Last week, some Harvard University scientists sparked widespread media attention about a possible alien origination for the mysterious interstellar object known as 'Oumuamua. At the end of a paper speculating about the object's observed movement, the authors presented "a more exotic scenario" suggesting that ‘Oumuamua may be "a fully operational probe sent intentionally to Earth vicinity by an alien civilization."

As we reported at the time, this outlandish theory may have been catnip for online news editors, but there just wasn't much evidence to take it seriously. Now, thanks to some previously unpublished observations by NASA, we can further discount the idea.

The object now called 'Oumuamua made its closest approach to Earth in September 2017, and astronomers first spotted it in October of that year as it began moving away. In November, when NASA trained its Spitzer Space Telescope on where astronomers expected to find 'Oumuamua, it found nothing over the course of two months of observations in the infrared portion of the spectrum.

"'Oumuamua has been full of surprises from day one, so we were eager to see what Spitzer might show," said David Trilling, lead author on a new study detailing these observations and a professor of astronomy at Northern Arizona University. "The fact that 'Oumuamua was too small for Spitzer to detect is actually a very valuable result."

The non-detection sets an upper limit on size. Although the interstellar interloper is believed to be a long, cigar-shaped object, the models used by astronomers with the Spitzer data produced estimates for the object's diameter as if it were a spherical object. Based upon different assumptions about the object's composition and the amount of light it reflects, the upper limit on 'Oumuamua's "spherical diameter" is 100 to 440 meters.

This is significant, because this smaller size may be able to explain one of the mysteries surrounding 'Oumuamua. Analyses based upon multiple telescopes aimed at the object late last year found that it accelerated away from our Sun significantly faster than could be explained by gravity alone.

The smaller size supports the theory that outgassing from the object—frozen gases heating up as 'Oumuamua neared the Sun—could have acted as thrusters to accelerate its movement (see short video, from NASA, above). In absence of other evidence, this natural theory seems more plausible than a visit from an alien probe.