“We must be careful not to believe things simply because we want them to be true,” physicist Richard Feynman once observed. “No one can fool you as easily as you can fool yourself.”

This is true in private life as it is in science. Consider two timely examples from the past few days.

First, mysterious drones have been spotted over New-Jersey. Many of them fly like human-made drones and others appear to be airplanes or helicopters. It is standard practice for the U.S. military to notify law enforcement authorities of any plans to fly drones over residential areas. Therefore, unidentified drones must have originated from civilians or an adversarial nation.

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Over the past few days, I was asked in many interviews, text messages and emails whether the mysterious drones could be extraterrestrial in origin. In response, I explained that all available data is consistent with the flight characteristics of human-made flying objects.

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Entirely independent from this story, a new paper led by the astronomer Darryl Seligman appeared a few days ago in Proceedings of the National Academy of Sciences. The paper reports about a population of dark comets with no detected cometary tail. The authors highlight the significance of the paper by stating that this population of dark comets shows “non-gravitational accelerations explainable by outgassing of volatiles, analogous to the first interstellar object 1I/’Oumuamua.” The implied interpretation is that `Oumuamua was simply a dark comet, namely a natural comet which was accelerated by invisible outgassing.

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Given that the typical speed of evaporated cometary gases is an order of magnitude larger, `Oumuamua had to lose about 10% of its mass to show the observed acceleration. Seligman thanked me for spotting the error in the acknowledgements at the end of the paper.

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The primary question that needs to be clarified is whether the coma of dark comets would remain invisible if their outgassing rate was boosted by several orders of magnitude?

Specifically, if the mass and corresponding brightness of the cometary tail increases by a factor of half a million for the least accelerated dark comet 2001 ME_1, will this comet remain dark? Probably not. Conversely, if a comet loses about 10% of its mass – as needed to explain the non-gravitational acceleration of `Oumuamua, would this comet be dark?

If Oumuamua does not look like a comet and does not accelerate as little as a dark comet, then it is probably not a comet. This is what common sense suggests about Oumuamua based on the available information.