Two More Explanations for Interstellar Asteroid ‘Oumuamua

More than a year has passed since the discovery of 1I/2017 ’Oumuamua, a bizarre body that burst onto the scene and then disappeared into the distance as quickly as it had arrived. During ‘Oumuamua’s visit, astronomers gathered some 800+ observations from telescopes around the world, which together reveal a strange light curve that raises more questions than answers.

A few things are agreed upon. ‘Oumuamua’s orbit indicates it originated outside of our solar system, making it the first visiting interstellar body we’ve witnessed. Its shape appears to be highly elongated, suggesting it’s more cigar-shaped than spherical. And its light curve reveals a periodicity of roughly 8 hours, potentially indicating the speed at which this odd body rotates.

But many unsolved questions remain. What are ‘Oumuamua’s structure, composition, and shape? Where did it come from? How was it launched onto its journey to our solar system?

Solar Push for a Fluffy Body?

One of ‘Oumuamua’s biggest mysteries relates to the discovery late last year that this asteroid wasn’t moving just under the influence of gravity; instead, ‘Oumuamua was experiencing a mysterious additional acceleration away from the Sun.

light sail

Artist’s impression of an artificial light sail, a thin spacecraft that can be propelled by radiation pressure. [Josh Spradling / The Planetary Society]

What could cause ‘Oumuamua’s added boost? Some scientists have suggested that radiation pressure — the push from solar photons hitting the object — could speed it up enough to explain observations. But for this to work, the asteroid would need an enormous surface-area-to-mass ratio.

One study suggested this could be achieved if ‘Oumuamua took the form of a giant light sail less than a millimeter thick (naturally reinvigorating the “is it aliens?” debate). But a recent study by Amaya Moro-Martín (Space Telescope Science Institute) suggests there might be another way: ‘Oumuamua could have a more ordinary shape, but an exceedingly low density.

cosmic dust

Image through a microscope of a porous interplanetary dust particle. Could ‘Oumuamua be an extremely low-density aggregate of icy dust? [Donald E. Brownlee (U. of Washington) and Elmar Jessberger (Institute for Planetology, Germany)]

The least dense manmade solid, aerographene, has a density on the order of 10-4 g/cm3, or ~10% the density of air. Moro-Martín suggests that an object with a tenth of this density, ~10-5 g/cm3, could get enough of a boost from the Sun to explain our observations of ‘Oumuamua.

Given this low density value, is this scenario actually likely? It turns out that fluffy, porous materials occur naturally in space, in the form of aggregates of icy dust particles. If the icy-aggregate explanation for ‘Oumuamua is correct, then the asteroid could have formed in the outer reaches of a nearby protoplanetary disk — and this could open a new window onto the study of the building blocks of planets around young stars. 

comet 67P/Churyumov-Gerasimenko

Image of comet 67P/Churyumov-Gerasimenko outgassing as it is heated by the Sun. Could similar processes be occurring on ‘Oumuamua? [ESA/Rosetta/MPS for OSIRIS Team]

Added Nudge from Migrating Jets?

There’s a more mundane explanation for ‘Oumuamua’s anomalous acceleration than radiation pressure, however: outgassing, which occurs as volatiles heat beneath a body’s surface and evaporate. This process is commonly seen in the jetted tails of comets, but there are some problems with using it to explain ‘Oumuamua’s motion.

First, no outgassing was observed from ‘Oumuamua; in fact, Spitzer observations placed strict limits on the amount of carbon-based material that could be evaporating from it. Second, calculations show that traditional comet-like outgassing would create torques that would spin ‘Oumuamua up rapidly, causing it to fly apart.

A new study may have found a way around these problems, however. A publication led by Darryl Seligman (Yale University) suggests that ‘Oumuamua may have been accelerated by outgassing not from a fixed point, but from migrating jets that follow the warmth, tracking the side of the asteroid closest to the Sun. Instead of spinning out of control, ‘Oumuamua’s motion might then resemble a pendulum, gently rocking back and forth to produce the ~8-hr period seen in the light curve.

'Oumuamua light curve

Light curves (left) and periodograms (right) for actual ‘Oumuamua observations (top row) and synthetic observations for three of the authors’ outgassing models, using three different aspect ratios for the body (next three rows). The bottom row reflects a flat photometry. Click to enlarge. [Seligman et al. 2019]

What about the Spitzer constraints on the visible evaporation? Seligman and collaborators suggest that the outgassing was primarily in the form of water vapor rather than carbon-based material. This could occur if the body had an unusually carbon-poor composition compared to a typical comet.

Ready for the Next Visitor

Could one of these explanations solve the mystery of ‘Oumuamua’s odd acceleration? Or could the true answer be a combination of proposed scenarios? With ‘Oumuamua long gone, we can’t be sure until we spot another interstellar visitor like it — but you can bet we’ll be prepared next time!


“Could 1I/’Oumuamua be an Icy Fractal Aggregate?,” Amaya Moro-Martín 2019 ApJL 872 L32. doi:10.3847/2041-8213/ab05df
“On the Anomalous Acceleration of 1I/2017 U1 ‘Oumuamua,” Darryl Seligman et al 2019 ApJL 876 L26. doi:10.3847/2041-8213/ab0bb5