In a recent scientific paper, two Harvard researchers make a very daring speculation. They suggest it's possible that an alien artifact, built by extraterrestrial intelligence, has flown through our solar system.
This speculation, while of potentially enormous import, is almost certainly wrong. However, the idea is sufficiently interesting that it is worth understanding what is being proposed.
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In 2017, newspapers reported the observation of a visitor to the solar system called Oumuamua, an unusual object that was hot dog-like in shape and estimated to be about a half-mile long and 80 meters wide. It dove in from interstellar space before buzzing by the sun and then rocketing outward past Mars.
When Oumuamua was initially observed, it was thought to be an asteroid. However, once the object's velocity and trajectory were determined, that possibility was ruled out. Instead, it appeared to be more like a comet, in which case it would make a single pass through the solar system and leave, never to return.
Oumuamua approached the sun from above the plane of the solar system, dove below the sun's southern pole and then used the sun's gravitational field to slingshot itself back above the plane of the solar system as it headed off into deep space. While its trajectory seemed comet-like, there was no evidence of the typical tail one sees in comets.
So, regardless of the improbable assertion of extra-terrestrial involvement, this is an interesting thing. An object without a tail, moving with sufficient velocity that it must have come from interstellar space, entered and will leave the solar system. In science, it is generally true that where there is one example of something, there are many. And so researchers were very excited at the prospect that Oumuamua could lead to a new branch of astronomy.
Scientists monitored the body, noting how its brightness and its velocity changed. The varying brightness can give some information on the object's shape and the manner in which it is rotating, while the velocity is expected to follow the law of gravity, which is very strict. It's that last measurement that has caused this recent hubbub.
Give an astronomer an object's position, velocity and trajectory, and he or she can predict far into the future where it will be and how fast it will be going. There's only one problem: The predictions for Oumuamua's velocity and ongoing measurements disagree.
It appears to be leaving the solar system more quickly than expected based on gravity. This is another way of saying that is that it is experiencing a small and unexplained acceleration away from the Sun.
Now, this acceleration is very small indeed; the effect of gravity is about 200,000 times bigger. In addition, it's being reported that this unexplained effect is changing in a very specific way. As the distance between Oumuamua and the sun increases, the unanticipated acceleration decreases as the square of that distance.
There is a very pedestrian explanation for this non-gravitational behavior: The object is "outgassing," which means various forms of ice are melting and puffing off into space. Basically, it's the same principle as a rocket ship. Near the sun, there would be lots of melting and lots of puffing.
Further from the sun, the melting would slow and that would result in less unexplained acceleration. This is a perfectly reasonable explanation for the object's movement through space -- except, of course, that there has been no observation of a comet-like tail, which is caused by exactly this sort of melting of ice. That doesn't mean that it doesn't exist, but it hasn't been seen and that means that astronomers cannot assume it is there.
This brings us to the most recent conjecture, made by two physicists at the Harvard-Smithsonian Center for Astrophysical Research. There is another possible source of the acceleration called "radiation pressure," which is a force that comes from light from the sun bouncing off Oumuamua. That force is clearly related to the amount of light bouncing off the object and thus it should depend on the distance Oumuamua is from the sun.
The Harvard researchers embraced the radiation pressure hypothesis and then calculated the mass and other properties that Oumuamua must have, consistent with all known measurements. They concluded that the object might be a solar sail of extraterrestrial origins, which is a possible technology that is being considered for humans to send probes to other stars. It's not a completely far-fetched idea.
The researchers went on to imagine a number of scenarios, including that Oumuamua was space debris from an extraterrestrial civilization or, even more provocative, that it was a fully operational probe that entered the solar system, did a survey, and is moving on. If that were true, then astronomers named Oumuamua very aptly, as the word means "scout" in Hawaiian.
Is this explanation likely? No, of course not. Is this hypothesis the result of hopeless crackpottery? No, that's not true either. Both the authors of the paper establishing the unexplained acceleration and the authors exploring the light sail idea are respected scientists with unblemished research histories and no hint of scientific misconduct or otherwise suspicious behavior.
So, what could it be? Well, I suppose the extraterrestrial origin is possible, but very unlikely. To better appreciate the scientific method, one should return to the original paper that reported the object's unexplained acceleration. It makes very strong claims that something in addition to gravity is going on, so we can be pretty certain about that.
Further, the paper proposes some plausible explanations for how outgassing might be the explanation, even though no outgassing was observed. And, on a more cautionary note, if you dig a bit deeper it says that while the way in which the acceleration changes relative to the object's distance from the sun is consistent with either the outgassing or radiation pressure hypothesis, they cannot rule out other behaviors. That would mean that astronomers need to rethink the entire situation.
"Extraordinary claims require extraordinary evidence" is an aphorism made popular by Carl Sagan, the legendary planetary scientist and science popularizer. The motion of Oumuamua is a perfect example of how it applies. It is almost certain that this mystery has an ordinary explanation, but it is at least physically possible that the probe explanation is true.
But, until I see extraordinary evidence, I'd bet against it.