By Jared Siegel, Fall 2019.
On October 17th of 2017, the Pan-STARRS1 telescope identified a small, peculiar object moving past the Earth. After analyzing the object’s extreme speed and its steep angle of inclination, researchers concluded that the object was not native to our solar system [1]. This interstellar visitor was officially named ‘Oumuamua–the Hawaiian word for “scout”–and became the first confirmed object to visit our solar system from another star.
The discovery of ‘Oumuamua set off a series of frenzied follow-up observations. The blistering speed of the object–peaking at 87.3 km/s during ‘Oumuamua’s closest approach to the Sun–meant that ‘Oumuamua was quickly passing through our solar system and that the window for study was limited [1]. In addition to continued tracking by the Very Large Telescope in Chile (one of science’s more creative names), the Hubble and Spitzer space telescopes also studied the object. With each observation, a more detailed and nuanced understanding of ‘Oumuamua emerged. In particular, by studying the periodic rise and fall of ‘Oumuamua’s brightness, researchers proposed that ‘Oumuamua must have an elongated shape and rotate on several axes: effectively tumbling through space [1]. Furthermore, these follow up-observations also revealed the mysterious behavior of ‘Oumuamua. As ‘Oumuamua continued to move away from the Sun and back into interstellar space, measurements from the Hubble space telescope revealed a slight acceleration pushing it away from the Sun. This acceleration component has previously only been associated with comets: as the Sun’s radiation vaporizes ice and dust on the comet’s surface, jets of gas and dust push the comet away from the Sun. Though this slight acceleration suggests that ‘Oumuamua has a comet-like composition, no outgassing has been observed. In fact, several simulations predict that the jets would lead ‘Oumuamua to rapidly rotate and tear itself apart [1]. This contradiction remains an enduring mystery surrounding ‘Oumuamua and has sparked continued research into the object’s true composition and dynamics.
The complex nature of ‘Oumuamua provides a glimpse into the discoveries that can be made by studying this new class of objects. Fortunately, several models have predicted that one interstellar object passes through our solar system each year, presenting a great opportunity for further study. In fact, a second interstellar visitor has already been identified: 2I/Borisov [2]. Unlike ‘Oumuamua, this object not only exhibits clear signs of being a comet –outgassing of ice and dust, as well as a tail– but was discovered on its way into our solar system. This early detection allows for a greater period for study and discovery. Current orbital projections predict that 2I/Borisov will remain within the orbit of Jupiter until mid-2020.
The observations of ‘Oumuamua and 2I/Borisov have opened entirely new paths of scientific research. These objects not only shed light on the dynamics of interstellar space, but also contain a great deal of information about their formation. In particular, the elongated shape of ‘Oumuamua has been studied in the context of planetary formation. By applying dynamical simulations, Raymond et al. 2018 found that the characteristics of ‘Oumuamua could be replicated through the close encounter of a planetesimal with a gas giant [3]. The resulting tidal forces would break up the planetesimal and deform the fragments. The researchers posit that if one of these fragments passed near their native star, the fragment could be ejected into interstellar space and lose its outer layers to the high radiation of the star. In this scenario, the resulting object would closely resemble ‘Oumuamua, with an elongated shape and no outer layer of ice and dust.
In addition to the theoretical implications of these interstellar visitors, there is also the exciting possibility of visiting one of these alien fragments. Over the past several years there have been multiple attempts to rendezvous with comets and asteroids, including the NASA mission OSIRIS-REx and the JAXA Hayabusa2 project. Both these missions have successfully entered orbit around their target asteroid and plan to return samples back to Earth. A mission of similar design to an interstellar object will be a remarkable scientific feat and has the potential to answer a myriad of questions about interstellar space and planetary formation [4]. Asteroids and comets, whether they are from our own solar system or an interstellar visitor, are particularly rich sources of scientific discovery. By remaining dormant for billions of years, they function as time capsules and retain vital information. Understanding the composition of these objects is critical to the study of the origins of solar systems and the nature of our stellar neighborhood before the Sun even formed. However, the extreme speeds of interstellar objects and the small window for rendezvous present several technological challenges. A rendezvous mission would not only require advances in rocketry, but also more dedicated surveys searching for interstellar objects in order to increase our understanding of their frequency and orbital dynamics.
Since the discovery of ‘Oumuamua, the study of interstellar objects has provided tantalizing clues into their scientific potential. As valuable tools for studying a range of subjects, from planetary formation to galactic evolution, interstellar objects present an exciting opportunity for discovery. As we continue to detect more of these objects and increase our ability to analyze them, our realm of study can begin to expand beyond the solar system and into interstellar space.
[1] `Oumuamua ISSI Team, Bannister, M. T., Bhand are, A., et al. 2019, NatAs, 3, 594.
[2] Andreoli, Claire. “Hubble Observes 1st Confirmed Interstellar Comet.” Goddard Space Center. NASA, October 16, 2019.
[3] Raymond, S. N., Armitage, P. J., & Veras, D. 2018, ApJL, 856, L7
[4] Hein, A. M., Perakis, N., Eubanks, T. M., et al. 2017, arXiv:1711.03155