Contrary to popular belief, there may be more visitors than permanent residents in the solar system’s comet reservoir

We have more questions than answers about ‘Oumuamua, the first interstellar object detected near Earth. The visitor was initially detected as it left the solar system, and the meager data collected by astronomical observatories has proven difficult to explain. What we do know is that ‘Oumuamua was neither a comet nor an asteroid, and no outlandish theories about its origin have been able to properly explain its features to this point.

Two years later, though, the second interstellar visitor was discovered—and it was unlike ‘Oumuamua in every way. Borisov had an amazing similarity to comets from the far reaches of our own solar system, despite the fact that it had an obviously hyperbolic orbit. Borisov’s parallels to known objects in the solar system, as the first interstellar comet, give for an interesting possibility not available to ‘Oumuamua: a direct comparison between the solar system and its cosmic neighbors.

We report for the first time in Monthly Notices of the Royal Astronomical Society: Letters that the discovery of the first interstellar comet revealed an unintuitive reality: the Oort cloud, our solar system’s vast reservoir of comets that extends halfway to the nearest star, hosts more visitors than permanent residents. Comets from within the solar system outnumber those from beyond it near the Earth, which explains why there has only been one clear invader since Gottfried Kirch discovered the first comet with a telescope in 1680. However, because our observations are influenced by our proximity to the sun, they are unrepresentative of most sites in the solar system.

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The sun preferentially attracts comets from within the solar system as a result of gravitational concentrating, like a lamppost inviting swarms of moths. Interstellar objects, on the other hand, zoom around the galaxy at great speeds and are practically resistant to the sun’s gravitational pull, thus they don’t cluster near the sun like Oort cloud objects do. Our new research demonstrates that, despite their speed, there are many more interstellar interlopers in the dark depths of the solar system than there are comets of local origin at any given time.

This result has far-reaching implications for future observations and ideas. It inspires new searches for objects in the Oort cloud, such as stellar occultation surveys like TAOS II, which scan the sky for blips in starlight caused by nearby objects and distant stars aligning by chance. Simultaneously, the new discovery calls into question our theoretical knowledge of how planets develop, because it implies that planetary systems must expel orders of magnitude more mass than previously assumed. In fact, our new work suggests that stars may have to evacuate at least as much mass as they retain, posing a novel limitation on the formation of planetary systems.

Future discoveries of interstellar objects will add to our knowledge of the solar system’s place in the galaxy. The Legacy Survey of Space and Time (LSST), which will begin operations in late 2023 on the Vera C. Rubin Observatory, is expected to discover at least one interstellar object per month, a rate that will help us pinpoint interstellar object origins and learn more about how stars and planetary systems form. Direct observations of interstellar matter, on the other hand, are expected to yield the most intriguing scientific discoveries about interstellar objects. What are the materials that these unexpectedly many things are made of? If an object like Borisov reaches the Sun at the correct time, speed, and direction, the European Space Agency’s Comet Interceptor mission might sample the gaseous tail of an object like Borisov in the 2030s for a few hundred million dollars.

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Another approach to hunt for interstellar objects, and even collect humanity’s first samples of materials from outside the solar system, is to do it at a minimal cost and from the comfort of Earth’s surface. Any material that comes into contact with our planet’s atmosphere burns up due to friction with the air, resulting in a meteor, which appears briefly as a streak of light in the sky. As a result, finding small objects in the atmosphere is considerably easier than finding them in space, where we would have to rely on reflected sunlight. While the atmosphere has a far lower search volume than the farthest reaches of space, the quantity of smaller interstellar debris should make looking for interstellar meteors an appealing prospect.

In fact, in 2019, when reviewing a publicly available U.S. government meteor data set, I discovered one documented hit that seemed to approach far too swiftly to be confined to the solar system. I couldn’t believe it when I learned that astronomers have been looking for an interplanetary meteor since at least 1950. This finding was eventually tentatively confirmed as the first interstellar meteor larger than dust, and Pentagon officials have expressed interest in perhaps declassifying the error bars linked with the observation, considering its tremendous scientific importance.

I am directing an endeavor to identify gram-scale interstellar meteoroids in our atmosphere utilizing unclassified and transparent sensor networks as the director of interstellar object investigations for the Galileo Project. Such discoveries, when combined with the interstellar objects seen by LSST in Earth’s neighborhood, would change our view of the solar system in relation to its peers. A kilogram-scale or larger item burning up above land would be the holy grail of interstellar meteoroids, as such events could leave easily recoverable meteorites—rocks that could constitute the first particles of interstellar matter ever retrieved by humanity. With a thousand globally distributed passive all-sky camera systems patiently waiting for the proverbial needle-in-a-haystack meteoroid to grace our planet, such a goal could be accomplished in a decade for only a few tens of millions of dollars—a budget ten times smaller than the Comet Interceptor mission—for a budget ten times smaller than the Comet Interceptor mission.

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One of the most appealing elements of studying interstellar objects is that it brings together so many distinct branches of astrophysics, ranging from planetary science to high-energy phenomena, as well as a wide range of detection technologies. Searches for interstellar objects, along with other branches of “multimessenger” astronomy that seek to supplement traditional methods of astronomical inquiry, such as gravitational-wave and neutrino surveys, could help reveal unprecedented insights that challenge our understanding of our place in the universe.

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