A recent paper studied the flux of interstellar objects entering the Solar system from the nearest star system, Alpha Centauri, located about 4 light years away. The authors calculated a probability of one in a billion for an `Oumuamua-size object from Alpha Centauri to be within the orbit of the Earth around the Sun.
However, the important point to keep in mind is delivered by Olbers’ paradox, namely the contradiction between the assumption of an infinite Universe filled with a uniform distribution of luminous stars and the darkness of the night sky. The paradox was formulated in terms of photons but it applies to any type of particles or objects ejected from star systems like Alpha Centauri.
The flux of interstellar objects arriving near Earth scales inversely with the square of the distance to the source star. But the number of stars scales with distance cubed for a nearly uniform local density of stars. This means that most of the interstellar objects arrive from the most distant source stars. For the Milky-Way galaxy, the dominant sources would be a few thousand light years away from the Sun, delineating the thickness of the Milky-Way disk of stars. There are of order a billion stars within that volume and their cumulative flux of interstellar objects is of order a billion times larger than that of Alpha-Centauri, even though the Alpha-Centauri system is a thousand times closer.
At distances larger than the thickness of the Milky-Way disk, the number of contributing stars grows like the square of the distance, representing the two-dimensional surface area of the disk, and so the flux of objects increases only logarithmically with distance. In summary, this new paper considers a part in a billion of the expected total flux of interstellar objects from all Milky-Way stars. Indeed, two interstellar objects on the scale of 100 meters, `Oumuamua and Borisov, were spotted near Earth in recent years.
Could some of these interstellar messengers carry an important message to humanity or should we ignore them since we are already familiar with rocks born within the Solar system?
When we go on vacation, we tend to silence our news feed from the rest of the world. Some people go through life on a vacation mode, in an attempt to minimize worries on issues beyond their control. With the advent of social media and artificial intelligence agents, it is now easy to live in a bubble of like-minded views and ignore course-correction from reality checks.
But the truth is that we cannot ignore reality because it affects our life even when we turn our back towards it. Jews cannot ignore the recent rise of neo-Nazis because of the far-reaching risk they pose. Israelis vacationing in Australia cannot ignore antisemitism, because an Australian nurse was just charged for claiming in a viral video that she would kill Israeli patients arriving at her hospital.
The asteroid 2024 YR4 reminded us last week that we cannot ignore the sky. It is often said that we should focus on down-to-Earth matters before turning our attention to the sky. This was indeed the mindset of non-avian dinosaurs. However, their privileged status at the top of the terrestrial food chain did not afford them the luxury of ignoring the sky 66 million years ago, when the Chicxulub impactor wiped them out from the surface of Earth.
Astronomers observe houses similar to ours on the cosmic street, in the form of Earth-Sun analogs within the Milky-Way galaxy. We should not ignore the possibility that some of these houses may have residents. The reason is simple. Most stars formed billions of years before the Sun and it takes a Voyager-like probe only a billion years to cross the Milky-Way disk from one side to the other. This implies that messengers from other Milky-Way residents might show up at our back yard. Ignoring that possible reality is a bad idea.
In recent years, witnesses in Congressional hearings testified under oath that the U.S. government hides information about materials retrieved in crash sites of extraterrestrial technological vehicles. If true, such a government finding would be equivalent to discovering a tennis ball from a neighbor in our backyard. The question would then be whether to inform our family members at the dinner table, who are not aware of any neighbors, about the finding or to hide the information from them. The answer is obvious: the U.S. government should not hide any information about what lies outside the solar system, for the same reason that the Vatican should not have put under wraps Galileo’s finding that the Earth is not at the physical center of the Universe. Delay tactic does not work. In 1992, the Vatican admitted that Galileo was right four centuries ago. But consider the counterfactual in which the Vatican would not admit this mistake and religious NASA engineers were to adopt the Vatican’s model in which Mars orbits around the Earth. In that case, NASA would have launched rockets that would have never reached Mars.
Nature is under no obligation to make us happy. It is better to adapt to reality, whatever it means. If we have neighbors, we better know about them because they might show up in our back yard even if we ignore them.
A few days ago, I received many kind messages for my birthday from friends and colleagues. I noted that as an optimist, I believe that our future can be better than our past if we only pay attention to reality. I am grateful to my team members for their help in realizing this hope.
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. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
