The object, which is currently being referred to as A11pl3Z, poses no threat to Earth, the ESA's planetary defence head Richard Moissl told AFP.

"It will fly deep through the Solar System, passing just inside the orbit of Mars," but not threatening to hit our neighbouring planet, he said.

Excited astronomers are still refining their calculations, but the object appears to be zooming at a speed up to 60 kilometres (37 miles) a second, which would mean it is not bound by the Sun's orbit, unlike comets, which all originate from within the Solar System.

Its trajectory also "means it's not orbiting our star, but coming from interstellar space and flying off to there again," Moissl said.

"We are not 100 percent certain at the moment, but anything else would be a surprise," he added.

Official confirmation is expected to come from the International Astronomical Union's Minor Planet Center, which has recorded more than 100 observations of the object so far.

After the object was discovered, astronomers across the world then searched through past data, tracing its trajectory back to at least June 14.

The object is currently estimated to be roughly 10-20 kilometres wide, though it could be smaller, Moissl said.

"It will get brighter and closer to the Sun until late October and then still be observable (by telescope) until next year," he added.

- Our third visitor -

It would be the third time humanity has detected something coming from the stars.

The first, 'Oumuamua, was discovered in 2017. It was so strange that at least one prominent scientist became convinced it was an alien vessel -- though this has since been dismissed by further research.

Our second interstellar visitor, 2I/Borisov, was spotted in 2019.

Moissl said it is not feasible to send a mission into space to intercept the new object.

It appears to be "moving considerably faster than the other two extra-solar objects that we previously discovered," Mark Norris, an astronomer at the UK's University of Central Lancashire, told AFP.

Norris pointed out that modelling estimates there could be as many 10,000 interstellar objects drifting through the Solar System at any given time.

If true, this suggests that the newly online Vera C. Rubin Observatory in Chile could soon be finding these dim objects every month, he added.

These visitors offer a rare chance to study something outside of our Solar System, Norris said.

For example, if we detected precursors of life such as amino acids on such an object, it would give us "a lot more confidence that the conditions for life exist in other star systems," he added.