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Letters to the editors

Vol. 6, NO. 3 / November 2021

To the editors:

In his recent book Extraterrestrial, Avi Loeb connects the discovery of ‘Oumuamua with the possibility of alien visitors. As evidence for his claims, Loeb points to ‘Oumuamua’s anomalous acceleration away from the Sun, along with its unusual reflectivity, shape, and curious ingress orbit. I don’t know why the latter is special, but it is likely related to ‘Oumuamua’s Earth-grazing orbit at only 0.16 au (astronomical units). Using the Drake equation, Loeb concludes that ‘Oumuamua may be an alien space craft and the long-awaited solution to the Fermi paradox.

In his review, Paul Sutter is critical of Loeb’s arguments, but does not quantify his statements. This is unfortunate because quantification would show that his skepticism is appropriate. I wish to add to Sutter’s case in two ways: first by applying our current understanding of the Drake equation, and second by confronting the lack of information about the interstellar visitor and conclusions drawn from scarce data.

The Milky Way galaxy contains between 200 and 400 billion stars, many of which are orbited by planets. Millions of these planets may be at the right distance from their parent star to have liquid water. The Kepler mission searched 530,506 stars and discovered 2,662 planets, 30 of which may have oceans. In our solar system, the only planet with liquid water also harbors life. It follows that the presence of liquid water on a planet may signify that life is present. Life on Earth emerged about a billion years after the formation of the planet, but it took another 3.5 billion years before intelligent life appeared. By this time, the universe was already more than 13 billion years old. This is a sufficiently large span of time that in some of the billions of planets, intelligent life may have emerged, followed later by an advanced society capable of traveling to neighboring stars. Once visits to nearby stars become possible, the hop to farther stars is the next step for an alien civilization. One can cross the Milky Way at light speed in a few hundred thousand years, leaving an advanced civilization plenty of time to populate the entire galaxy. Where are these extraterrestrial visitors? This is the basis for the Fermi paradox.

But there is convincing evidence against this perspective. On Earth, species typically survive for 2 or 3 million years before becoming extinct, and there are no solid arguments that intelligence saves them. A few million years is a rather short timeframe to colonize the entire galaxy, even for an advanced civilization. In that case, a simple solution to the Fermi paradox is that there simply are no advanced civilizations that populate the entire galaxy. In 1021, crossing the Atlantic was a nontrivial problem for seagoing Norsemen, and in 1969 we could barely travel to the nearest body in the solar system. Traveling to the next star may well be beyond the capacity of any intelligent living organism.

The simple solution to the Fermi paradox, that intelligent life does not spread across the galaxy, is often neglected. If that solution is accepted, it means that humans are somewhat unique, which contradicts the Copernican principle. After addressing the Drake equation as a Poisson process through Monte Carlo sampling, I can only conclude that life in the galaxy is omnipresent, but rather ill-equipped for interstellar travel. If ‘Oumuamua was indeed built and launched by an extraterrestrial civilization, its people are probably extinct. Traveling through space at a speed of 5.7 au per year, the first star one would encounter in the direction of Gamma Pegasi and Alpha Andromedae is Gaia EDR3 291720741454402944. ‘Oumuamua will not reach it for about 25 million years. One can ask quite a number of interesting questions about the motivation of this purported alien civilization.

Regardless of whether life exists elsewhere in the universe, there are several hard-to-explain phenomena related to ‘Oumuamua. Among those are the four mentioned earlier. ‘Oumuamua was observed for eleven hours, and astronomers will probably never see it again, unless we populate the entire galaxy looking for the needle in a haystack—in effect, defining our own role in somebody else’s Fermi paradox. In seeking to determine the object’s origin, all astronomers have to work with are the data gathered during that brief observation window. Some phenomena in science have been studied much longer and still remain elusive. These include the nature of gravity, the wave-particle duality of light, the nature of consciousness, and whether computers can think. In each of these cases, we have few answers and many questions. Science is as much about answering questions as sailing is about the direction of the wind. A scientific question can become like Pandora’s box: you never know how many new questions arise from one answered. For all these reasons, we should not be surprised to find that four questions about the origin of a newly discovered object remain unanswered. In fact, many questions were raised and answered by the discovery alone. The most important one, from my perspective, is whether interstellar asteroids exist. My answer to the question of whether life exists elsewhere in the galaxy is yes, probably. But why would they care about us?

Simon Portegies Zwart is Professor of Computational Astrophysics at Leiden Observatory.

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