EARTH-LIKE PLANETS MIGHT BE PLACES YOU WOULD NEVER WANT TO VISIT...
The air composition is similar to that of Earth's, the sea-level pressure is that of +90 meters, temperature is stable near 16°C... So far, humanity could thrive here......Or is it? ô-0 *cue music*
Michael Stevens, VSauce YT channel
See, there's very much of an appeal to Earth-like conditions on other planets, in principle that would make easier for humans to settle around, like, it's way easier to move from Washington to Tokyo, than say move from Washington to somewhere in the mid Antarctic.
We would look for matching temperatures, atmosphere, mass and size to that of Earth's, that's what we've been doing so far.
Is rather fun to see weird creatures interacting in a world like Star Wars, a dozen species sharing the same bar and maybe a hundred sharing the same planet in the great Z Empire or X Republic. BUT there is something rather overlooked that is an interesting point to include sometimes - microbes.
The problem with a place just like Earth around another star, is ironically the same very thing that attracted us there in the first place - it's just so friendly to life, it becomes too dangerous to ever go beyond low orbit.
When the astronauts from Apollo 11 returned to Earth, they were in quarantine for 21 days, just to make sure they hadn't come back carrying harmful lunar microbes that might live on the Moon, later on it was found that the Moon was sterile, and following lunar expeditions were safe to come home back right away.
But differently from the Moon which is cosmically just kissing Earth, we cannot be sure of life on other planets from here, if not via very strong signals like atmospheric pollution spectral analysis.
Samplings and tests against known antibiotics and development of vaccines against the local microorganisms might be the very first step into colonizing another planet.
Knowing with certainty that life is present on such a place, decades or even centuries of microbial research would be required before the first civil settlement is established.
A planet may be totally devoid of plant and animal life, but it's air and water might be teeming with bacterial films and viruses.
For Earth, the amount of bacteria (source1, source2) floating in air is between ~800 thousand bacterial and ~1.2 million viral spores per cubic meter, fungi about 1K spores per cubic meter. Or if you examine a square meter of soil after a day it will be under 800 million airborne viral spores. That's not even mentioning aeroplankton.
If a planet present similar numbers to ours, an astronaut taking a single breath would be exposed to about (a human breathes 7~8L of air per minute, giving us ~125mL per breath) ~100K bacterial, ~150K viral and ~125 fungi spores, foreign lifeforms that our bodies haven't evolved to counter or even chemically detect. Imagine taking a 1 second breath and catching a thing similar to syphilis down your throat and lungs, you wouldn't want that to happen.
Entire native-American populations of humans were vanished from the map in a couple decades via diseases brought by European explorers - which despite never having previous contact with still evolved stranded to the same biological machinery the pathogens were used to explore.
In some cases, one may argue that we as 'superior' beings, would be immune to most primitive alien biospheres. What does make sense as we are continuously dealing with viruses and bacteria in our own planet that may happen to be way more dangerous than what we would ever found in an alien planet and thus our major concern would be Forward Contamination - to carry our pathogens to this place.
And you're right to a certain point, but what do you mean by humans as "superior"? Your body may have powerful antibodies compared to those tiny viruses and protozoans in this alien atmosphere, so we are safe... Right?
WЯÖNG!
Actually, your body as 100% human matter does pretty well against most pathogens, but you should be concerned about Immunodeficiency-causing pathogens, the ones that don't directly target your cells like skin and muscle, but target your local microbiota, we humans depend on chemicals and vitamins extracted from food by microbes that live within our tissues and guts. Just imagine the physiological catastrophe that would be if a virus happens to prey on our Mitochondria, or the bacteria that syntheses vitamin B and K, that's sensible machinery, that may even trigger our immune system to destroy our own cells while trying to stop the spread of the virus or infection.
Viruses raiding a cell
There will be worlds without penicillin, that on Earth is naturally produced by fungi, and those worlds may have natural chemicals we would consider synthetic because our fauna or flora isn't able to produce it, and many infectious diseases are so harmful because the pathogen releases a dangerous chemical as byproduct of it's life-cycle or in response to the immune system attacks, now imagine an alien infection cause it's host to die via an overdose of aspirin, or Benzylpenicillin, not exactly a lethal dose to our nervous and lymphatic system, but enough so the lack of our microscopic comrades, or even the spread of local bacteria due to change in the gut chemistry would cause severe injury if not death.
Even if viruses are not able to use our DNA machinery, they would still trigger over-allergic reactions from the body, or worse - to just destroy the cell without being able to properly use it.
It would be quite ironic if that image was inverted to a heat-ray lifting a dead human hand instead. War of the Worlds (2005)
If we are including other friendly aliens or alien food, we must also be concerned about their pathogens and their resistance against ours, unless one or both of us have to wear hazmat or space suits during reunions and visits.
Maybe everybody should have their helmets on...
Nature is force, a force that drives creatures forward in competing with others, and either theirs or our own pathogens will out-compete one another in some areas given time and exposure, in a manner that, if equilibrium is possible, any, and every outworld colony should be in constant quarantine - for the security of their own citizens and the security of the rest of humanity.
- M.O. Valent, 06/02/2020
