Panspermia is the theory that life on our planet arose from other planets. The most plausible variant of the theory is the hypothesis of directed panspermia, the idea that life was seeded on our planet eons ago by some extraterrestrial intelligence, which may or may not have been robotic.
Panspermia seems to be a useful concept, because it may help explain something that is otherwise very hard to plausibly explain. The problem with the origin of life is that before you can have anything at all get started from an evolutionary standpoint, a very high level of sophistication has to be reached – in other words, a very high hurdle has to be jumped. This threshold is apparently the simultaneous appearance of a sophisticated genetic code and the existence of self-replicating molecules such as RNA or DNA. There is a severe, unsolved problem of explaining how that happened on the early Earth.
How can the panspermia concept help with that? I can help explain it with an analogy. Imagine you robbed $100,000 from a bank, and wanted to explain your new riches to your girlfriend without raising suspicion. If you told her that you just started playing the lottery yesterday, and won $100,000 on your first try, she would never believe you. But if you told her you had been playing the lottery every day for 10 years, and finally won $100,000, she would be much more likely to buy the story. The panspermia concept has pretty much the same effect. We can help “reduce the miracle” of life appearing (making it seem like not quite so much a miracle) if we imagine that there are billions of planets in our galaxy on which life had an opportunity to arise, and that on one of these life did arise, eventually evolving to some form that then spread the seeds of life throughout the galaxy. The more trials or chances there are for an improbable event to occur, the greater the chance of eventual success.
Let's look at various possible findings that may increase or decrease the plausibility of the panspermia idea.
Things That Might Make Panspermia Look More Likely
Below are some things that may add to the credibility of the panspermia theory.
The existence of abundant planets revolving around other stars. The more planets that exist in our galaxy, the more attractive the idea of panspermia is, because the more it accomplishes the effect of reducing an origin of life “miracle” by increasing the number of trials or chances.
The existence of planets much older than Earth. If there are planets in our galaxy billions of years older than Earth, than panspermia becomes a more attractive theory, because if allows us to say that even if life did not have enough time to naturally arise on Earth four billion years ago, it might have arisen on other planets during a much larger time window spanning billions of years, and then spread to our planet. Also, the earlier that life arose in our galaxy, the more likely that it may have spread to other solar systems.
Possible breakthroughs allowing interstellar travel. If interstellar travel is impossible, then panspermia is ruled out. But if there are breakthroughs that allow interstellar travel, then the idea of panspermia becomes more popular.
Evidence of artificiality in the genetic code. In this paper, some scientists have claimed to find evidence of artificial features in the genetic code. If such claims are ever confirmed, they would give weight to the hypothesis of panspermia (although others might argue that some divine hand was involved).
Things That Might Make Panspermia Look Less Likely
Below are some possible developments that might decrease the credibility of the panspermia theory, making it less likely to be true.
The artificial re-creation of self-replicating molecules in conditions simulating the early Earth. Imagine if scientists were ever to rig up some laboratory setup that mimicked the conditions of our planet four billion years ago. If the scientists were ever able to produce self-replicating molecules such as RNA in such a laboratory setting, that would help show that the origin of life wasn't such an improbable miracle. There would then be much less need for any theory of panspermia.
The artificial re-creation of the genetic code in conditions simulating the early Earth. Again, imagine if scientists were ever to rig up some laboratory setup that mimicked the conditions of our planet four billion years ago. If the scientists were ever able to produce something like the genetic code in such a laboratory setting, that would help show that the origin of life wasn't such an improbable miracle. There would then be much less need for any theory of panspermia.
Things That Would Not Make Panspermia Look More or Less Likely
Below are some possible developments that would not clearly either lend credibility to the panspermia theory, or make it seem less plausible.
The discovery of life on a planet revolving around another star. Imagine if scientists were to find the tell-tale chemical signature of life in some planet revolving around another star. No doubt some would claim that this was proof that life will naturally arise wherever it gets the chance, and that there is therefore no need for a theory of panspermia. But such logic would be incorrect. We would have no idea of whether that life discovered on that distant planet had naturally arisen on that planet, or had been seeded on that planet through the same process imagined by the panspermia theory.
The discovery of life elsewhere in our solar system. Imagine if scientists were to discover life on one of the moons of Jupiter, or in a comet in our solar system. No doubt some would claim that this was proof that life will naturally arise wherever it gets the chance, and that there is therefore no need for a theory of panspermia. But such logic would be incorrect. We have no idea how panspermia might have worked if it occurred billions of years ago. Rather than it involving only a placement of life on our planet, it might have involved a biological seeding of large parts of our solar system, including a large part of our planet's cometary system.
How is the Panspermia Theory Standing Up These Days?
So far, the panspermia theory seems to be holding up quite well. Little has come in recent decades from scientists' attempt to recreate the key components of life in a laboratory setting. When science writers write about such a thing, they usually discuss the Miller experiments, but those experiments occurred way back in the 1950's, and didn't take things very far, producing only some of the building blocks of proteins rather than self-replicating molecules. Attempts to get much farther haven't been very successful. The origin of the genetic code and the first self-replicating molecules is still a gigantic mystery, a mystery that panspermia theorists hope to lessen.
In the past ten years astronomers have discovered more than 1000 extrasolar planets, leading them to conclude there are many millions or billions of planets in our galaxy on which life can evolve. Last week astronomers announced the discovery of five earth-sized planets in a solar system (Kepler-444) that is 11.2 billion years old, almost 7 billion years older than our solar system. That's exactly the type of finding that lends weight to the idea of panspermia. If there were billions of planets for life to evolve five billion years before Earth even existed, that increases the chance that life may have been spreading throughout the galaxy while our solar system was forming; and in such a case some of that life may be the parent of all earthly life.
But it is an open question whether the theory of panspermia eliminates the miracle of the origin of life, or simply makes it seem somewhat less miraculous. There may be some possibilities that still deserve to be called miraculous even if you allow a billion trials and 10 billion years for their occurrence.