NASA has — quite recently — stopped being snarky about aliens. That does not make them true. It just means we don’t have to live with the snark now. Which makes it easier to think.

Will Lockett makes the point at Medium that if extraterrestrials did land on Earth, they would probably
starve to death: The way life forms work (at least, the only ones we know) involves chirality:

All living organisms are based upon certain ‘mirror’ isomers of amino acids. Although normal chemical reactions produce right and left mirrors in equal amounts, often called a ‘racemic mixture’, the proteins which constitute the organelles in living cells are composed entirely of right handed forms of sugars and left handed forms of amino acids (called ‘enantiomers’). Because such chemicals exist exclusively in one form, they are referred to as being ‘homochiral’.

““Chirality”” at All About Science

So what will happen to life forms that don’t have our chirality if they land on a planet where all other life forms do?

That means that alien life has a 50% chance of being the other chirality to us, making it incompatible with Earth life. This is known as Mirror Image Life. When I say incompatible, I don’t mean it will be a bit annoying. I mean Earth will be deadly to them and vice versa.

Firstly, some fundamental biological interactions can’t happen. Just like with our glucose example, our food would have zero calories to a Mirror Image alien. So at the very least, Mirror Image aliens would starve to death here.

But, just because a compound is an opposite chirality doesn’t mean it glides through passively. Some opposite chirality compounds interact with the wrong mechanism, causing massive damage in the process.

Will Lockett, “Aliens Could Starve To Death On Earth” at Medium (June 23, 2021)

He thinks that’s one reason why aliens have not visited us.

Now, we should keep in mind that maybe no alien life form would have that problem. We actually don’t know that our chirality is chance, as opposed to a natural law like the fact that time flows in one direction only. We can write science fiction about what would happen if we reversed time but nature doesn’t reverse it. Maybe nature does not reverse chirality either.

We can’t know whether chirality is natural law or not because we cannot examine life forms that originated apart from Earth. If it is in fact a natural law, no life form in our universe will have the problem Lockett addresses. So we must wait and see.

You may also enjoy these accounts of why we do not see the aliens:

1.Are the Aliens We Never Find Obeying Star Trek’s Prime Directive? The Directive is, don’t interfere in the evolution of alien societies, even if you have good intentions. Hence the Zoo hypothesis. Assuming the aliens exist, perhaps it’s just as well, on the whole, if they do want to leave us alone. They could want to “fix” us instead…

2.How can we be sure we are not just an ET’s simulation? A number of books and films are based on the Planetarium hypothesis. Should we believe it? We make a faith-based decision that logic and evidence together are reasonable guides to what is true. Logical possibility alone does not make an idea true.

3.Did the smart machines destroy the aliens who invented them? That’s the Berserker hypothesis. A smart deadly weapon could well decide to do without its inventor and, lacking moral guidance, destroy everything in sight. Extinction of a highly advanced civilization by its own lethal technology may be more likely than extinction by natural disaster. They could control nature.

4.Researchers: The aliens exist but they are sleeping… And we wake them at our peril. The Aestivation hypothesis is that immensely powerful aliens are waiting in a digitized form for the universe to cool down from the heat their computers emit.

5.Maybe there are just very few aliens out there… The Rare Earth hypothesis offers science-based reasons that life in the universe is rare. Even if life is rare in the universe, Earth may be uniquely suited to space exploration, as the Privileged Planet hypothesis suggests.

6.Does science fiction hint that we are actually doomed? That’s the implication of an influential theory, the Great Filter hypothesis, as to why we never see extraterrestrials. Depending how we read the Kardashev scale, civilizations disappear somewhere between where we are now and the advanced state needed for intergalactic travel.

7.Space aliens could in fact be watching us. Using the methods we use to spot exoplanets. But if they are technologically advanced, wouldn’t they be here by now? The Hart-Tipler conjecture (they don’t exist) is, of course, very unpopular in sci-fi. But let’s confront it, if only to move on to more promising speculations.

8.Is the brief window for finding ET closing? According to some scenarios (the Brief Window hypothesis), we could be past our best-before date for contacting aliens. Of course, here we are assuming a law of nature as to how long civilizations last. Can someone state that law? How is it derived?

9.What if we don’t see aliens because they have not evolved yet? On this view, not only did we emerge during a favorable time in the universe’s history but we could end up suppressing them. The Firstborn hypothesis (we achieved intelligence before extraterrestrials) lines up with the view that humans are unique but sees that status as temporary.

10.The aliens exist—but evolved into virtual reality at a nanoscale. That’s the Transcension Hypothesis, the latest in our series on science fiction hypotheses as to why we don’t see extraterrestrials.
On this view, after a Singularity the ETs become virtual intelligences, exploring inner space at an undetectably small scale.

11.Is intelligent life in the universe living in interior oceans of planets and moons? The Ocean Planets Hypothesis is that intelligent beings may flourish in the interior oceans of the moons of gas giant planets — or within exoplanets — but they are trapped there.
If intelligent life forms are trapped in the interior oceans of rocky moons and planets, Earth is a special planet—much better suited to space exploration.

12.Is real-world space travel just too daunting for ET? That’s the Percolation Hypothesis as to why we don’t make contact with aliens. They can’t overcome the laws of physics, any more than we can. If there is a purpose behind the universe, maybe the aliens and we weren’t intended to meet. That’s worth considering, given the physics barriers.

13.The Aurora hypothesis: ET could risk only rare contact with us. Given the difficulties and risks of space travel, extraterrestrials with advanced technology may have visited Earth only one in a million years, researchers say.