Scientists have been trying to figure out what the 1,312ft (400m) object named Oumuamua might be after it was first spotted by a telescope in Hawaii in October 2017.
The object’s flat, elongated shape and reddish colour is from outside our solar system, according to the researchers.
It was moving at 59,030mph when it was first tracked by scientists.
Oumuamua’s unusual trajectory and high speed sets it aside from other space objects such as asteroids and comets.
Harvard researchers have now said it “may be a fully operational probe sent intentionally to Earth vicinity by an alien civilisation”.
In a letter published in The Astrophysical Journal Letters on 12 November, the researchers add that Oumuamua could be a spacecraft pushed along by light falling on its surface.
They add in the paper that the object could be a “lightsail of artifical origin”.
Lightsails are a proposed method of spacecraft propulsion which uses radiation pressure exerted from sunlight or large mirrors.
The researchers are not saying outright that Oumuamua is a sign of extraterrestrial life.
Avi Loeb, chairman of Harvard’s astronomy department and co-author of the paper, told NBC News: “It is impossible to guess the purpose behind Oumuamua without more data.”
If the object is a lightsail, the paper adds it might have been floating in interstellar space when our solar system ran into it “like a ship bumping into a buoy on the surface of the ocean”.
Mr Loeb is an adviser to Breakthrough Starshot, an initiative that plans to send a fleet of tiny laser-powered lightsail craft to the nearest star system.
Alongside Shmuel Bialy, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics, acknowledges that the alien spacecraft theory is an “exotic” one.
Seth Shostak, a senior astronomer at the SETI (Search for extraterrestrial intelligence) Institute in Mountain View, California, said: “It’s certainly ingenious to show that an object the size of Oumumua might be sent by aliens to another star system with nothing but a solar sail for power.
“But one should not blindly accept this clever hypothesis when there is also a mundane explanation for Oumuamua – namely that it’s a comet or asteroid from afar.”
Coryn Bailer-Jones, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, said: “In science, we must ask ourselves ‘Where is the evidence?
“Not ‘where is the lack of evidence so that I can fit in any hypothesis that I like?”
Mr Bailer-Jones led a group of scientists earlier this year who identified four dwarf stars as likely origin points for Oumuamua.
He raised questions in particular about the object’s tumbling motion.
He said: “If it were a spacecraft, this tumbling would make it impossible to keep any instruments pointed at the Earth.”
Mr Loeb called his findings “purely scientific and evidence-based” and added: “I follow the maxim of Sherlock Holmes: when you have excluded the impossible, whatever remains, however improbable, must be the truth.”
Oumuamua has now left the solar system and is no longer visible even with telescopes.
When the object was first spotted scientists believed it might have been travelling through space for hundreds of millions of years.
In December 2017 it was announced the astronomers were going to scan Oumuamua for signs of alien life.
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