Mar 18 2021
Oumuamua Explained
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.