Science

The Solar System’s Oort Cloud May Harbor an Astonishing Number of Objects from Other Stars

`Oumuamua, the first interstellar object discovered near the Earth, left us with more questions than answers. The visitor was first observed during its exit from the solar system, and the limited data that astronomical observatories were able to collect have proved challenging to explain. What we know is that `Oumuamua was neither a comet nor an asteroid, and none of the exotic theories regarding its origin to date has been able to fully explain its properties.

Two years later, though, the second interstellar visitor was spotted—and it couldn’t have been further from `Oumuamua in nature. Borisov exhibited an uncanny resemblance to comets originating from the distant reaches of our own solar system, yet it travelled on a clearly hyperbolic orbit. As the first interstellar comet, Borisov’s similarities to known objects in the solar system allow for an exciting opportunity never afforded by `Oumuamua: a direct comparison between the solar system and its cosmic neighborhood.

In a new paper in Monthly Notices of the Royal Astronomical Society: Letters, we report for the first time the unintuitive reality that the discovery of the first interstellar comet revealed: the Oort cloud, our solar system’s vast reservoir of comets, which extends halfway to the nearest star, harbors more visitors than permanent residents. Near the Earth, comets originating from within the solar system outnumber those from outside of the solar system—hence the fact only one definitive interloper since the first comet was detected with a telescope by Gottfried Kirch in 1680. But this observation is unrepresentative of most locations in the solar system, since it is biased by our proximity to the Sun.

Due to gravitational focusing, the Sun preferentially attracts comets from within the solar system, like a lamppost beckoning swarms of moths. On the other hand, interstellar objects, which whiz about the galaxy at high speeds, are nearly impervious to the Sun’s gravitational pull, and so don’t cluster near the Sun in the same way that Oort cloud objects do. Our new work shows that there are so many of them that, despite their speed, there are far more interstellar interlopers in the dark reaches of the solar system at any given time than there are comet of local origin.

This conclusion has profound effects for future observations and theories alike. It motivates new searches for objects in the Oort cloud, including stellar occultation surveys like TAOS II that scan the sky for blips in starlight resulting from the chance alignments of nearby objects and distant stars. At the same time, the new finding directly challenges our theoretical understanding of how planets form, since it implies that planetary systems must throw out orders of magnitude more mass than previously thought. In fact, our new paper shows that stars may have to expel at least as much mass as they keep—a striking new constraint on planetary system formation.

Future discoveries of interstellar objects will continue to inform our understanding of the solar system in its galactic context. The Legacy Survey of Space and Time (LSST) on the Vera C. Rubin Observatory, scheduled to begin operations in late 2023, is expected to discover at least one interstellar object per month, a rate that will help us pinpoint the origins of interstellar objects, and learn more about how stars and planetary systems form. The most exciting scientific revelations regarding interstellar objects, however, will likely come from the direct study of interstellar matter. What are these surprisingly abundant objects made of? At the cost of a few hundred million dollars, the European Space Agency’s Comet Interceptor mission might be able to sample the gaseous tail of an object like Borisov in the 2030s, if one approaches the Sun at the right time, speed, and direction.

There’s also another way to search for interstellar objects, and even to obtain humanity’s first samples of matter originating from outside of the solar system, at relatively low cost and from the comfort of Earth’s surface. Any material coming into contact with our planet’s atmosphere burns up due to friction with the air, appearing briefly as a streak of light in the sky—a meteor. As a result, it is much easier to find small objects in the atmosphere than in space, where we would have to rely on reflected sunlight. And while the atmosphere provides a much smaller search volume than the reaches of space, the abundance of smaller interstellar objects should be great enough to make searching for interstellar meteors an attractive idea.

In fact, while analyzing a publicly accessible US government dataset of meteors in 2019, I found one recorded impact that seemed to have approached far too quickly to have been bound to the solar system. I could hardly believe this, as astronomers have been searching for an interstellar meteor since 1950 or earlier. This discovery would later be tentatively confirmed as the first interstellar meteor larger than dust, and since then Pentagon officials have expressed interest in potentially declassifying the error bars associated with the detection, given its immense scientific value.

As Director of Interstellar Object Studies of the Galileo Project, I am leading an effort to discover gram-scale interstellar meteoroids in our atmosphere, using unclassified and transparent sensor networks. In concert with the interstellar objects that LSST will detect in Earth’s neighborhood, such discoveries would revolutionize our understanding of the solar system in the context of its peers. The holy grail of interstellar meteoroids would be a kilogram-scale or larger object that burned up above land, since such events might leave easily recoverable meteorites—rocks that could represent the first pieces of interstellar matter ever obtained by humanity. 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—with a thousand globally distributed passive all-sky camera systems patiently waiting for the proverbial needle-in-a-haystack meteoroid to grace our planet.

One of the most beautiful aspects of the study of interstellar objects is that it connects so many disparate fields in astrophysics, spanning planetary science to high-energy phenomena and incorporating an equally diverse array of methods for detecting them. Along with other branches of `multi-messenger’ astronomy that seek to supplement traditional methods of astronomical inquiry, like gravitational-wave and neutrino surveys, searches for interstellar objects could help reveal unprecedented insights that challenge the way we understand our place in the universe.

This is an opinion and analysis article; the views expressed by the author or authors are not necessarily those of Scientific American.

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