Renowned astrophysicist Avi Loeb has calculated that there could be quintillions of objects like the interstellar asteroid Oumuamua flying around the solar system, which Loeb himself believes is a ship of an alien civilisation.
Five years ago astronomers spotted a small but highly unusual object. Judging by its trajectory and speed, it came to us from interstellar space, becoming the first known asteroid to arrive from outside the solar system. However, Harvard astrophysicist Avi Loeb, known not only for his considerable scientific achievements but also for his rather eccentric views, has come up with his own original hypothesis, calling 1I/Oumuamua a probe of an alien civilisation.
In his new work, Loeb takes this idea further by estimating the possible number of such objects in the solar system. His paper, co-written by Carson Ezell, is available at the arXiv open online library of preprints. By some simple calculations, the authors come up with a staggering figure of four quintillion (billion billion, or 1018).
Scientists rely on available and very scarce data on observations of interstellar bodies in the solar system. Only four have been detected so far: in addition to Oumuamua, they are meteorites CNEOS 2014-01-08 and CNEOS 2017-03-09, as well as the interstellar comet 2I/Borisov. They were all observed within eight years, between 2014 and 2022. Adjusting for the limitations of existing observational tools, Loeb and Ezell conclude that there could be a total of about 40 decillion (1,033) such bodies in the galaxy.
The scientists then estimate how many of them could pass through the inner regions of the solar system, within the habitable zone of our star. This is the name given to the space where temperatures are moderate enough to allow water on the surface of celestial bodies to exist in liquid form, providing comfortable conditions for the development of life in the forms we know. This brings the number down to four quintillion.
The authors stress that we are not talking about a colossal armada of alien probes. Most of these celestial bodies may well be interstellar comets or asteroids. But some fraction of their mind-boggling number may be spacecraft - or at least fragments of them.