To the editors:

On October 19, 2017, the first object from outside the solar system was discovered near earth by the Pan-STARRS telescope in Hawaii. It was given the name ‘Oumuamua, which means “a messenger from afar, arriving first” in the Hawaiian language. I was intrigued by this first interstellar object because a decade earlier I had co-authored a paper forecasting that, based on what is known about the solar system, Pan-STARRS will not detect any rock from other stars.¹

Astronomers initially assumed that the object was a comet, because comets are most loosely bound to the sun in the Oort cloud at the periphery of the solar system where they can be easily sent to interstellar space by the perturbation of a passing star. But there was no visible cometary tail around ‘Oumuamua. Moreover, a cometary origin would imply that ‘Oumuamua inherited the motion of its parent star. Instead, it was found to originate from the so-called local standard of rest. This frame averages over the motions of all stars near the sun, and only 1 in 500 stars is so much at rest as ‘Oumuamua was in that frame, before the sun kicked it gravitationally.²

These were just the initial anomalies that made ‘Oumuamua different from all the comets and asteroids seen before in the solar system. As it tumbled every eight hours, the brightness of the sunlight reflected from it changed by a factor of ten. This meant that it has an extreme shape, which can be described as disk-like with 90% confidence.³ The Spitzer Space Telescope did not detect any carbon-based molecules or dust around ‘Oumuamua, setting a tight limit on ordinary cometary activity.⁴ The lack of heat detectable in the infrared spectrum placed an upper limit of about 200 meters on its size. But most remarkably, ‘Oumuamua exhibited an excess push away from the sun which would have required it to lose approximately 10% of its mass if the push was caused by the rocket effect from normal cometary evaporation.⁵ An extensive evaporation of this magnitude was absolutely ruled out by the Spitzer Telescope data. Moreover, the repulsive force declined smoothly with distance from the sun, showing no change in spin and no sudden kicks from jets on the surface of comets, as routinely observed.⁶ Finally, there was no apparent cut-off in the push at the distance beyond which evaporation of water ice by sunlight is expected to stop.

The excess force without a cometary tail convinced me that this object is not a familiar rock. Since the push away from the sun was consistent with a smooth inverse-square law, I reasoned that it may result from the reflection of sunlight from a thin object.⁷ For the reflection of sunlight to exert a strong enough force, the object had to be thinner than a millimeter, like a light sail. Since nature does not make thin objects, I suggested that it might be artificial in origin.⁸

This possibility was intended to encourage scientists to obtain a high-resolution image of ‘Oumuamua-like objects in the future. It is often said that a picture is worth a thousand words. In my case, a picture is worth 66,000 words, the number of words in my book, Extraterrestrial. I would never have written my book if we had a megapixel image of ‘Oumuamua.

In September 2020, another object was discovered by Pan-STARRS,⁹ sharing ‘Oumuamua’s anomalies of no cometary outgassing and excess push by reflection of sunlight. It was given the astronomical name 2020 SO and later found to be a rocket booster from a 1966 mission to the moon. It had thin walls and hence a large area for its mass. It was not designed to be a light sail but was thin for a different purpose. Its discovery illustrates that the difference between a rock and an artificial object can be inferred from the unusual dynamics of an object. We know that humanity manufactured 2020 SO. The question is, who manufactured ‘Oumuamua?

A cave dweller finding a cellphone would argue that it is a rock of a new type, in the same way that earthlings who studied the anomalies of the first interstellar object ‘Oumuamua suggested that it is a comet of a type “never seen before.” They suggest it may be an iceberg made of pure hydrogen or pure nitrogen, even though these possibilities face “serious difficulties,” in the words of some of their proponents.¹⁰ When a colleague of mine, specializing in solar system rocks, heard about ‘Oumuamua, he said: “This interstellar object is so weird … I wish it never existed.” His statement explains why innovation is often suppressed in the face of anomalies. Mainstream scientists would prefer these anomalies go away so that they can keep their reputation of being able to forecast all data with their existing knowledge. Anomalies pose a threat to their status as experts in the field.

In his review, Paul Sutter dismisses the anomalies of ‘Oumuamua as a credible reason for considering its artificial origin. But these anomalies will not go away if we ignore them and use authority or groupthink to argue abstractly for a natural origin. ‘Oumuamua will never be a regular rock, just as the earth continued to move around the sun even when philosophers refused to look through Galileo Galilei’s telescope. They insisted that everyone knows the sun moves around the earth and placed Galileo under house arrest.

Just as for Galileo’s discovery, ‘Oumuamua’s data was collected by looking through the best available modern telescopes. Practicing scientists who attempt to explain the anomalies of ‘Oumuamua by a natural origin are all forced to contemplate objects that have never been seen before, with major quantitative challenges. These possibilities are: a porous structure with a mean density a hundred times lower than air,¹¹ which is unlikely to maintain its integrity after being heated to hundreds of degrees by the sun; fragments from tidal disruption,¹² whose shape would be more likely to be that of a cigar than a disk as inferred for ‘Oumuamua; an iceberg of molecular hydrogen¹³—which evaporates too quickly along its interstellar journey¹⁴; or a nitrogen iceberg chipped off the surface of a planet like Pluto around another star¹⁵—a mechanism that cannot supply enough material to explain the implied abundance of objects like ‘Oumuamua.¹⁶

Given these challenges to the natural origins of ‘Oumuamua and the similarity in the anomalous dynamics of 2020 SO and ‘Oumuamua, the possibility of an artificial origin should be left on the table. Thanks to generous donations from people who were inspired by the vision of my book Extraterrestrial, I was able to inaugurate in July 2021 the Galileo Project.¹⁷ One of the major goals of the project is to search for ‘Oumuamua-like objects in future surveys, like with the upcoming Vera Rubin Observatory. An early alert for ‘Oumuamua-like objects would allow for the design of a space mission to intercept their trajectories and take close-up photographs. Such data could resolve their nature and unambiguously determine whether they are natural or artificial in origin.

Finding equipment from an extraterrestrial technological civilization would have a major impact on the future of humanity. Here’s hoping that we will be open-minded enough to search for objects that resemble the equipment our technological civilization is launching to space. Half of the sun-like stars host a planet the size of the earth roughly at the same separation.¹⁸ Many of these stars formed billions of years before the sun, allowing for the possibility that numerous probes were sent to interstellar space. Ridiculing the notion that ‘Oumuamua may have been artificial in origin will not get rid of our neighbors.¹⁹ As Galileo demonstrated four centuries ago, the nature of celestial objects must be found through telescopes rather than philosophical prejudice.