In 2017 astronomers spotted a very unusual object approaching Earth. What was most unusual about it was that it was on a trajectory that would take it out of the solar system. Given its path it could only have come from outside the solar system – our first ever discovered extrasolar visitor, named Oumuamua. For an extrasolar object, it came improbably close to the Earth and the Sun, which gave us a great opportunity to take a close look at it. And then, as it passed by the sun and headed out of the solar system it became even more unusual. First, we could see that it was an very long and flat object, not typical for a comet or asteroid. Second it accelerated as it moved away from the sun, like a comet would from sublimation of ice into gas acting like a rocket. But we could not see a comet-like tail, and the albedo was off. Curiouser and curiouser.

This lead some to speculate wildly that Oumuamua may be an alien artifact, most famously Avi Loeb, a Harvard scientist who has now even published a book – Extraterrestrial: The First Signs of Intelligent Life Beyond Earth. This is a clear case of the “aliens of the gap” fallacy – any astronomical phenomenon we do not currently fully understand must be evidence of alien technology. Of course, all natural explanation must first be excluded. But even then, we don’t have aliens, we have an unknown phenomenon that needs further exploration.

Oumuamua is now yet another great case in point. Two Arizona State University astrophysicists, Steven Desch and Alan Jackson, have come up with a plausible explanation for Oumuamua’s funky properties. Perhaps, they hypothesized, our attempts so far to explain the object’s behavior and properties failed because we were making false assumptions about what kind of ice it might contain. We assumed it would have a profile of ice similar to the comets we know. But what if the ice is made of something else, because Oumuamua is not a typical comet. When they looked at the properties of nitrogen gas – bingo. This would nicely fit the data, including the combination of the rate of acceleration from ice sublimation near the sun and the low albedo – not as much reflective ice would have been necessary to cause the acceleration.

Nitrogen ice might also explain Oumuamua’s strange shape. If it is not a typical comet but rather a piece of a Pluto-like planet that broke off from a collision, it might contain mostly nitrogen ice. As the outer layers of ice then sublimated away during its long journey due to cosmic rays, that would have the effect of flattening it out. Desch and Jackson further calculate that a chunk of solid nitrogen ice could have survived an interstellar journey for half a billion years, long enough for such a journey to reach Earth.

There are so many examples of this sort-of thing happening in the history of science we need to keep it in the front of our minds when considering new scientific mysteries. There may be assumptions we don’t know we are making. There may be new phenomena out there we have not discovered yet. There is simply too much unknown to make any sort of argument from ignorance.

My favorite historical example is that Lord Kelvin calculated the age of the Earth to be mere millions of years old based on his thermodynamic equations and the rate of Earth’s cooling. But he did not know about radioactive decay, which serves to heat the Earth’s crust and slow its cooling over time. This is why scientists need to keep an open mind, and consider all possible logical hypotheses for any phenomena. Only by going systematically through all possibilities can we be at all confident in our conclusions, and even then we have to acknowledge the possibility of unknown unknowns.

But keep the context in mind here – I am stating why the argument from ignorance is so weak. This is not an excuse to turn the possibility of unknown unknowns into yet another argument from ignorance, or a refutation of all possibility of meaningful knowledge. The key is to understand the difference between building a positive case for one hypothesis vs just assuming a hypothesis based entirely on what we don’t known or the possibility that we don’t know.

Plausibility and Occam’s razor are also important concepts here. There are potentially unlimited hypotheses to explain any phenomenon, if we let ourselves speculate wildly and introduce any idea, no matter how implausible and contrived.

Oumuamua, in addition to being a very cool astronomical find, will go down as yet another cautionary tale about the alien of the gaps fallacy.