Most of the volume of a sphere lies near its boundary. If the sphere is an onion made of shells with a fixed width, the volume of each shell scales as the square of its radius. The volume interior to each shell scales as the cube of its radius.
Now, consider the Solar System as an onion which extends out to 60,000 times the Earth-Sun separation, a fifth of the distance to the nearest star, Proxima Centauri. The region interior to 10,000 times the Earth-Sun separation can be regarded as the core of the Onion, carrying just half a percent of the total volume. What does the outermost 99.5% of the Solar System volume contain?
We have some clues in addressing this question. Some of the resident objects in that distant volume travel towards the center of the Solar System and arrive close enough to our lamppost, the Sun, for them to be visible from Earth. These are long period comets. These ancient building blocks of the Solar System are made of rock and water ice which evaporates near the Sun and leaves detectable cometary tails of gas and dust that scatter sunlight.
Astronomical surveys provide a complete census of all new comets bigger than a few kilometers. Here, the adjective `new’ means that the tide of our Milky-Way galaxy nudged these comets to their first passage through the region occupied by planets within 40 times the Earth-Sun separation. After entering this region which resembles a pinball machine, these new comets were kicked gravitationally by one of the planets towards the Sun. We can trace back the planetary kick and correct for it so as to infer the original distance from where each new long-period comet originated.
New comets with a diameter larger than 2.3 kilometers are all visible out to 2.5 times the Earth-Sun separation, where their appearance rate is once per year. Most of the long-period comets originate from an aphelion distance (farthest point from the Sun) of 60,000 Earth-Sun separations, and have an orbital time of 5 million years. The probability distribution of their orbital eccentricity scales with eccentricity squared. Combining all these factors suggests that the full reservoir of comets at the outskirts of the Solar System contains about 30 billion comets of this diameter. Each of these comets carries a few billion tons of ice and rock. The entire reservoir — called the Oort Cloud after the Dutch astronomer Jan Oort who inferred its existence in 1950, carries a few percent of Earth’s mass in comets with a diameter of about 2.3 kilometers. Adding up comets of all sizes could bring the total to roughly an Earth mass, with an order of magnitude uncertainty.
The same volume also contains interstellar comets which are not bound gravitationally to the Solar System. So far, only one interstellar comet was discovered, by the amateur astronomer Gennadiy Borisov in 2019. The core of comet Borisov was estimated to have a diameter of order 0.7 kilometers, carrying a mass of about a hundred million tons. Estimating crudely the abundance of such comets yields one such comet within a distance that is 3 times farther than the Earth is from the Sun. Within the volume occupied by long-period comets of 60,000 Earth-Sun separations, there should therefore be of order 10 trillion interstellar comets of this size, carrying altogether about 15% of Earth’s mass. Adding interstellar comets of all masses could bring their total reservoir within the Oort cloud to about 10 Earth masses with a large uncertainty.
There are other forms of matter within the volume of the Oort cloud. The local density of interstellar gas and the local density of dark matter, account for several hundred Earth masses each, altogether two orders of magnitude more than the cumulative mass of comets from the Solar System and beyond.
Finally, we arrive at the most fascinating and speculative component: interstellar objects manufactured by advanced technological civilizations. How many interstellar probes, active or defunct, might be floating within a distance of 60,000 times the Earth-Sun separation?
We do not know. Just as with long-period comets, these objects could be detected by our telescopes when they arrive very close to the lamppost of the Sun. Future astronomical surveys could monitor the rate of appearance of new interstellar probes, in the spirit of detecting new comets. Here, artificial propulsion could provide an additional source of non-gravitational acceleration beyond the rocket-effect from cometary evaporation.
Functional devices that charge their batteries by sunlight or use liquid water as fuel, might concentrate in the habitable zone near the Sun, just like bumble bees collecting pollen from Sunflowers. In that case, the number of extraterrestrial gadgets near us cannot be easily used to infer their overall population.
For now, we need to find the first definitive case for such probes. We might do so with the Rubin Observatory or the Galileo Project’s sky surveys or expeditions in the coming years. Here’s hoping that we will learn much more about all residents of the Solar System in the coming years.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
