I have bad news for science fiction writers, including myself. Actually, the news is only half bad for us, since we ignore reality anyway in the pursuit of good stories. However, this news is one hundred percent bad for people who think we will one day be able to visit or colonize other planets in the galaxy.
I’m not talking about the physical limits to space travel. I think there is a lot about the universe and about physical law that we don’t know, so I’m not convinced that someday we won’t discover a way to traverse enormous distances in a short time. And I’m not talking specifically about superluminal (faster-than-light travel); there may be ways to move quickly through the space-time continuum without violating that stricture.
What I’m talking about is much more mundane, so mundane, in fact, that H.G. Wells figured it out well over a century ago. I’m talking about microorganisms, specifically bacteria, viruses, and protists (and even some fungi). If you recall, what killed the invaders in The War of the Worlds was nothing more exotic than the common cold virus (which is a coronavirus, by the way, lending the tale a rather eerie similarity to today’s COVID problem). The same fate would undoubtedly greet any humans landing on another Earth-like planet.
To explain, let’s first look at what kinds of planets we might be tempted to colonize. They would have to be similar in size to the Earth and in the “Goldilocks Zone,” the region around their star where liquid water can exist. They would also need an active magnetic field to protect the surface against the solar wind.
That’s a lot to ask, but assuming these criteria are met, there are three possible options: 1) barren planets where life has not developed; 2) planets with life not based on carbon and/or DNA; 3) planets with life based on carbon and DNA. What are the chances we could colonize each of these?
- Because of the tenacity of life on this planet and the wide ranging environments in which it exists and even flourishes (hot water springs, inside rocks, deep inside the earth, at the bottom of glaciers, at the bottom of the deepest ocean), I think the existence of an option-one planet is unlikely, unless it is only a few hundred million years old. Such planets might exist, but because they are barren of life they wouldn’t have an oxygen-containing atmosphere, which would make them essentially uninhabitable except for sealed enclosures and environmental suits for anyone leaving the enclosure. They would also not have an ozone layer to protect against ultraviolet radiation. So, not a very nice place to live. If there is oxygen in the atmosphere, it indicates the existence of photosynthetic life. Which takes us to the next two options.
- If a planet has life not based on carbon and/or DNA, it would be enormously inimical to us. The soil would not contain the kinds of organic matter and microorganisms necessary for growing crops. There would be nothing on the planet that we could eat. In fact, the kind of life on the planet (whether intelligent or not) could very well be hazardous to our health. The place would be no good for colonizing, and attempts to terraform it (which is of dubious morality if there’s already life there) would be difficult to the point of impossible. Our potential colonists would be restricted to the kinds of sealed enclosures and environmental suits described above for an option-one planet (assuming it would even be possible to completely eradicate the soil-based microorganisms in the enclosures).
- That leaves us with planets that have carbon-and-DNA based life, with the same kinds of biochemistry as Earth life. Sounds ideal, doesn’t it? Not so fast.
If we look at our own little planetary home, one of the first things that stands out is how much of the biomass here is just bacteria and viruses. While there are about 2.6 gigatons of carbon locked up in all the animals on Earth, there are 70 gigatons in bacteria. In other words, the weight of all the bacteria on Earth is more than 27 times the weight of every single animal; fish, insects, marine invertebrates, birds, reptiles, mammals—all of them.
There are also seven gigatons of archaea (ancient organisms similar to bacteria), four gigatons of protists (single-celled organisms, such as amebae and foraminifera), 12 gigatons of fungi, and 0.2 gigatons of viruses. But don’t let that small mass of viruses fool you; there are more individual viruses on Earth than there are stars in the universe—by a 100 times over. The total number of species is unknown, but it surely numbers in the millions. One bat was found to harbor 58 different viruses, many of them previously unknown. Based on that, it was estimated that there could be as many as 100,939,140 different virus species—most of them still unknown—that infect vertebrates. As we well know, many viruses make humans ill, and the COVID pandemic was a reminder that there are undoubtedly many, many more we don’t know about yet that will do the same.
As for bacteria, they are the most abundant life form on the planet (since viruses aren’t technically alive), with an estimated five million trillion trillion of them (that’s a 5 followed by 30 zeros). The total number of species is unknown (the figure of one billion has been tossed around), but it is estimated that over 99% have yet to be discovered and scientifically described. Only five percent of the known species are pathogenic to humans, but that’s more than enough when you consider all the terrible diseases those five percent cause.
Then there are also numerous pathogenic fungi (300 species) and protozoa (the ones that cause malaria and giardiasis being just two examples). In short, there is no shortage of microscopic creatures that like to feast on people.
Any planet that harbored life compatible with our own would host an untold number of viruses and bacteria (and probably fungi and protists) for which we would have absolutely no immunity. In fact, if we discover life on another planet, chances are it will just be bacteria. If there is more complex life, the number of species of microorganisms increases accordingly (because there are more niches for them to inhabit). Here on Earth, no complex life could exist without bacteria, and the same would be true anywhere else.
If the percentage of pathogenic microorganisms on another planet is the same as it is here, the sheer number of deadly bugs would be huge. Any human setting foot on that planet wouldn’t last a week.
Some people might say that we could construct hermetically sealed enclosures, after first sterilizing all the soil inside and killing off any other life, but that’s hardly the way to colonize another world. And as we saw with countries that tried really hard to keep COVID out, sooner or later something would get in.
The conclusion is inescapable: There is no way we could ever colonize another planet, so matter how Earth-like. Frankly, even Mars carries substantial risks, because if there is any life there it is almost certainly bacterial. The truth is, this planet we’re on is the only home we will ever have.
Maybe we should think about taking better care of it.
