, but in a way based more on the terrain itself, and with some sort of different gameplay response required.  In addition, I also have some backstory in mind that this system is being used as a "testing ground" in some way, attracting different groups of aliens to it over some time, although the exact details are not something I've fully worked out.