One of the possible scenarios for ‘Oumuamua’s acceleration is radiation pressure. Sunlight is made of photons, which can exert force. But in ‘Oumuamua’s case, photons could only exert enough oomph if the object had an extremely low density compared to its size — about 1/100,000 the density of water. Amaya Moro-Martín of the Space Telescope Science Institute proposed, in a February 2019 paper published in The Astrophysical Journal Letters, that ‘Oumuamua could be a “fluffy aggregate” of icy dust particles. This type of structure, which could have such a low density, would indicate ‘Oumuamua formed beyond its host star’s snow line — the region past which water is found only in ice form — and, the study says, provide a unique window into how the building blocks of planets form. 

Still, Moro-Martín’s paper concludes, “There are many open questions that need to be addressed in order to assess the viability of this scenario,” such as how such a porous object might survive not only ejection from its solar system, but also its journey through interstellar space. And Meech’s team deemed that unlikely: “We have no examples of anything like that in the solar system. While theoretically plausible on a mathematical scale, it’s not physically sensible,” Meech says.