The visitor has long been the subject of theories it is an extraterrestrial probe

The "interstellar object" which passed close to the Sun last year may be nearly three times older than our solar system, astronomers have revealed.

The mysterious visitor, named 3I/ATLAS, is only the third interstellar object of its kind ever detected by humanity.

It's now on its way out of our neighbourhood - but its exceptional brightness has given scientists a rare chance to examine material originating from elsewhere in the Milky Way.

First observed in July last year, the space rock sparked online speculation from prominent Harvard professor Avi Loeb, who suggested it might be an alien spacecraft.

The world's most powerful telescopes have now provided fresh insights into the remarkable cosmic traveller.

Research published in the journal Nature suggests 3I/ATLAS could be up to 12 billion years old. Our solar system is around 4.5 billion years old in comparison.

Lead author Martin Cordiner of Nasa's Goddard Space Flight Centre said it is "maybe the oldest object to have been observed in our solar system".

He also revealed the visitor may have spent billions of years on "vast unimaginable trajectories around our galaxy".

It could even be a "relic" from an era known as "cosmic noon" when many stars were being made, he added.

Data from the James Webb space telescope and Chile's Alma observatory revealed the comet contains 10 times more deuterium than objects in our solar system.

"That high abundance of heavy water can only really happen, according to our understanding of astrochemistry, in a very cold environment," Cordiner explained.

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The isotopic evidence indicates the visitor was formed at close to absolute zero - or -243C.

Prof Loeb, who previously courted controversy by proposing that the earlier interstellar object Oumuamua might be alien technology, made similar claims about 3I/ATLAS.

As well as the denial from Nasa, last month the Search for Extraterrestrial Intelligence Institute (Seti) confirmed it had found "no evidence of extraterrestrial technology" on the comet.

The two previous interstellar objects detected by astronomers, 1I/'Oumuamua in 2017 and 2I/Borisov in 2019, were too dim to allow isotopic analysis.

This made 3I/ATLAS uniquely valuable for scientific study due to its unusual brightness.

Astronomer Peter Veres, who helped identify the comet at the International Astronomical Union's Minor Planet Center, described the research as "exciting".

"The comet is now leaving the solar system and will never return, so future observations will become increasingly difficult," he told AFP.

Scientists anticipate discovering many more interstellar objects in coming years, particularly through the new Vera C Rubin observatory in Chile.

"This is just the beginning of an exciting new field," Mr Cordiner added.

"We've got a lot more to learn about these things and what they can tell us about our galaxy."