We live in a galaxy teeming with wandering tiny worlds, and some astronomers predict they’ll be detecting them at least once per year.

It sounds almost like science fiction: a tiny world that formed around another star, visiting our cosmic neighborhood for us to study. And yet thats exactly what has happened, twice now as of the last few months. It will only happen more often this decade.
Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research develop­ments and trends in mathe­matics and the physical and life sciences.
The first known interstellar objectmeaning it formed outside of our solar systemdropped by in late 2017. Named 1I/Oumuamua, scientists werent sure what to make of it at first. They didnt 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 Oumuamua. 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 exteriorthe 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 weve 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 systems planet-forming disk, they grow stately tails of gas and dust under our stars gentle heat. Imprinted in the light of these wisps of subliming vapor are the fingerprints of a comets 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: ammonias daughter, NH2, and waters daughter, hydroxyl (OH). 2I/Borisov shares this chemistry, we learned as it approached the sun last month, and its also kin to our comets in terms of sizea little under a kilometerand its rates of slow dissolution.
The Hubble Space Telescope captured interstellar object 2I/Borisov as it raced through the solar system, only the second known cosmic visitor to do so.Video: NASA, ESA and D. Jewitt (UCLA)
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 Oumuamua 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, weve 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 Observatorys 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.