It sounds almost like science fiction: a tiny world that formed around another star, visiting our cosmic neighborhood for us to study. And yet that’s exactly what has happened, twice now as of the last few months. It will only happen more often this decade.
The first known interstellar object—meaning it formed outside of our solar system—dropped by in late 2017. Named 1I/ʻOumuamua, scientists weren’t sure what to make of it at first. They didn’t have much time with the object, only two weeks of detailed observations as it raced away from Earth, before it became too faint. These scraps of data form our understanding of ʻOumuamua. It was a skyscraper-size, tumbling little world, fairly elongated and smaller than most asteroids or comets that we regularly observe. It also had a reddish exterior—the sort found on surfaces far out in the solar system, where an unyielding rain of starlight interacts with carbon-rich molecules to give them a ruddy hue.
In contrast, the second interstellar traveler we’ve observed, 2I/Borisov, is an obvious comet, a lot like the ones that formed in the icy extremes of our own system. When comets come close to the sun, they blossom: Heavy with ices, kept chilled from the earliest times of our solar system’s planet-forming disk, they grow stately tails of gas and dust under our star’s gentle heat. Imprinted in the light of these wisps of subliming vapor are the fingerprints of a comet’s chemical composition. Some of the frozen molecules and ions in comets are familiar ones, like ammonia and water, and we infer their presence by detecting their offspring, split off by sunlight: ammonia’s daughter, NH2, and water’s daughter, hydroxyl (OH). 2I/Borisov shares this chemistry, we learned as it approached the sun last month, and it’s also kin to our comets in terms of size—a little under a kilometer—and its rates of slow dissolution.
These interstellar ambassadors are exciting to researchers because the frozen samples in a comet tell the story of the varied chemistry of its home. Now, with 2I/Borisov, we finally have a chance to learn directly about the neighborhood surrounding another star. Already we can surmise that the processes that form a comet within the solar system take place outside of it as well. And while ʻOumuamua was seen too late to be studied in great detail, Gennadiy Vladimirovich Borisov discovered his namesake comet much earlier in its one-way trip through the solar system, allowing for months of study.
Even so, both objects have been relatively faint, which has made their observation and characterization challenging. In fact, we’ve likely had many such visitors in the past but can only now spot them thanks to technological advances. The telescopes that track the skies to hunt near-Earth asteroids and other varied populations within the solar system are also skilled at finding interstellar objects. Wide-angle cameras continually photograph broad swaths of the skies, and software evaluates the faint dots of light for any that wander, highlighting newcomers. Our sky surveys are always improving, and even deeper searches are coming. The Rubin Observatory’s upcoming Legacy Survey of Space and Time (LSST) will make a movie of the whole southern sky every three nights, over a decade. Expectations run high, and astronomers think this survey will sift out visiting interstellar worlds at least once per year.