Nasa has revealed more about Oumuamua, the piece of space rock so strange that some scientists have suggested it could be an alien spacecraft.

The space agency revealed that it could not see the object with its Spitzer Space Telescope. And that could reveal important clues about what it actually is.

Oumuamua passed by Earth in September 2017, becoming the first known interstellar visitor ever to make its way to our solar system from another one. As it flew by, researchers rushed to learn more about it, pointing telescopes and other instruments towards it in an attempt to learn as much as posisble before it disappeared out the other side of the solar system.

Spitzer tried to pick out the rock in November, around two months after its closest approach. It failed to see it – but that failure puts a limit on how big the rock can be, since if it was large enough it would have been spotted, according to a new paper published in the Astronomical Journal and coauthored by scientists at NASA's Jet Propulsion Laboratory in Pasadena, California.

That helps to lead credence to the theory that the relatively small object is being pushed along by gas that was being thrown out of the object. That gave the effect of adding thrust as it travelled through the solar system, speeding it up.

It was that strange speeding up behaviour that led scientists to suggest that it could be an alien probe, sent past Earth by a distant civilisation. The extra propulsion could be caused by the object working as a lightsail, designed to be carried along by solar radiation, Harvard scientists recently suggested.

The alternative and more accepted theory of frozen gases inside the object being expelled and pushing it along was dependent on Oumuamua being smaller than typical comets inside of our solar system. With the determination that it probably is, the research seems to suggest theories about it being an alien spacecraft are less likely.

"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 new 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."