Revisiting Interstellar Asteroid `Oumuamua

Oumuamua is back in the headlines again.When the minor object 1I/2017 Oumuamua was discovered inOctober of 2017, it was already speeding away from the Sun. Oumuamuas path, however, quickly identified it not as a solar-system body, but as an asteroid that originated from somewhere beyond our tiny corner of the universe.Since Oumuamuas discovery, scientists have scrambled to interpret the bizarre observationsof this body. For starters, Oumuamuahas an unusual shape: though we have no high-resolution images, light curves suggest that this tumbling asteroid is at least five times as long as it is wide. More recently, however, its Oumuamuas acceleration that has captured the attention of scientists.Though the asteroids trajectory through our solar system was originally assumed to be solely governed by ordinary gravitational forces, recent research suggests that theres something else at work. Oumuamuas motion cant be explained by gravity alone; instead, the asteroid seems to be experiencing an additional acceleration away from the Sun thats dependent on the Sunasteroid distance.What does some of the latest research say about Oumuamua in light of this new development?Oumuamuas OriginEncounter distance vs. velocity between Oumuamua and the four main home candidate stars (plotted in blue, green, red, and yellow), for different models of Oumuamuas trajectory. [Bailer-Jones et al. 2018]Previous studies have extrapolated Oumuamuas path backwards, searching for a nearby star system from which this asteroid may have originated, with no luck: Oumuamuas trajectory did not seem to line up with any nearby stars.But the extra acceleration measured for Oumuamua changes this picture. Past studies assumed that Oumuamua was only influenced by gravity as it traveled, but a new study led by Coryn Bailer-Jones (Max Planck Institute for Astronomy, Germany) has traced its path backwards while also taking into account the asteroids observed extra boost.The result? Using Gaia DR2 data, Bailer-Jones and collaborators found at least four plausible host stars for Oumuamua within a few light-years, and future observations are likely to reveal additional candidates.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]Could Oumuamua Be a Comet?Though puzzling for an asteroid, the additional acceleration measured for Oumuamua is naturally expected for icy comets. Comets passing near the Sun heat up, causing outgassing; this evaporating material then adds a boost to the bodys motion away from the Sun.Observations of Oumuamua contradicted the comet theory early on: theres no observational evidence of outgassing, no coma or dust tails, and no cometary emission lines. But perhaps we just missed the signs, and Oumuamua is somehow a comet in disguise?Thats unlikely, according to author Roman Rafikov (University of Cambridge, UK and Princetons Institute for Advanced Study). In a recent study, Rafikov examined what would happen if Oumuamua were outgassing like a comet. He found that, for reasonable assumptions about Oumuamuas properties, torques caused by outgassing would cause the bodys spin to evolve rapidly. This process would have spun Oumuamua up to the point where it couldnt hold itself together anymore, causing it to fly apart before it passed through our solar system.How About a Giant Sail?So if Oumuamuas acceleration cant be explained by gravity, and it doesnt seem likely that its outgassing, then what is causing the extra boost? Authors Shmuel Bialy and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics) propose an alternative: solar radiation pressure. In a new study, Bialy and Loeb suggest that Oumuamua could have an even more unusual shape than we initially thought: it may be tens of meters in surface area, but less than a millimeter thick.Bialy and Loeb demonstrate that if this is true, the pressure of photons from the Sun pushing on this large surface could be enough to explain Oumuamuas extra acceleration, and they demonstrate that such a body could have survived interstellar transit without being eroded or broken apart.Artists impression of an artificial light sail, a thin spacecraft that can be propelled by radiation pressure. [Josh Spradling / The Planetary Society]If Bialy and Loebs model is correct, then what is Oumuamua? Such a large, thin object would represent a new class of interstellar material that we havent previously observed in our solar system, but perhaps something of this shape could arise naturally in the interstellar medium or in protoplanetary disks. Bialy and Loeb also consider the possibility of an artificial origin, proposing that Oumuamua could be a piece of tech debris a solar sail discarded by an alien civilization and now tumbling through interstellar space.The Puzzle ContinuesSo: is it aliens? Probably not, folks. But does Oumuamua present a large number of puzzles that force us to continue to carefully employ the scientific method, evaluate possible uncertainties, and search for new explanations? Definitely.We still have so much to learn about this bizarre asteroid. But in many ways, the puzzle of Oumuamua beautifully encapsulates the process of science both in the excitement of an unsolved mystery and in the importance of critical thinking and methodical evaluation of possibilities.Keep following along as our understanding of Oumuamuacontinues to evolve!CitationSpin Evolution and Cometary Interpretation of the Interstellar Minor Object 1I/2017 Oumuamua, Roman R. Rafikov 2018 ApJL 867 L17. doi:10.3847/2041-8213/aae977Plausible Home Stars of the Interstellar Object Oumuamua Found in Gaia DR2, Coryn A. L. Bailer-Jones et al 2018 AJ 156 205. doi:10.3847/1538-3881/aae3ebCould Solar Radiation Pressure Explain Oumuamuas Peculiar Acceleration?, Bialy and Loeb 2018 ApJL, in press. https://arxiv.org/abs/1810.11490