It Came From Beyond

13 August 2023

Illustration of an interstellar object approaching our solar system. Rubin Observatory/NOIRLab/NSF/AURA/J. daSilva
Illustration of an interstellar object approaching our solar system.

Most of the comets we see in the sky were born in our solar system. They may have formed deep within the Oort cloud, and for some, it is their first visit to the inner solar system, but they are distinctly children of the Sun. We know of only two objects that came from beyond our solar system, Omuamua and Borisov. There are likely other interstellar objects visiting our solar system, we just haven’t found them. But that’s likely to change when Rubin Observatory comes online.

One of the things we know about stellar systems is that they can toss debris into interstellar space. Planetary orbits can shift dramatically during the early period of a system, and near misses between asteroids and even planets can give them enough kinetic energy to escape their star’s gravitational pull. We have discovered several rogue planets, and smaller bodies such as asteroids and comets can escape more easily. So there are plenty of interstellar bodies drifting through the galaxy.

But how many of these interstellar bodies visit our solar system? Oumuamua and Borisov were only discovered by chance because they passed within the inner solar system. Oumuamua had a maximum magnitude of 19, while Borisov was even brighter with a magnitude of 15. And it was only through a series of observations that we could determine their interstellar trajectories. Given random statistics, interstellar bodies are more likely to be found in the outer region of our solar system. That means they would be much dimmer and harder to find.

One of the projects for Rubin Observatory is the Legacy Survey of Space and Time (LSST), which will catalog objects in our solar system. It’s expected to gather data on more than 5 million asteroid belt objects, 300,000 Jupiter Trojans, 100,000 near-Earth objects, and more than 40,000 Kuiper belt objects. Since Rubin will be able to map the visible night sky every few days, many of these objects will be observed hundreds of times. We will have plenty of data to calculate their positions and orbits. Even in the outer regions of our solar system, interstellar objects will stand out.

The Rubin Observatory team estimates that within the first year of operation they will discover dozens of interstellar visitors. From their motion and spectra, we will be able to study the origins of these bodies, as well as their chemical compositions. It will give us a deeper understanding of how planetary systems form, including our own.

Years ago the first projects to search for exoplanets transformed our understanding of planetary systems. We’ve gone from knowing only a handful of planets to thousands. Rubin’s LSST project will transform our understanding of our solar system in a similar way. It will create the most detailed database of the Sun’s family, as well as the occasional visitors from beyond.