Press enter or click to view image in full sizeThe 2011 Perseid meteor shower. (Credit: Babek Tafreshi/SSPL/Getty Images)
It is commonly thought that the Universe outside the Solar system has little impact on life on Earth. Interstellar objects, like the recent visitor 3I/ATLAS, challenge this notion, as they invade our space unexpectedly. The biggest impact to humanity would be in the form of a visit by alien technology. But even natural visitors from our Galactic neighborhood could have had an impact on terrestrial life.
For example, when a star passes through the outskirts of the Solar system, it can gravitationally kick Oort cloud comets towards Earth and generate a major meteor shower that would significantly affect terrestrial life. Did a passing star influence the evolution of early humans?
This possibility was discussed in a new paper that I wrote with the visiting student, Zhuoya Cao, and my former postdoctoral fellow, Morgan MacLeod (accessible here), which was just accepted for publication in the prestigious Nature magazine. We studied the particular case of the nearby star HD7977 which crossed the Oort cloud about 2.5 million years ago, around the time that the human species emerged on Earth. Our calculations showed that this star passage likely repopulated the Inner Oort Cloud and potentially triggered a significant comet shower on Earth.
What is the Oort cloud and how do we know that it exists? The characteristics of comets with very long orbital periods around the Sun suggest that a distant reservoir of small bodies is bound to the Sun and defines the outskirts of the solar system, extending up to 100,000 times the Earth-Sun separation (defined as an Astronomical Unit, abbreviated as AU). The realization that comets are fundamentally “dirty snowballs” was pioneered by the Harvard College Observatory astronomer Fred Whipple in 1950 as he walked through Harvard Square on a snowy day. This suggests that comets are icy remnants from the solar system’s formation, with their nuclei stored in a nearly spherical reservoir around the Sun, called the Oort Cloud after the Dutch astronomer Jan Oort who suggested it as the origin of long-period comets in 1950.
The inner edge of the Oort Cloud is at ∼20,000 AU, beyond which it is called Outer Oort Cloud (OOC). The massive reservoir inside the OOC is named the Inner Oort Cloud (IOC). The IOC dynamically feeds comets into the OOC via stellar and planetary perturbations, and also the galactic tide. Comets from IOC cannot always be observed in the inner solar system because when orbits reach a high eccentricity, comets are destroyed by sublimation or scattered away by gravitational encounters with the planets
Our paper shows that the close flyby of the star HD 7977 had the potential to substantially perturb the IOC, potentially leading to an enhanced comet flux during the global climate change associated with the Pliocene-Pleistocene transition, marked by the Earth’s transition from a warm, relatively stable climate to the glacial-interglacial cycles of the Pleistocene a few million years ago. Given the actual mass, trajectory, and timing of HD 7977, we studied under what conditions could it have generated a significant comet shower, and what would the subsequent implications for Earth have been?
Press enter or click to view image in full sizePanels (a)-© show snapshots from a simulation of the gravitational effect of a passing star on comets in the Oort cloud of the solar system. Colors represent the angular momentum value of each comet. Those with blue colors have a possibility of entering the inner solar system. The pentagram symbol denotes the Sun, the gray trajectory represents the flyby path of HD7977, and the purple trace illustrates the orbit of the selected comet. The insets inside each plot show the magnified orbit evolutionary of a selected representative comet. Panel (d) is the distribution of angular momentum change of Inner Oort cloud comets.
We found that the comet shower’s intensity strongly depends on HD7977’s distance of closest approach to the Sun (labeled as the. parameter b), with possible flyby distances ranging from 2,300 AU to ∼ 13,000 AU. For the closest approach of 2,300 AU, the terrestrial impact probability of 1-kilometer comets increased by an order of magnitude compared to the steady state before the passage of the star. These findings suggest that HD7977’s flyby may have caused an enhanced comet flux during the Pliocene-Pleistocene transition, which could plausibly be related to the environmental changes on Earth during this era.
Press enter or click to view image in full sizeRate of comets with diameter larger than a kilometer colliding the Earth during the 3.5 Myr timescale. Year 0 represents the present time (2025 A.D.). There was a peak of comet shower shortly after the passage of the HD7977 (-2.47 million years). The transparent shaded region indicates the uncertainty in the results, which is relatively small.
Future geological studies that identify a peak in the lunar cratering rate around 3 million years ago could provide supporting evidence for a comet shower induced by HD 7977’s flyby. We also surveyed terrestrial impact craters with nominal ages up to 10 million years to identify potential candidates linked to the proposed increase in impactor flux. Both recorded and unrecorded impacts may have something to do with the comet shower caused by HD 7977. While the prevailing view attributes the Pliocene–Pleistocene transition primarily to asteroidal impacts, the findings presented here suggest a plausible alternative scenario involving cometary bombardment. Further geological investigations will be needed to evaluate the potential role of such a mechanism.
The detectability of a comet shower associated with HD 7977 in the geological record depends on its intensity exceeding the background rate of asteroid impacts. Distinguishing between asteroidal and cometary impact craters is challenging. Furthermore, some comets may disintegrate in the atmosphere, leaving no crater. Future geological studies that confirm such a signal would provide support for the potential role of the HD 7977 flyby in the Pliocene–Pleistocene transition.
In summary, our new paper shows that the HD 7977-induced comet shower may have influenced Earth’s impactor flux during the Pliocene-Pleistocene transition, and could have been a factor in the environmental perturbations associated with this geological boundary interval.
The more we know about our cosmic neighborhood, the more we learn on how it could have influenced the history of humanity. It is possible that HD 7977 triggered environmental effects that promoted the emergence of humans. In such a case, the human species owes its existence to the impact of a random star on a collection of dirty snowballs in the outer solar system.
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Figuring things out is a work of art. Advancing the scientific understanding of our place in the Universe resembles the artistic creation of a painting or a sculpture. Such creations are first in the mind of the scientist or the artist, but after materializing — they give us a new perspective on the physical reality that we find around us.
At the foundation of creative science and art is the humility to learn, not the arrogance of expertise. I pointed this out today in front of a hundred people who came to celebrate the creativity of the remarkable artist Greg Wyatt, who donated 51 watercolors of distinguished scientists and 2 bronze sculptures of Galileo Galilei to my office at the Harvard College Observatory. The creative composer, David Ibbett, performed an original piece of music inspired by the latest images of 3I/ATLAS.
Press enter or click to view image in full sizeGreg Wyatt (right) next to two sculptures of Galileo Galilei and 51 watercolors which he donated to the office Avi Loeb (left). (Credit: Loeb’s photo collection)
But the biggest gift that Greg gave me is the foundation for a new bronze sculpture along with the plastalina that will allow me to shape it and bring it to life. I plan to create a bronze sculpture of an alien being, titled “The Alien”, either out of pure imagination or — if I discover a real alien anytime soon — as part of my scientific effort to describe it.
Press enter or click to view image in full sizeThe foundation, sculpting tools and plastalina for a new bronze sculpture “The Alien” gifted to Avi Loeb by Greg Wyatt. (Credit: Loeb’s photo collection)
Galileo Galilei produced a set of six watercolors of the Moon in its various phases “from life,” as he observed the Earth’s satellite through a telescope in the autumn of 1609. They represent the first realistic depiction of the Moon in history.
Galileo Galilei’s (1564–1642) Drawings of the Moon, November-December 1609, Florence, Biblioteca Nazionale Centrale, Ms. Gal. 48, f. 28r. (Credit: Museo Galileo)
As Galileo illustrated in his Moon drawings, the boundary between science or art is fuzzy at the frontiers of our knowledge.
ABOUT THE AUTHOR
Press enter or click to view image in full size(Image Credit: Chris Michel, National Academy of Sciences, 2023)
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.