Oh, I think it’s almost the opposite. If there is life already there, we probably can’t live there. First a bit of context about what exactly they hope to find.
WHAT TO ASTRONOMERS HOPE TO FIND IN THE NEAR FUTURE E.G. WITH THE JAMES WEBB TELESCOPE
Astronomers when they are looking for habitable planets mean any kind of life including microbes. Our Earth only had microbes for much of its history - so maybe nearly all planets have microbial life. Hard to say with only one example, but it’s our only starting point for now.
But would we detect a planet with only microbial life, even if it is a very habitable ocean world like ours? There’s doubt about that, because, life on Earth was probably undetectable from a distance with the tools we have available until terrestrial life evolved - because everything happened in the sea and the atmosphere was more or less in equilibrium. Sometimes oxygen excess, earlier on perhaps methane excess, but never both together in any quantities until recently. The problem there is that you can get either an oxygen or a methane excess easily by natural processes. What is hard to get without life is both of those together (or two other gases that react with each other quickly) though even that is also possible to some extent without life.
The main ways we have for detecting life at a distance is through changes in the atmosphere - or - unusual colours and spectra from the land. Not much we can do about detecting life that’s in an ocean directly (apart from really huge planet scale algae blooms or some such). And not much we can do to detect it indirectly either - changes in the ocean chemistry won’t be anything like as easy to detect from a distance as changes in atmosphere chemistry. Life in the oceans does affect the atmosphere, but in our Earth’s history anyway - it did it in a way that could be mimicked by natural processes.
So if we detect life unambiguously around a distant Earth like planet, it’s probably going to have spread to the land anyway and quite probably multicellular - just to go by what happened on Earth at the stage when life here got (fairly) easy to detect from afar. They would also try other things to search for including the “red edge” of chlorophyll vegetation - or the unusual colours of vegetation of all sorts, or fluorescence etc. All that again likely to work best if it has spread to land. Another possibility is to detect industrial pollutants in the atmosphere, or of course a signal from ET or megarchitecture, in which case it’s got intelligent life like ourselves.
Apart from that it’s a rather strange situation - where they would search maybe a thousand planets, but can’t say of any particular one that it has life. But statistically some of them probably do because of excesses of oxygen or methane that just seem on average to be more than you’d expect from natural processes, to have so many with such large excesses.
So, maybe they detect terrestrial life at a similar stage to Earth or later. Or else if that’s very rare, maybe they find lots of planets with microbial life but they’d need a large sample of those to have a decent chance of concluding that some of them are inhabited (without knowing which). But they could find anything of course, Earth might or might not be a good example of what to expect. Or they may even find intelligent life like ourselves that’s also got technology (intelligent life without technology wouldn’t be detectable, and you could have intelligent sea creatures just as hard to detect as the almost undetectable microbes in the sea).
More on this at the end, anyway back to the question.
IF THERE IS ADVANCED MULTICELLULAR LIFE THERE
First, if there is advanced life there, plants, animals etc, probably we can’t eat much except for sugar and alcohol.
Unless, that is, we are both results of seeding by the extraordinarily advanced Star Trek Ancient humanoids :).
Some of this food could easily be toxic to humans, or vice versa, even if we are all biologically closely related. In the Star Trek universe then they have an underlying hypothesis that all the planets with the various humanoids on them were seeded by the Ancient humanoids, so it's reasonable that they all use the same amino acids as Earth life. That could also be the situation if the ETs and us have a common shared microbial ancestor, e.g. all our planets seeded by life around earlier stars that pass through the forming nebulae.
In Star Trek they go one step further (rather improbably) that the humanoids are so closely related they can actually interbreed (again a result of the amazing mastery of the processes of evolution of the ancient hominids, able to seed the worlds with just the right organisms to ensure that hominids would co-evolve on all these worlds pretty much simultaneously several billion years later), which would suggest that they can eat just about anything we can. Still, even then, they need to take some care.
Humans vary a fair bit in their tolerance to foods and in allergic reactions that can be deadly. A Thanksgiving Look At Great Meals In Star Trek History
IF THEY ARE INDEPENDENTLY EVOLVED - WHY THEIR PROTEINS MAY NOT BE NUTIRITOUS FOR HUMANS
Humans need amino acids to stay healthy. When we digest food, any proteins are broken down first into polypeptides, then peptides, then into their component amino acids. Protein Digestion and Absorption Process - Video&Lesson Transcript | Study.com
So, it seems that what matters are what the amino acids are in the food. There are nine essential amino acids that humans need in their food. Essential amino acid
There are an estimated around 4,000 possible biologically reasonable amino acids in one computer search. Alien Life Could Use Endless Array of Building Blocks. And many of them occur in nature. Out of those, life uses 20 (or 21 or 23 depending how you count them) (Amino acid). Of course many of those won't occur in nature - but Earth life also uses some amino acids that don't occur naturally either. Also other searches might change or refine those numbers.
It's clear that there are many more amino acids than those used by life. For instance, 52 amino acids have been identified in the Murchison meteorite. Amino acids in meteorites.
So, the food is likely to be missing essential amino acids. It's also likely to include extra amino acids that our body is not used to.
And then, the amino acids are asymmetrical, and any of those amino acids can be in either its left or right form to build the proteins. We can only use it in its "left hand" form.
OTHER ESSENTIAL NUTRIENTS
Humans also have many other essential nutrients e.g. vitamins, that have to be in our food or eventually we'll die. Essential nutrient
Their equivalent of our vitamins, for instance, instead of being health giving, may be Antimetabolites for us - drugs that interfere with the normal functioning of the cell.
And vice versa, our vitamins might interfere with their metabolism equally.
And the trace elements we need might some of them be deadly to them and vice versa.
IF IT IS NOT RELATED TO EARTH LIFE
So, unless related to us, it's not going to be able to give us all the amino acids and vitamins and other essential nutrients we need to survive, bar an extraordinary coincidence.
Barring an extraordinary coincidence, it wouldn't have toxins that are targeted at humans or animals specifically. But there are many things that are poisonous for us, so it doesn't need to specifically target vulnerabilities in humans to do that.
For instance, maybe your ET just loves to eat hydrogen cyanide, or arsenic, or perchlorates. These are delicacies for them, but deadly to us.
COULD BE TASTY HOWEVER
Apart from that - well it could be tasty even if not actually nutritious. And it could have carbohydrates to give you energy.
SWEETENERS
Sugars seem quite possible.
Glucose, the form of sugar you have in fruit juice,
Seems reasonably possible they'd have something like that, simple carbohydrates that we may be able to eat. So you'd be able to use ET food as a source of energy at least.
At least you could hope to eat the icing of their cakes.
As other possibilities, maybe it naturally produces an artificial sweetener, e.g. Aspartame which is a combination of two amino acids.
Or Saccharin
Or lead acetate, which of course would give you lead poisoning if you used it a lot.
See Sugar substitute
ALCOHOL
Ethanol is simple enough, so good chance you can at least share a drink with ETs. That is, if they happen to also like alcohol, which of course depends on their metabolism.
Romulan Ale
TASTY POISONS
But tasting nice is no guarantee that it is good for you, or indeed not poisonous. E.g. antifreeze, as simple as ethanol chemically, just ethanol with an extra hydroxyl grouping, Ethylene glycol, tastes sweet.
And is so simple you can easily imagine an ET food including it.
But it is moderately toxic, and because it is sweet, sometimes children for instance drink it in large quantities, which can lead to Ethylene glycol poisoning and death.
OTHER CONDIMENTS
Of course also salt is tasty - sprinkling ET originated salt (maybe with trace elements) on your food might well be fine unless it contains mercury, arsenic or some such.
So there might well also be tasty condiments from ET planets.
Salt on Mars. But this isn't table salt, these are sulfates. Salts on Mars are always highly oxidized, so not much chlorides. Sulfates, chlorates, or perchlorates dominate them.
But you'd need to be careful, for instance, salt deposits on Mars consist of sulfates, chlorates and perchlorates with almost no chlorides. The perchlorates, particularly, are poisonous to humans.
So, if an ET host offers you salt, you need to ask "what kind of salt?". In case they have a taste for perchlorates, for instance, as a condiment.
COULD CAUSE ALZHEIMERS OR OTHER TANGLE DISEASES
It could also contain chemicals that are similar enough to ones our body uses that it takes it up as food - but they are not quite identical.
Example:
L-serine, resembles
BMAA which is created by green algae.
It’s been suggested that BMAA can be misincorporated to cause tangle diseases like Alzheimers.
So - if it is similar enough so that you find it tasty - might be that your predilection for ET food gives you Alzheimers in your later life.
BE SURE TO STERILIZE YOUR FOOD AND NOT SIT IN THE SAME ROOM AS YOUR ET HOST - BEST - USE TELEROBITIC AVATARS
As well as that, you'd need to be sure to sterilize the food because the biggest danger I think would be from ET microbes. Which you might also get from the ETs themselves.
You might think that ET microbes would have no effect on a human, but the thing is - that though they wouldn't be adapted to us, our immune system also wouldn't be adapted to them. Just as you can have e.g. artificial implants that your body doesn't reject because it doesn't trigger your immune system, so also ET life could quite possibly invade your body and your immune system doesn't even notice it because it doesn't produce the chemicals characteristic of life. This has been suggested as a motivation for developing synthetic life in the laboratory - that you could create implants that the body wouldn't recognize as life and so wouldn't reject. See Xenobiology: A new form of life as the ultimate biosafety tool
Joshua Lederberg who took a special interest in this said about Mars life, which might not produce the same peptides and carbohydrates as Earth life:
"On the one hand, how could microbes from Mars be pathogenic for hosts on Earth when so many subtle adaptations are needed for any new organisms to come into a host and cause disease? On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens. Thus, although the hypothetical parasite from Mars is not adapted to live in a host from Earth, our immune systems are not equipped to cope with totally alien parasites: a conceptual impasse"
Then these microbes themselves, once they are able to make a home on and in your body, may make various chemicals that are toxic to you. Or interfere with your body functions. And the other way around, your microbes may make chemicals toxic to them.
Or another thought - just to think about - might they even just slowly eat you? What if you are edible to some of the microbes, general purpose feeders - and your cells don't realize what's going on? They will respond to the trauma but their reaction may have no effect on the ET microbes that just gobble up everything your cells create.
Bowl of yoghurt, photo by Schwäbin
If your host offers you live yoghurt, be sure to sterilize it first. Unless you have proved that our microbes and theirs are compatible first.
But as well as that - after visiting an ET and sharing a meal with them, your lungs, sinuses, eyes, stomach, the surface of your skin, and every accessible part of your body may be inhabited by trillions of ET microbes that your body has no idea needs to be eliminated. Microbes that are used to living on the skin and inside the stomach etc of an ET host that has a different biology from you.
And of course would be the same likewise for them, they'd surely find that at least a few of the hundred trillion microbes in ten thousand species that make human bodies their home have jumped over to them and are inhabiting their body as well.
They may be well behaved bacteria, perfect symbionts for the ET. But are they going to be as well behaved for you with the different biochemistry of your body, which may not recognize them as life?
This is from my What Food Can You Share With An ET?
IF WE CAN’T LIVE THERE RIGHT AWAY, COULD WE GROW FOOD THERE THAT WE CAN EAT?
Now, that doesn’t really address the question completely. Maybe Earth life can be compatible with other forms of life. Or maybe some biochemistries are compatible and play nicely with us and others don’t. We can’t eat their food, except things like alcohol, salt, sugar etc, but perhaps if we are lucky we can plant our own crops there and they grow fine and we eat those. And perhaps we can breathe the air and there is nothing in it that’s poisonous to us, and no microbes that eat our alien (to them) biology.
Or it might be that they are so incompatible that our crops just can’t grow there at all - the microbes in the soil just don’t play nicely with them. Or they have pests that just gobble up our crops and the crops have no defenses and maybe they gobble us up too with our bodies mounting no defenses. Any of that seems possible.
Even if there is only microbial life there - would it be compatible with Earth life to the extent we can have a mix of Earth and ET life? Or is the only way to grow food on another world to somehow sterilize their entire planet first?
Perhaps it depends on the planet. Some have compatible life that plays reasonably nicely with Earth life and some just don’t?
That might be an issue even with Mars. We don’t know enough about it yet to know if there is life there now, or was in the past. It may well have had life in the past though we don’t know, and if it did, there’s a decent chance it is still there. So would it play nicely with Earth life, or not? There’s no way to know. Can we have both forms of life on Mars at once, or must it be only the one or only the other? Again nobody knows yet. Though we’ve sent many missions to Mars we haven’t searched for life there since Viking in the 1970s. All the other ones since then have been mainly geology missions, and nothing we’ve sent since Viking would be able to detect life in, say, the driest part of the Atacama desert. Also we haven’t sent anything to the places that scientists now think are the most likely to have habitats on present day Mars (it’s difficult sterilizing our robots well enough to visit such places).
Luckily there are plenty of other places we can live even in our solar system. To start with, the Moon, ice at the poles. Also materials from the asteroid belt and later on from comets. It’s far easier to live on Earth of course, and always will be surely, as we evolved here. It’s the place we are adapted to. Anywhere else has to be forced into Earth’s mold, “terraformed” in some way. But we could live comfortably perhaps in large habitats, kilometer scale, if those can be made low maintenance, with closed system ecology inside. I’m skeptical about whether we will ever live on other planets except inside habitats like that myself. Terraforming seems easy on paper - but I think the challenges would be formidable, trying to speed up what took hundreds of millions of years on Earth, and with no guarantees that it will get to where you want, it could go in many surprising directions.
ABOUT WHY LIFE ON A PLANET LIKE EARTH WOULD PROBABLY BE HARD TO DETECT THROUGH MOST OF ITS HISTORY UNTIL IT DEVELOPED TERRESTRIAL LIFE
The problem is that for much of Earth's history, before the land was colonized, a lot of the biological activity happened in the ocean and the ocean sediments.This would have been largely decoupled from the atmosphere.
Sometimes there was an excess of oxygen in the atmosphere, but the amount of oxygen present is hard to estimate. It should have been easily detectable for the last 500 million years, and also there may have been an oxygen "overshoot" from about 2.2 billion years ago to 2 billion years ago. Apart from that, howeer, although there was oxygen in the oceans, the amounts in the atmosphere could have been too low to spot remotely.
At other times, there was an excess of methane in our atmosphere (from about 3.8 billion years go to 2.5 billion years ago). However it is difficult to have large seasonal variations in methane without a terrestrial biosphere, so it wouldn't be a clear cut case that it was the result of life, as seen from a distance.. It's also hard to have both methane and oxygen in the atmosphere together in detectable quantities, for an ocean planet without a terrestrial biosphere.. Ozone might be easier to detect.
Their general conclusion is that it's hard to detect atmospheric biosignatures in an ocean world like Earth, even though this would count as one of the most habitable types of planet. It may only become easy to detect life on a world like ours once it develops a terrestrial biosphere. Of course we only have our one example to judge this by, but our Earth only developed one at a very late date. Another civilization looking at our Sun from a distant star probably would not have managed a definite detection of any life here for most of Earth's history, at least, not using the methods we are likely to have available to us in the near future.
Quoting from a paper by Christopher Reinhard et al, published in April 2017, "False Negatives for Remote Life Detection on Ocean-Bearing Planets: Lessons from the Early Earth" (emphasis mine)
As a proof on concept, we briefly summarize the remote detectability of O2/O3, CH4, and the O2-CH4 disequilibrium throughout Earth's history. Our analysis suggests that the O2 - CH4disequilibrium approach would have failed for most of Earth's history, particularly for observations at low to moderately high spectral resolving power (R≤10,000). In addition, it is possible that O2 /O3 may only have been applicable as a potential biosignature during the last ~10% of Earth's lifetime. As a result, most of our planet's history may have been characterized by either high abundances of a single biogenic gas that can also have significant abiotic sources (e.g., CH4 ) or by a cryptic biosphere that was widespread and active at the surface but remained ultimately unrepresented in the detectable composition of Earth's atmosphere. Finally, we argue that cryptic biospheres may be a particularly acute problem on ocean-bearing planets, with the implication that many of the most favorable planetary hosts for surface biospheres will also have high potential for attenuation of atmospheric biosignatures.
and from the final Discussions and Conclusions section:
"Our analysis suggests that a planet with a biosphere largely (or entirely) confined to the marine realm will in many cases remain invisible to remote detection as a result of biosignature filtering by ocean biogeochemistry—a difficulty that may apply to both presence/absence and thermodynamic techniques. Our analysis suggests that the possible detection of oceans at a planet's surface (Robinson et al., 2010; but see Cowan et al., 2012) is a critical piece of contextual information for validating potential atmospheric biosignatures, and that planets with terrestrial biospheres (e.g., partially or entirely subaerial in scope) may be the most readily detected and characterized because of their more direct geochemical exchange with the overlying atmosphere. Ironically, in some cases planets that are very conducive to the development and maintenance of a pervasive biosphere, with large inventories of H2O and extensive oceans, may at times be the most difficult to characterize via conventional biosignature techniques."
In this earlier paper from 2014, "The future of spectroscopic life detection on exoplanets" by Sara Seager, she makes a similar point, that even with present day Earth then the methane signal is far weaker than the oxygen signal and that in the past, when one of these signals is strong, usually other is very weak. She recommends that to have a decent chance of detecting life on a planet like Earth, we need a large pool of planets to observe. She also suggests that we also search for a single active gas well out of equilibrium, by much more than you'd expect from the dissociation effects of the sunlight, and that this might give results at an earlier stage, but would lead to false positives, so the result would just be a probability that one of the planets being studied has life..
"The Lederberg–Lovelock approach could be useful at the time when hundreds or thousands of rocky exoplanets have observed atmospheres—to increase the chance that two spectroscopically active gases that are redox opposites might simultaneously exist in the lifetime evolution of a planet. In the shorter term, a different approach is needed to optimize our chances to detect biosignature gases, if they exist, around a handful of accessible potentially habitable worlds. (Note that subsurface life is problematic for astronomical techniques because remote sensing may not be able to detect weak signs of life by biosignature gases coming from the interior.)"
"An idealized atmospheric biosignature gas approach is to detect a single spectroscopically active gas completely out of chemical equilibrium with the atmosphere that is many orders of magnitude higher than expected from atmospheric photochemical equilibrium. False positives will, in many cases, be a problem, and in the end, we will have to develop a framework for assigning a probability to a given planet to have signs of life."
In her conclusion she suggests that we have a sample of at least 1,000 Sun like stars, in order to have decent statistical evidence of the presence of life on some of them, though because of the problem of false positives, one might not be able to say of any of them definitively that it has life.
This last bit is from the section
in my OK to Touch Mars? Europa? Enceladus? Or a Tale of Missteps?
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