Some astronomers and X-Files fans wanted to believe that a cigar-shaped space rock named ‘Oumuamua was a probe from an advanced alien civilization after it was detected in our Solar System in October 2017. While that “exotic scenario,” to quote a pair of astronomers, would change the world forever, research published Wednesday offers a sense of this mystery object’s size — and chips away at that alien theory.

The new research notes that ‘Oumuamua (“visitor from afar arriving first” in Hawaiian) was too small to be seen by the Spitzer Space Telescope when it was trained on the object in November 2017. This finding lends credence to the theory that the first interstellar object we’ve observed in our solar system is more like a comet than an alien probe. The study was coauthored by scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California, and was published in The Astronomical Journal.

‘Oumuamua’s small size supports a theory that it’s a comet and that its changing speed was the result of frozen gases inside it heating up when it traveled near the sun; when those gases were expelled, ‘Oumuamua thrusted forward in space.

“‘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 the study 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.”

Oumuamua
'Oumuamua was first detected by the University of Hawaii's Pan-STARRS 1 telescope in Haleakala, Hawaii.

The alien theory received new life in November, when a preprint paper posted on arXiv saw researchers from the Harvard Institute for Theory and Computation — Shmuel Bialy, Ph.D. and Professor Abraham Loeb, Ph.D. — offer an explanation for Oumuamua’s speed increase that scientists had pointed out could not be due to gravity alone. The video below illustrates the object’s sharp acceleration they investigated:

In their paper, Bialy and Loeb describe the possibility of solar radiation pressure — the idea that photons from the sun might have pushed Oumuamua along — but sneak in a far more creative idea at the end:

Alternatively, a more exotic scenario is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization.

Using estimates from the amount of sunlight reflected off it, scientists estimated that the mystery object was 2,600 feet long. But using estimates from the Spitzer Space Telescope — which looks for infrared heat radiated by an object to determine size — they thought it might have a spherical diameter of 1,440 feet, 460 feet, or even as small as 320 feet.

Like “alien megastructures” before it, the mystery object generated worldwide attention for the fact it might be, but probably wasn’t, alien-related. Breakthrough Listen, the alien-searching initiative headed up by Russian billionaire Yuri Milner even announced in December 2017 that it would observe the object for radio bands or signals that would indicate “extraterrestrial technology.”

Since it flung by Earth in September 2017 ‘Oumuamua has hurtled well beyond our planet on its way out of the Solar System. NASA’s Jet Propulsion Laboratory announced Wednesday that the object is nearly as far away from the sun as Saturn, and far beyond the eyes of any telescopes, which may mean ‘Oumuamua’s composition will forever remain a mystery.

But it is probably just a weird-looking comet.


More ‘Oumuamua stories:

With additional reporting by Yasmin Tayag.

Photos via NASA