Interstellar visitors
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ʻOumuamua (1I/2017 U1)

In October 2017, astronomers using the Pan-STARRS1 telescope in Hawaii detected a small, fast-moving object entering the solar system. Initially classified as a comet and later designated 1I/ʻOumuamua (meaning “scout” or “messenger” in Hawaiian), it became the first confirmed interstellar object ever observed passing through our planetary neighborhood.

ʻOumuamua displayed a number of unusual properties. Its trajectory clearly indicated that it was not gravitationally bound to the Sun, proving its extrasolar origin. Unlike most comets, it showed no visible coma or tail, even when it passed within 0.25 AU of the Sun. However, subtle non-gravitational accelerations in its path suggested some form of outgassing, though none was directly detected.

Another striking feature was its shape. Based on its brightness variations, astronomers inferred that ʻOumuamua was highly elongated—perhaps up to 10 times longer than it was wide—or possibly disk-like. Its reddish surface color hinted at space weathering, similar to outer solar system objects. Because of its uniqueness and ambiguity, scientists proposed a wide range of theories for its nature: an exotic hydrogen iceberg, a fragment of a tidally disrupted planetesimal, or even a thin sheet-like object expelled by another star system. While the more speculative “alien probe” hypothesis attracted public attention, most experts favor natural explanations.

ʻOumuamua’s visit was fleeting. It sped away at about 87 km/s relative to the Sun, leaving no chance for a spacecraft to intercept it. Despite its brief appearance, it fundamentally changed planetary science by confirming that interstellar objects can and do enter our solar system.

Borisov (2I/Borisov)

Just two years later, in August 2019, Ukrainian amateur astronomer Gennadiy Borisov discovered a comet with a distinctly hyperbolic orbit, later confirmed as 2I/Borisov—the second recognized interstellar visitor and the first clearly cometary one. Unlike ʻOumuamua, Borisov displayed familiar comet-like behavior, including a bright coma and tail, which made it easier to study and compare to native solar system comets.

Borisov’s nucleus was estimated to be around 0.4–1 km in diameter, with a composition rich in carbon monoxide and water ice. Its chemical makeup suggested that it formed in a cold region of its home star system, likely beyond the frost line where volatile ices could condense. The similarities to comets in our own solar system indicated that planetary formation processes may be broadly similar throughout the galaxy.

Borisov passed closest to the Sun in December 2019, reaching just under 2 AU, and then continued outward, never to return. Observatories worldwide—including Hubble and ALMA—monitored it extensively. Unlike ʻOumuamua, which sparked confusion, Borisov behaved in line with expectations for a comet, making it a valuable reference point for understanding extrasolar small bodies.

Significance

The discoveries of ʻOumuamua and Borisov opened a new field of astronomy: the study of interstellar objects (ISOs). ʻOumuamua highlighted the diversity and unexpected nature of such visitors, while Borisov demonstrated their potential similarity to familiar bodies. Together, they provide a glimpse into planetary systems beyond our own and suggest that countless fragments of ice, rock, and dust are constantly traveling between the stars. Future surveys, particularly with the Vera C. Rubin Observatory, are expected to detect many more such wanderers, deepening our understanding of the galactic environment.