No, astronomers did not just claim that ‘Oumuamua is an extraterrestrial probe

Title: Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?

Authors: Shmuel Bialy & Abraham Loeb

First Author’s Institution: Harvard Smithsonian Center for Astrophysics, Cambridge, MA

Status: Submitted to The Astrophysical Journal Letters, closed access

If you have been paying attention to space news recently you may have seen stories suggesting that the interstellar object ‘Oumuamua, which passed through our solar system just over a year ago, could have been an extraterrestrial probe. Some headlines are a bit sensational (e.g. Scientists say mysterious ‘Oumuamua’ object could be an alien spacecraftMysterious interstellar object floating in space might be alien, say Harvard researchers or An Alien Spacecraft May Have Passed Through Our Solar System Last Year, Claim Scientists) while other articles are a little more doubtful (e.g. Interstellar object may have been alien probe, Harvard paper argues, but experts are skeptical). Today’s paper is the source of these articles and, while the premise is a little less far-fetched than you might think, the conclusion lacks transparency.

Figure 1: Artist’s impression of ‘Oumuamua based on its inferred composition and shape. Credit: ESO/M. Kornmesser

So what is so odd about ‘Oumuamua (other than its odd shape, see Figure 1, and the fact that it came from outside our solar system!)? A paper published earlier this year noted a “non-gravitational acceleration” in the trajectory of ‘Oumuamua as it passed through our solar system. This acceleration, detected at 30σ significance, corresponded to a radial (with respect to the sun) acceleration proportional to r-2. This behavior is actually very common for solar system comets mainly due to outgassing as the comet warms, though other weaker forces such as the Yarkovsky effect can play a role. Outgassing is what causes the beautiful coma and tail usually associated with comets. This feature is notably missing from ‘Oumuamua, leading the authors to suggest that, since outgassing is the most likely source of the acceleration, the composition of ‘Oumuamua must be slightly different than solar system comets. If the dust grain sizes were much larger than normal they would not have been detected.

Since radiation pressure follows a similar r-2 dependance to the acceleration seen in ‘Oumuamua’s path, it could be a simple explanation. The authors of the earlier paper rule out radiation pressure as the source of the extra acceleration, arguing that it would imply an unreasonably low density for the object. Undeterred, the authors of today’s paper investigate what would be required if it were the source. They use some relatively straight forward physics to determine that ‘Oumuamua would have a mass to area ratio of ~0.1 g cm-2. Assuming a typical density of 1–3 g cm-3 the authors determine that the object would have a thickness of ≲1mm. This is extremely thin for an object that has a radius somewhere between 10 and 1000m (given the observed brightness and a range of albedos)!

Figure 2: Maximum possible distances traveled, as limited by collisions causing slow down and vaporization, as a function of mass to area ratio. The dashed magenta line shows the m/A constraint based on the observed acceleration. Figure 1 in the paper.

The authors then go on to study how traveling through interstellar space would affect such an object. Given the typical density of the interstellar medium (ISM), it is possible for such an object to travel Galactic distances without slowing down due to momentum transfer or lose a significant fraction of its mass due to vaporization (see Figure 2). (While vaporization is not a significant effect for the relatively slow speed of ‘Oumuamua, ~26 km/s, this is a real concern for the breakthrough starshot project which will be traveling at a significant fraction of the speed of light.) The authors also find that even relatively weak cometary material would be able to hold the thin object together under its own rotation and the tidal forces experienced during perihelion passage.

But how did this thing form? Well, we don’t know of any natural processes which could form such a large and yet extremely thin object. This leads the authors to propose that it could be a lightsail, either floating through space as debris or sent toward our solar system by an advanced civilization. While of course this is possible, it is an extraordinary claim which, as Carl Sagan said, requires extraordinary evidence. Weighing the “extraordinariness” of a comet of slightly different composition than solar system comets (with outgassing causing the acceleration) versus a large and extremely thin object which we have no known formation mechanism other than intelligent construction (with radiation pressure causing the acceleration) I think the choice is obvious. The authors do appropriately stress the need for future observations to find similar objects, since we can no longer observe ‘Oumuamua.

In conclusion, while speculative studies like this are interesting, it is extremely important to emphasize caveats associated with extraordinary claims such as those made in this paper. For example, this paper only mentions outgassing as an impossible source of the acceleration and does not mention the previously proposed solution. It is important to recognize the evidence for and against a particular explanation, and emphasize the difference between a possible explanation (which has no evidence refuting it) and a probable explanation (which has evidence supporting it). Journalists should also be careful not to extrapolate conclusions and distort claims made by the authors, especially when a covering topic of public interest. For example, the words “alien civilization” are mentioned once in the entire paper, yet many news stories focus on this explanation above all others.

About Samuel Factor

Sam Factor is a 5rd year Ph.D. candidate at The University of Texas at Austin studying direct imaging of extrasolar planets and low mass binary stars. He uses an interferometric post processing technique to allow the detection of companions below the diffraction limit of the telescope.


  1. Thanks for this nicely written recap.
    I was interested to hear your further thoughts on explaining ‘Oumuamua’s acceleration by its cometary activity, because in addition to the cometary activity that is not observed, there is another problem with that explanation.
    In his paper, Rafikov (2018, ) shows that the observed acceleration cannot result from cometary activity because if cometary activity is responsible for the observed acceleration, there must be also torques associated which will spin-up the comet to high rotational frequency, above what is observed.
    Following Rafikov, the spin up will become noticeable, already after few days of observations. See their Eq. 11.
    He also suggests that the final spin would have been so high that it would have resulted in the comet fission by centrifugal forces.
    Anyway, I will be very interested to hear your thoughts on Rafikov’s work, if it sounds reasonable to you, as well as how likely/unlikely it is that ‘Oumuamua is a comet while we do not see any signs of cometary activity (taking into account the sensitivity of the instruments).
    This led us to seek an alternative explanation, however unlikely and extraordinary, for the acceleration.
    What do you think?

  2. I would like to point out that in the paper “Considering an artificial origin, one possibility is a lightsail floating in interstellar space as debris from an advanced technological equipment (Loeb 2018). Lightsails with similar dimensions have been designed and constructed by our own civilization, including the IKAROS project and the Starshot
    Initiative.” So, yes, they are claiming some alien intelligence created it.

    • The key word here is possibility. To me, their claim (and the title of the paper) is that radiation pressure could have caused the observed acceleration. This then lead them to determine that ‘Oumuamua must be very large and thin. Since we know of no natural process which could have formed such an object, one possibility is artificial origin. They don’t say that it likely is, only that it might be. The conclusions of my post was meant to point out that the authors could have done a better job at emphasizing this and all the caveats leading up to this (extreme) possibility. I think that you will agree that when dealing with subjects like this scientists must be very careful yet strong with our wording so as not to be misinterpreted.

  3. Thanks for an excellent summary; I read this paper out of interest and noted that they excluded outgassing as a cause of the non-Keplerian orbit, but didn’t have time to follow up so didn’t know about the proposed explanation of a comet with different composition. Very interesting.


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