Could Solar Radiation Pressure Explain 'Oumuamua's Peculiar Acceleration?

Abstract: Oumuamua (1I/2017 U1) is the first object of interstellar origin observed in the Solar System. Recently, \citet{Micheli2018} reported thatOumuamua showed deviations from a Keplerian orbit at a high statistical significance. The observed trajectory is best explained by an excess radial acceleration $\Delta a \propto r^{-2}$, where $r$ is the distance of Oumuamua from the Sun. Such an acceleration is naturally expected for comets, driven by the evaporating material. However, recent observational and theoretical studies imply thatOumuamua is not an active comet. We explore the possibility that the excess acceleration results from Solar radiation pressure. The required mass-to-area ratio is $(m/A)\approx 0.1$ g cm$^{-2}$. For a thin sheet this requires a thickness of $\approx 0.3-0.9$ mm. We find that although extremely thin, such an object would survive an interstellar travel over Galactic distances of $\sim 5$ kpc, withstanding collisions with gas and dust-grains as well as stresses from rotation and tidal forces. We discuss the possible origins of such an object. Our general results apply to any light probes designed for interstellar travel.

• The paper explores the hypothesis that solar radiation pressure explains `Oumuamua's anomalous acceleration, calculating that it would require a very thin structure and analyzing its resilience to interstellar conditions.
• The study calculates that `Oumuamua would need to be between 0.3 mm and 0.9 mm thick with a mass-to-area ratio around 0.1 g cm⁻² for solar radiation pressure to account for its acceleration.
• The implications suggest such objects could challenge conventional understanding of interstellar matter, potentially representing natural byproducts or artificial constructs, emphasizing the need for future comprehensive sky surveys.

An Examination of the Hypothesis: Solar Radiation Pressure and the Peculiar Acceleration of `Oumuamua

The research paper explores the intriguing question of whether the anomalous acceleration observed in `Oumuamua, the first interstellar object detected in our Solar System, can be explained by solar radiation pressure. Authored by Shmuel Bialy and Abraham Loeb at the Harvard Smithsonian Center for Astrophysics, the study presents a mathematical and theoretical exploration of this hypothesis.

Key Observations and Theoretical Framework

Oumuamua exhibited non-Keplerian motion as it passed through the Solar System, characterized by an excess radial acceleration that was unexpected for an inactive object. Even though such accelerations are common in comets due to outgassing, various studies have shown thatOumuamua lacks typical cometary activity like a visible tail or gas emissions, necessitating alternative explanations.

The hypothesis explored in this paper suggests that solar radiation pressure could account for this peculiar acceleration. The authors calculate that Oumuamua would need a mass-to-area ratio of approximately 0.1 g cm$^{-2}$ for solar radiation pressure to be viable. They also suggest that an object would need to be exceptionally thin, with a thickness between 0.3 mm to 0.9 mm. This thesis is grounded in solid astrophysical theory, focusing onOumuamua's reflectivity and the efficiency of solar radiation pressure in imparting acceleration.

Structural Integrity and Interstellar Travel Viability

The resilience of such a thin object in the harsh interstellar environment is critically analyzed. Bialy and Loeb inspect whether an object with these physical constraints could survive collisions with gas and dust grains, as well as endure stresses induced by rotational and tidal forces. They conclude that despite its fragility, `Oumuamua's hypothesized structure could endure a journey of several kiloparsecs through the interstellar medium without suffering from significant mass loss or structural failure.

Implications and Broader Context

The implications of this study are multifaceted. On a theoretical level, it challenges conventional understandings of interstellar objects and the potential processes that could create such a phenomenally thin object naturally. The paper speculates on both natural and artificial origins, proposing that such objects could be a byproduct of unrecognized astrophysical processes or, more provocatively, artificial constructs, potentially analogous to solar sails developed by human technology.

This research profoundly influences the approach toward identifying and studying future interstellar visitors. The authors advocate for comprehensive surveys with upcoming observational facilities like the Large Synoptic Survey Telescope to discover more objects with similar characteristics. This could offer insights into the frequency of such objects and refine the estimated number of similar-size interstellar bodies that are undetectable with current technology.

Conclusion

This paper provides a rigorous scientific exploration of one of the most curious astronomical phenomena observed in recent years. It lays a significant foundation for future studies and poses intriguing questions about the nature and origin of Oumuamua. While neither definitively proving nor dismissing any particular theory aboutOumuamua's anomalous properties, it encourages further exploration and discussion within the astrophysical community. The hypotheses regarding radiation pressure and potential artificial origins, if not directly addressed, still spark interest and suggest promising directions for future research and observation.