Rock or Alien Probe? The Science Behind ‘Oumuamua

There's a mysterious rock traveling through our solar system, and it's making headlines again. Could it in fact not be a rock at all, but an alien probe? Let's look at what the evidence has to say. 

Sabrina Stierwalt, PhD,
Episode #308

A few days ago, I got an early morning text from a friend: “news says we have an alien probe. Please advise.” Part of my duty as an astrophysicist is, of course, to be able to warn my friends of an alien invasion—so I was worried I had failed. The press often sensationalizes headlines in order to get eyeballs on their articles, but a quick search told me this claim was even coming from astronomers. This deserved a proper investigation!

There was much excitement when astronomers discovered the first interstellar visitor—that is, the first object to visit our solar system from another star system—in October of 2017. Where had it been? What had it seen? Where was it headed? I mentioned the discovery here on Everyday Einstein. The strangely elongated rock was traveling too fast, around 25 kilometers per second, to be a member-in-good-standing of our own asteroid belt so it must be just passing through. It was named ‘Oumuamua (apostrophe intentional) meaning “first scout” in Hawaiian.

So what’s all the fuss about now? A year later, ‘Oumuamua is making headlines again due to a paper suggesting that it could be an alien probe inserted here by an outside intelligent civilization. Let’s take a look at the evidence for why this may (or may not) be the case.

What do we know about ‘Oumuamua?

The truth is ‘Oumuamua is weird. While we are used to seeing mostly round or nearly round objects in space, this rock is elongated at an estimated 800 by 100 feet in size. It dims and brightens in a regular cycle, suggesting it may be hurtling through space end-over-end. (Think of a tumbling baton.) Also weird: when first discovered, astronomers thought it was most likely a comet. But observations with the Hubble Space Telescope showed the object did not have the typical tail of gas that comets have when their surface ices are turned to gas by the sun’s heat.

And, of course, its unusual orbit suggests that it has been traveling through interstellar space before entering our solar system, only to leave us again in the future. We know this because it is traveling too fast for the gravitational pull of all the major objects in the solar system to be able to hold on to it.

Here’s where the new theory-based paper comes in. The authors calculated whether light from the Sun could be forceful enough to be responsible for pushing ‘Oumuamua along. When photons exert a pressure on an object, it is called radiation pressure, which is a well-studied and well-understood phenomenon—so nothing unusual. Their calculations showed that it would be rather hard for radiation pressure to exert enough force to push along a wide, elongated space baton (remember that size estimate of 800 by 100 feet), but radiation pressure could do the trick for an object that was shaped more like a very thin, flat sheet. Maybe we were wrong about the size.


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