One year ago, in October 2017, astronomers detected the first confirmed interstellar visitor to our solar system – an asteroid dubbed Oumuamua. The name is Hawaiian for “scout”, as if the asteroid is a messenger from a distant system. A Hawaiian name was chose because the object was discovered by the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) in Hawaii. Determining that Oumuamua was an interstellar object was not difficult – the determination was based on its trajectory. It was traveling really fast, too fast for any object originating from our own system. It’s velocity would also take it out of our system – it was moving too fast to be captured by the gravity of our sun.

All of that is cool enough, but astronomers carefully analysing the trajectory of Oumuamua discovered (and published their findings in June 2018) that its acceleration could not be explained entirely by gravity. Some force was pushing, ever-so-slightly, on the object. This acceleration could be explained by outgassing, if there were any volatiles on Oumuamua that were heating up as it got closer to the sun. These gases would be like tiny rocket engines. Observations of the object did not detect any comet-like tail, which is why it was thought to be an asteroid. But if this new observation were correct, then it would have the ices and gases associated with a comet.

Oumuamua was discovered 40 days after its closest approach to the sun, when it was already on its way out of our solar system. At this point it should have been slowing down a bit from the pull of the sun’s gravity, but instead it was speeding up slightly. This could be explained by outgassing caused by heat from the sun.

This led to a debate about whether or not Oumuamua was an asteroid with a small amount of ice, or a comet that had lost most of its ice. It seems that the object exists in the gray zone between asteroid and comet (and we run into yet another definition demarcation problem). Further, close analysis showed that Oumuamua is very elongated (often described as “cigar-shaped”) and quickly tumbling end-over-end.

Now, Shmuel Bialy and Prof. Abraham Loeb have published online a paper hoping to take the debate into another direction. They offer an alternate hypothesis to cometary outgassing – that Oumuamua’s acceleration was due to a “solar sail” effect. Their main argument is a technical one:

If ‘Oumuamua were in fact a comet, why then did it not experience outgassing when it was closest to our sun? In addition, they cite other research that showed that if outgassing were responsible for the acceleration, it would have also caused a rapid evolution in ‘Oumuamua’s spin (which was not observed).

They explain further:

“We explain the excess acceleration of `Oumuamua away from the sun as the result of the force that the sunlight exerts on its surface. For this force to explain measured excess acceleration, the object needs to be extremely thin, of order a fraction of a millimeter in thickness but tens of meters in size. This makes the object lightweight for its surface area and allows it to act as a light-sail. Its origin could be either natural (in the interstellar medium or proto-planetary disks) or artificial (as a probe sent for a reconnaissance mission into the inner region of the solar system).”

So if Oumuamua has a certain shape, mainly a sufficient ratio of surface area to total mass (meaning it has to be very thin) then the pressure from the solar wind itself could explain its acceleration. They add two possible ways that it could have achieved this shape – either through natural forces, or by design, making it an alien artifact.

In their paper they calculate that such an object (one so thin) could survive interstellar space. This makes the solar sail hypothesis at least plausible. But actually their most interesting argument is the fact that Oumuamua came within 0.25 AU of the sun and 0.15 AU of the Earth. If this were random, they calculate that stellar systems on average would have to eject 10^15 such objects – which is 100 million times more than projected. I guess that means that our chance encounter with Oumuamua was a 1 in 100 million chance – that’s like winning the interstellar lottery.

But further, we did not detect Oumuamua until it had already passed the sun. It was a lucky find, and we could have missed it. This means we could be missing more such objects. Bialy and Loeb solve the “intersellar lottery” problem by arguing that Oumuamua might be an alien probe – so it was targeted at us deliberately, perhaps to survey the Earth for signs of life.

I am not saying I buy their argument. I have pointed out before than whenever astronomers see something strange, or something they cannot quite explain, someone brings up the alien hypothesis. So far, every time it has turned out that a natural explanation was found. That does not mean that at some point we won’t encounter a genuinely alien phenomenon, it just means that we should be appropriately skeptical and not jump to the alien hypothesis too quickly.

Astronomers did point radio telescopes at Oumuamua, and it was not sending out any radio signals. Bialy and Loeb argue that perhaps it was a defunct alien probe, more of a shipwreck, but this is just special pleading. I will say the lack of radio emissions does not mean it isn’t an alien probe – any such artifact would not necessarily be using any technology recognizable to us. But still – we have a lack of any evidence that Oumuamua was technological.

What is clear is that Oumuamua is an interstellar object. It is currently heading out of our system, and in another three years will cross the orbit of Neptune. In 11,000 years it will be one light year away. I don’t think the alien hypothesis will hold up, but the argument about the odds of this encounter are at least interesting. I will need to hear what other astronomers think of this.

And eventually (probably) like most of these one-off astronomical encounters, the debate will be settled when we encounter further examples. If this was just a random encounter with an ordinary interstellar comet or asteroid, then we should encounter others. Right now we have one data point. When we have more we will be able to make more informed statements about probability.