On August 30th, an amateur Ukrainian astronomer, Gennady Borisov, spotted something through his telescope that no one else had noticed. Borisov had his telescope aimed towards the Earth’s horizon, a direction most optical equipment cannot be pointed because of their sensitivity to the brightness of the sky often found there. However, his decision proved prosperous.

The peculiar object, a bright and fuzzy blob moving across the sky, turned out to be a comet hurtling through our solar system at speeds close to 93,000mph. The comet has been cataloged as C/2019 Q4, also known as the Borisov comet after the man who discovered it. What makes this comet particularly interesting, and what has garnered attention worldwide is not where it is going, but where it came from.

This image of C/2019 Q4 is the first obtained by Gemini Observatory at Maunakea in Hawaii. The characteristic glowing coma can be seen as well as the tail of the comet. Because of the comet’s high speed, background stars appear as blue and red streaks. Credit: Gemini Observatory/Association of Universities for Research in Astronomy

The Borisov comet’s specific trajectory, the curve of its path as it makes its way through space, as well as its incredibly high speed suggest that it is coming from beyond our solar neighborhood: the comet is from a different solar system altogether, making it the first interstellar comet ever identified. The last time we had an interstellar visitor was in 2017 when an asteroid, later named ‘Oumuamua, was spotted leaving our solar system. Unfortunately, ‘Oumuamua was difficult to study since it had already made its closest approach to us and the Sun. C/2019 Q4, however, is still inbound and is bigger and brighter than ‘Oumuamua. The comet will make its closest approach to the Sun in early December and to the Earth later that month, before making its way out again. This gives astronomers plenty of time to study the comet, up to a year from now, particularly as it makes its way to parts of the sky that are easier to observe.

Comets have its distinctive physical properties. They are essentially icy masses containing a nucleus of frozen matter, often covered in organic material. When they come into proximity of a heat source – in this case the Sun – their surfaces form a coma: a glowing atmosphere of gas and dust. As comets travel through space, often at very high speeds, they also form tails as the gas and dust is pushed behind them and away from the Sun. The Borisov comet is estimated to be between 1.2 and 10 miles in diameter, but other characteristics (like size and rotation) are yet to be determined.

Why are scientists so interested in studying interstellar material like ‘Oumuamua and the Borisov comet? We know that “domestic” asteroids and comets originate from the asteroid belt, the Kuiper belt and the Oort Cloud; and are leftovers from the very earliest stages of our solar system’s formation. Collectively, they help us determine what happened here almost 4.6 billion years ago. Interstellar objects therefore provide remote insight into other solar systems within our galaxy. By studying these visiting objects, scientists can determine the conditions, evolution and compositions of other stars and the objects (including exoplanets) that surround them without having to physically travel to them. Chemical analyses can determine how similar or different these other stellar worlds are from ours. Furthermore, since other stars are so distant, the current conditions of these comets and asteroids can tell us about what lies in between us and our neighbors. Objects like ‘Oumuamua and the Borisov comet help astronomers address questions about remote parts of our universe that would otherwise remain unanswered.

— Zeenath Khan