When Oumuamua was spotted past year, scientists observed that its speed had increased after passing the Sun. But 'Oumuamua didn't have a "coma", the atmosphere and dust that surrounds comets as they melt.
The object was discovered a year ago with the Pan-STARRS 1 telescope atop Haleakalā on Maui and was given the Hawaiian name ʻOumuamua, meaning 'a messenger from afar arriving first'.
"This unusual object ... is less than [400 metres] in diameter and is moving remarkably fast", NASA said at the time. That means the object would be made of some thin material that could absorb radiation from the sun - either a naturally created material we've never seen before, or something made by aliens.
Oumuamua has now left the solar system and is no longer visible even with telescopes.
Astronomers from the Harvard Smithsonian Center for Astrophysics (CfA) have detected an unexpected boost in speed and shift in trajectory in the red object named Oumuamua as it passed through the inner solar system.
Since scientists are not able to explain the unusual features of a celestial body and its origin, giving it finally a new class of "interstellar objects".
"Astronomers are urgently working to point telescopes around the world and in space at this notable object".
"Observational anomalies like we see with Oumuamua, combined with careful reasoning, is exactly the method through which we make new discoveries in astrophysics - including, perhaps, truly incredible ones like intelligent life beyond the Earth", he wrote.
Their theory - and this is really, really cool - is that it might be a light sail, powered by solar radiation and of an "artificial origin" (i.e. built by aliens).
"There was something affecting its motion other than the gravitational forces of the Sun and planets", Marco Micheli of the European Space Agency, which was tracking the object, said in a June press statement.
According to Harvard scientists, we may not be alone in our solar system. Not only is it the first-ever interstellar asteroid or comet detected by astronomers while passing through the solar system, its shape is unusual, its acceleration is unexpected, and a cometary tail - the signature trail of particles seen behind shooting stars - is conspicuous in its absence.
To explain its movement, the report's authors, Abraham Loeb, a professor and chair of astronomy, and Shmuel Bialy, a postdoctoral fellow, argue that Oumuamua's non-gravitational acceleration may be explained by solar radiation pressure.
But Loeb called the conjecture "purely scientific and evidence-based", adding, "I follow the maxim of Sherlock Holmes: When you have excluded the impossible, whatever remains, however improbable, must be the truth".