Over the past few months a lot of airtime has been given over to the interstellar ‘visitor’ ‘Oumuamua. This faint object, falling through our solar system on a hyperbolic orbit, has excited astronomers because, well because it’s super unusual.

The baseline interpretation of this object’s trajectory, color, and time-varying brightness suggests a highly elongated, vigorously tumbling piece of reddish, perhaps metal-rich asteroid-like material that has spent a very long time moving through interstellar space. Presumably escaped from another planetary system somewhere in our galaxy, ‘Oumuamua’s passage through our Sun’s gravity well revises theoretical predictions for the number of such comparatively large (perhaps a few hundred meters across) interstellar objects upwards by a factor ranging from a hundred to ten million times.

In other words, the simple fact that we spotted ‘Oumuamua may be revealing a large population of interstellar chunks that now become very interesting things to understand and to try to study more for insights to planet formation and the exchange of material across galactic distances.

Another intriguing feature of ‘Oumuamua is that on its outward orbital trajectory from our solar system there was evidence for it accelerating – with that acceleration declining as if it was associated with the Sun’s own radiation. Or, more specifically, the acceleration varied by the inverse-square of ‘Oumuamua’s distance from the Sun. Because there was been no direct detection of material ‘out-gassing’ (sublimating) from ‘Oumuamua in a cometary fashion – which could provide this kind of acceleration – astronomers remain slightly puzzled about what was going on.

But what about non ‘baseline’ interpretations? In late 2018 Shmuel Bialy and Abraham (Avi) Loeb at Harvard University published a paper taking a look at that hard-to-explain acceleration behavior by asking whether it could be due to solar radiation pressure alone – the pitter-patter of stellar photons exerting a force.

Without making any crazy assumptions they show, using a simple calculation of mass, area and radiation pressure, that the observed acceleration would require ‘Oumuamua to consist of a roughly millimeter thickness material with an overall size (cross-section) of the order of several tens of meters and with a mass around a thousand kilograms.

They then posed the question (and I should say the paper is a really good paper in terms of being methodical and logical) of whether this could conceivably be consistent with a non-natural, interstellar probe utilizing a light-sail.

Reading the paper I’m struck (and rather surprised) by how many characteristics of ‘Oumuamua can – in principle – be quite neatly explained by exploring this hypothesis.



The reddish color? Interstellar dust accumulation at a level that might be expected for a probe traversing large distances for a long time.

The time-varying brightness – attributed to a severe tumbling of the object in the baseline explanation? A tumbling light-sail that could be a sheet, a cylinder, a cone, either abandoned long before falling to the Sun, failed, or (my addition) unhooked from a small probe before entering our solar system.

They also posit that if this was a deliberately targeted probe, that would alter the baseline extrapolations (see above) that suggest a vast increase in the number of natural interstellar bodies like this. Instead those numbers could remain as previously predicted from our understanding of planetary formation and interstellar detritus.

It is, and I find myself a bit surprised to be saying this, an unexpectedly compelling argument – made perfectly soberly.

But of course, and to cut a long story short, this paper ended up generating headlines like ‘Alien Probe Claim by Harvard Scientists’ and other juicy and outrageous banners. Followed by several cycles of various scientists being equal parts irked, amused, and outraged.

I’ll admit that I’ve come to this a bit late. I was entertained by the furor, but didn’t pay too much attention because it seemed that Bialy and Loeb, while being deliberately provocative, were also trying to articulate a much more nuanced thought, namely: if we don’t even consider exploring seemingly outrageous hypotheses, there’s a chance that one day we’ll miss something really important.

Re-reading their original paper firms up my agreement with that sentiment. I think one can fully agree with the baseline interpretation – and the many excellent scientists backing that up with data and decades of insight to the workings of stars, planets and galactic dynamics. But at the same time one can agree that there are aspects of the ‘extreme’ interpretation (alien probe) that work rather well, and do meet the fundamental scientific criterion of being testable – on any future objects that are similarly unusual.

So, on reflection (and I’m not alone in this general camp, and others have been far more articulate and nuanced in their discussions), I think that overall Bialy and Loeb have done a real service to science by forcing us to admit that – even if we think such things are hugely unlikely – it is probably the case that the only way we will ever discover evidence for other technological life in the universe (if it exists) is to be willing to ask the question of our observations in the first place. If you don’t ask, you probably won’t find. Waiting for a neon sign that says ‘Alien’ to fall in our lap is unlikely to be how this happens, we have to push ourselves harder than that.

Of course, these authors also talked up their work to various news outlets, and as is the way of such things they ended up making statements about their confidences in hypotheses in that are good soundbites, but infuriate other scientists. I don’t really want to comment to the ‘correctness’ of doing that, and being faced with journalists’ questions can be daunting. But all things being considered I think that generating a conversation – even if with a smattering of outrage – is ok. In the end, nature will have the final say regardless of what we think.



