‘Oumuamua: was it aliens? (spoiler: no)

Imagine this.  You’re drinking your morning coffee. A small blob zooms at top speed through the kitchen, into the hall and out an open window. You only catch a quick fuzzy glimpse before it’s gone.

What was that thing?

What’s your first guess? A neighborhood cat? A squirrel? Maybe something more exotic like a raccoon or a fox?

Or was it … an alien?


A parallel story has been playing out in astronomy.

Something zoomed past the Earth in October 2017. Astronomers scrambled to observe the object but within about a week it was too faint for almost all of our telescopes.

It was so small that we couldn’t see its shape. The only thing we knew right away is that the object was moving so fast that it must have came from outside the Solar System. It was named ‘Oumuamua.

Some scientists proposed that ‘Oumuamua was an asteroid kicked out of its home planetary system. Others (like me) thought it was more likely to be comet-like. Some suggested it could have been ejected from a dying star or be a remnant of a planet that was shredded to pieces.

Then two scientists from Harvard claimed it was an alien spacecraft and the media of the world went bonkers.

‘Oumuamua’s trajectory through the inner Solar System. Credit: Tom Ruen via Wikimedia Commons.

Over the past year I’ve had the great pleasure to be a part of a group of scientists thinking about ‘Oumuamua, sponsored by the International Space Science Institute.

Our team includes the discoverers of ‘Oumuamua, scientists who specialize in detecting and modeling the evolution of asteroids, comets and outer Solar System objects, and experts in how stars and planets form (like me).

Our first team meeting took place shortly after those two astronomers cried “alien”! (In astronomy-land this is like the boy calling “wolf”.)

Our team’s first order of business quickly became, as best we could, to un-ring the alien bell.

The ‘Oumuamua ISSI team. From left to right: Robert Jedicke, Sean Raymond (that’s me), Matthew Knight, Asmita Bhandare, Piotr Dybczynski, Karen Meech, Susanne Pfalzner, Michele Bannister, Aurelie Guilbert-Lepoutre, Alan Fitzsimmons, Colin Snodgrass, and Andrew McNeill.  Missing: Quanzhi Ye, David Trilling.

Our first paper – just published in Nature Astronomy – explains how natural processes can explain what we know about ‘Oumuamua.

To quote Carl Sagan (and precursors Laplace and Hume), Extraordinary claims require extraordinary evidence.

The claim that ‘Oumuamua is an alien spacecraft is as extraordinary as it gets. But it is lacking in extraordinary evidence, or really, in any compelling evidence at all.

In this post I will discuss how all of ‘Oumuamua’s characteristics can be explained without aliens. There is still plenty of debate; we aren’t sure whether it was a squirrel or a cat that ran through the house. But there is no reason to think that it was aliens.

There are five pieces of “evidence” used to claim that ‘Oumuamua is an alien spacecraft.

None stands up to even the gentlest scrutiny.

Let’s go through them one at a time.

  1. There are too few natural objects in interstellar space: ‘Oumuamua should never have been discovered. It must have been aimed at Earth deliberately.

This argument relies on knowing – by other means than simply looking – how many natural objects are out there. And that relies on knowing where they came from.

Planetary systems are the most obvious source of interstellar objects. Planet formation is messy, and leftover building blocks are commonly tossed out of their systems.

We know quite a bit about planets around other stars and how planets form (videos here). We can leverage this to estimate that every star probably throws out somewhere like Earth’s mass in planetary leftovers out into interstellar space.

How many objects depends on their sizes, which we don’t know.  It could be a few big ones or a zillion small ones.

Let’s imagine that interstellar objects have the same distribution of sizes as our asteroids. In that case the big objects dominate and it is indeed very unlikely that we would have detected ‘Oumuamua.

The asteroid size distribution.  Even though there are only a handful of large asteroids, those objects outweigh all of the small ones. Credit: Marco Colombo, DensityDesign Research Lab via  Wikimedia Commons.

But if the size distribution of interstellar objects is tilted a bit more toward smaller sizes then it is no surprise that we found ‘Oumuamua.

So interstellar objects probably don’t have the same size distribution as the asteroids.

This is interesting! It may be telling us about the growth of the building blocks of exoplanets, or the processes that shape these objects.

But not evidence for aliens.

  1. ‘Oumuamua entered our Solar System on a very unlikely orbit, which came closer to Earth than to any other planet. It must have been aimed toward Earth by intelligent aliens.

Objects from interstellar space enter and leave the Solar System all the time. Studies find that, indeed, ‘Oumuamua is on an unusual orbit when compared with the typical trajectory of an object entering the Solar System.

But which interstellar objects can we actually find? These objects are faint and we can only detect those that come close to Earth.

It turns out that ‘Oumuamua’s orbit is completely average when compared with detectable interstellar objects. The ones we could find are mostly on “special” orbits.

The odds of running into a Japanese person in Paris are pretty low. They are in a small minority. But they are no longer a small minority if you are at the airport, by the gate for a flight to Tokyo. It all depends on the context.

In the right context, ‘Oumuamua’s orbit is nothing special. No need for aliens.

  1. Its shape is too strange not to have been shaped by aliens.

‘Oumuamua’s shape is weird. In my mind, this is its most mysterious characteristic.

The thing is: we don’t know ‘Oumuamua’s actual shape. All we know is that its brightness bobbed much farther up and down than a typical asteroid. To explain these large oscillations in brightness, we think that it must be very stretched out.

It might look like a cigar:

A simulation of how a cigar-like shape for ‘Oumuamua would match its large observed brightness oscillations.  Credit: nagualdesign via Wikimedia Commons.

..or it might look more like a pancake (I really want to say that it looks like the Millennium Falcon but the whole “alien spacecraft” media ruined that kind of analogy):

Artist’s view of the potential, pancake-like shape of ‘Oumuamua. Credit: William K. Hartmann.

Why is it so stretched out?

We aren’t sure but there are a few promising ideas.

Maybe ‘Oumuamua is a fragment of a larger object that was torn to shreds before it was ejected from its home planetary system, and that process stretched it out.

Maybe high-speed collisions with interstellar dust during its voyage between the stars changed its shape from a potato into a shard.

Maybe slow collisions in the outer reaches of its home planetary system produced its strange shape.

Or maybe another process we haven’t thought of yet is responsible.

It’s an unsolved problem. But the idea that we need aliens to solve it is like saying that we should wait for aliens to cook us dinner. Come on, people, we can do this ourselves!

  1. ‘Oumuamua is too shiny to be an object similar to asteroids or comets

The only measurement of ‘Oumuamua’s reflectance used NASA’s Spitzer Space Telescope a month or so after ‘Oumuamua was discovered. Unfortunately, the observations were unable to detect ‘Oumuamua because it was too faint.

The study concluded that ‘Oumuamua’s reflectance could be anywhere from one to fifty percent, with a most likely value of about 10%.

Most Solar System objects are dark. Cometary nuclei – chunks of rock and ice that give off water vapor to produce spectacular cometary comas and tails – only reflect 2-7% of sunlight. Water-rich asteroids are a little less dark, but still only reflect 5-20% of sunlight.

If ‘Oumuamua really reflected 50% of the light that hit it, that would be really weird. But cometary nuclei and water-rich asteroids each fit nicely within the most likely range.

No need for a shiny alien spacecraft.

  1. Its non-gravitational acceleration – the observed changes in ‘Oumuamua’s orbital speed not due to the Sun’s gravity – cannot be explained without it being an alien space ship.

This is the most controversial point and it’s a little technical. Let me explain.

Within a week of ‘Oumuamua’s discovery it became too faint to be detected by any but the most powerful telescopes. The Hubble Space telescope was able to track its position with a few precise measurements over more than two months, until January.

The data showed that ‘Oumuamua did not follow a purely gravity-determined trajectory. Rather, in addition to gravity it seemed to have been gently pushed away from the Sun. This extra push is called non-gravitational acceleration.

‘Oumuamua’s trajectory through the Solar System as it was affected by non-gravitational acceleration.  The data show that ‘Oumuamua did not just follow the Sun’s gravity (green line). From this paper. Credit: ESA.

This is nothing terribly shocking in itself. Many comets undergo non-gravitational acceleration. In comets this happens because as comets heat up, ices near their surfaces turn to vapor and create jets, which push the comet.

But jets also change comets’ spins.

How comet outgassing through jets — if asymmetric and misaligned with the comet’s spin — can act to change the comet’s spin. Credit: Nature.

Exactly how strongly jets change comets’ spins depends on how many jets there are, how strongly they push, and how they are positioned compared with the comet’s spin axis.

In the case of ‘Oumuamua, we know about how strong any jets could have been pushing because of its measured non-gravitational acceleration.

If there was just one jet, a study showed that odds ‘Oumuamua should likely have been pushed to spin faster and faster until it was torn apart.

It wasn’t torn apart, of course.

How do we make sense of this?  There are two possibilities. Either the jet model used in that study did not capture how ‘Oumuamua’s jets actually worked. Or we need to throw the whole jets idea out the window.

The alien-spacecraft-camp threw the jets model out the window.

When all else is eliminated, they wrote in Sherlock Holmesian fashion, what remains must be the truth. [The problem is that all else was not eliminated — we’ll come back to that.]

Another theoretically possible explanation for ‘Oumuamua’s non-gravitational acceleration is pressure from the Sun’s radiation. Light has momentum, which can push an object in space.

But light only has very little momentum. For light to push an object, that object must collect a lot of light but have a very small mass. It must have a very very low density.

One study proposed that ‘Oumuamua could be a very loosely bound pile of dust pushed around by sunlight. This seems unlikely as such an object would probably have been torn to pieces by its passage close to the Sun.

Another solution is that ‘Oumuamua is the size of a soccer field but only a few millimeters thick. That would make it a solar sail, which uses starlight as a means of propulsion.

Considering an artificial origin, one possibility is a lightsail, floating in interstellar space as debris from an advanced technological equipment. — Avi Loeb.

This is the crux of the alien-spacecraft-camp’s chain of logic: this solar sail presumably represents a spacecraft launched toward Earth by intelligent aliens.

Artist’s concept of a solar sail.  Credit: Josh Spradling/ The Planetary Society.

Solar sails are nothing to scoff at. Radiation pressure has been used as a means of attitude control and a number of upcoming space missions plan to use solar sails for propulsion.

But there are some pretty titanic problems with the idea that ‘Oumuamua is a solar sail.

‘Oumuamua spins every 7-8 hours in a non-repeating way.  But why would ‘Oumuamua be spinning at all if it were a solar sail?  That is not how solar sails work — they need to keep their light collectors pointed toward the star!  If ‘Oumuamua were a functional solar sail we would not have seen it spinning.

‘Oumuamua also entered the Solar System at a modest galactic speed.  If aliens really wanted to say hello, why did they send it toward us so slow?  Any aliens that actually want to send out interstellar probes could be assumed to send them a bit faster (e.g., the Breakthrough Starshot solar sail project aims at reaching 20% of lightspeed)!

The simplest way to explain away these problems is if ‘Oumuamua is a broken-down solar sail that was used as an alien probe.

Sort of like the giant cylinder in Rendezvous with Rama but designed by aliens that didn’t think ahead and couldn’t build too well.

Plus, statistically speaking, each star would need to toss out more than one quadrillion (that’s one million billion) of these alien probes for us to find one.

If it’s an alien spacecraft, it’s the Brexit of alien spacecraftAlan Fitzsimmons

(Editorial note: in that quote, “Brexit” = massive screw up)

Now let’s go back and take the other path: what if ‘Oumuamua’s jets simply did not follow the assumptions made in that study?

How comets’ spins change due to outgassing is an active area of research.  And it’s more complicated than you might think.

The good news is that there are simple configurations that match ‘Oumuamua’s spin and outgassing.  Here is a model by Darryl Seligman showing how ‘Oumuamua’s spin would change if its jet (the moving blue arrow) always pointed toward the Sun.

Following this idea, ‘Oumuamua would have lost about 10% of its total mass as it passed by the Sun.  Its spin would have evolved in a way that matches what was observed.

Without aliens.

Is that model the right one?  We don’t know — it’s being studied right now.  But it clearly shows that comet-like outgassing can explain ‘Oumuamua’s non-gravitational acceleration and spin at the same time.

That wraps up this point: there is no need to invoke aliens to explain ‘Oumuamua’s observed acceleration.

If it’s not an alien spacecraft, then what is ‘Oumuamua?

Since its discovery, there have been a lot of ideas thrown out about where ‘Oumuamua came from and what it really is.

There is no universally accepted answer, but signs point toward ‘Oumuamua being similar to Solar System comets.

Comet-like objects are tossed out of their home planetary systems all the time.  It is usually Neptune- to Jupiter-mass giant planets that are doing the ejecting.  But passing stars can liberate comets also.

If ‘Oumuamua is comet-like (or perhaps a cometary fragment), the number of interstellar objects works out. Its appearance and non-gravitational acceleration make sense.

But some questions remain.

If ‘Oumuamua underwent outgassing, why didn’t we see it?  Most comets give off dust along with gas. Telescopes should have detected small dust grains but they didn’t. Maybe ‘Oumuamua simply didn’t give off any observable dust (as is often the case for comet 2P/Encke).  Or perhaps its dust was somewhat larger in size (as for some known long-period comets).

And why does ‘Oumuamua’s have such a weird shape?  It is far more stretched out than most asteroids and comets in our Solar System. There are some ideas to explain its shape but none that is mic-drop compelling.

It is possible that some of ‘Oumuamua’s weirdnesses are simply due to its small size. We don’t actually know the shapes or properties of very many ‘Oumuamua-sized objects even in our Solar System. Small objects are so faint that they are simply hard to observe.

Conclusion: signs point to ‘Oumuamua being a natural object.  Maybe something similar to a comet. But no aliens, unless they are pretty dumb.

Looking ahead, when will we find the next ‘Oumuamua?  Will it be as weird as ‘Oumuamua?  Are there many different flavors of interstellar objects?  (And will we try to intercept it?)

A final philosophical note.

We’ll never prove that it wasn’t aliens. ‘Oumuamua is beyond the orbit of Saturn and there is no hope of getting new data.  There is no way to demonstrate conclusively that it is a natural object not an alien artifact.

Where do we go from here?

We have to stay true to the data. The simplest explanation based on what we know right now is that it is a comet-like object. There is plenty of debate to be had — for example, maybe it is more asteroid-like, maybe it came from a binary star system, maybe… — but no evidence for aliens.

The most useful approach is to keep an open mind. If some new argument convincingly disproves the comet-like hypothesis, then we should move on.

And if, somehow, new evidence to support the alien-spacecraft claim comes to light, we shouldn’t immediately dismiss it.  Unless it’s zooming through the kitchen disturbing our morning coffee…



68 thoughts on “‘Oumuamua: was it aliens? (spoiler: no)

  1. Two more facts may be added that hint that Oumuamua may be a natural object after all:
    1 ) Before entering the Solar system, Oumuamua had a kinematics similar to the molecular clouds in the Milky Way, that is, these clouds rotate around the Milky Way almost on circular orbits, contrary to most stars. So it seems more likely that Oumuamua originated in a molecular cloud than in a planetary system around a star.
    2) Several papers now discuss the possibility that Oumuamua and other interstellar objects could consist in part of molecular hydrogen ice (solid H2), which would be possible deep inside the opaque coldest molecular cloud cores, where temperature may be very low ~3K and stellar radiation heating is almost absent. Outgasing molecular hydrogen is very efficient to push an asteroid, and was not looked for in observations, because very difficult to detect.

    The first paper mentioning the possibility of interstellar objects made of solid H2 has my student Andreas Füglistaler as first author:

  2. Asking for my own education: if indeed there was a light sail floating around the Solar System, what would it look like when observed from a telescope such as Pan-STARRS? What could be a telltale sign that it’s something artificial?

  3. Good question! The sad thing is that I don’t know of anyone who is even doing this sort of modeling. I feel like the alien-spaceship camp isn’t even really testing their own hypothesis! The point being: I would love to see such models.

  4. Thanks for answering both my questions! Indeed, proposing a solution without proper modelling is not a strong argument to me. The “alien” camp is probably running short of people who can actually do this kind of studies, but I was indeed surprised anyway of the lack thereof. On the other hand, I’ve seen models of Dark Matter as particles that allegedly explains the kinematics of the Bullet Cluster, but only if you stretch the parameters (see also http://backreaction.blogspot.com/2017/01/the-bullet-cluster-as-evidence-against.html)…but since the alternative doesn’t fly either, the DM-as-particle model is de facto accepted by the majority, but IMO not for really compelling reasons.

  5. Hi, nice article and, beautiful animation of Oumuamua’s trajectory! I agree that there is insufficient evidence to invoke an alien hypothesis but, I do have a query:

    I’ve always wondered about the late, great Sagan’s ‘extraordinary claims require evidence’. Does Science really require two types of evidence? Isn’t it simply ‘evidence’? The ‘extraordinary’ seems emotive and redundant. Is there extraordinary evidence for the multiverse or dark matter? Or, any scientific theory for that matter? Surely there’s simply a body of evidence in support or refutation over time?

  6. Hi, nice article and, beautiful animation of Oumuamua’s trajectory! I agree that there is insufficient evidence to invoke an alien hypothesis but, I do have a query:

    I’ve always wondered about the late, great Sagan’s ‘extraordinary claims require extraordinary evidence’. Does Science really require two types of evidence? Isn’t it simply ‘evidence’? The ‘extraordinary’ seems emotive and redundant. Is there extraordinary evidence for the multiverse or dark matter? Or, any scientific theory for that matter? Surely there’s simply a body of evidence in support or refutation over time?

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