Research on a tiny portion of the 5.4 grams of rock and dust collected from the Ryugu asteroid, some 300 million kilometres (185 million miles) from Earth, offers new insights into how the solar system looked in the early life of our planet.

Scientists believe the reason Earth has oceans, lakes and rivers is because it was hit with water-carrying asteroids four to 4.5 billion years ago, making it a habitable planet.

However, while previous work showed water existed on asteroids in the solar system's early years, it was not clear how long that water lasted, said Tsuyoshi Iizuka, associate professor of the Department of Earth and Planetary Science at the University of Tokyo.

"Here we learned for the first time that water existed (in the form of ice) for one billion years," Iizuka, an author of the study published in the journal Thursday, told AFP.

The finding "forces us to rethink the starting conditions for our planet's water system."

The findings are only the latest in a trove of research done on the tiny sample retrieved from Ryugu by Japan's Hayabusa-2 probe and returned to Earth in 2020.

The new study suggests asteroids may have retained water in the form of ice and hydrated minerals.

This means that asteroids striking a young Earth could have carried water in quantities two to three times larger than previously thought, significantly affecting the planet's early oceans and atmosphere, the study said.

Ryugu is thought to be formed from fragments scattered after the destruction of its parent body, but researchers have yet to definitively date its age.

However, using a radioactive decay system, they found evidence that water flowed on Ryugu more than one billion years after the formation of its parent.

That flow may have been triggered by an impact that generated heat, melted ice and opened rock fractures, they said.

"I was surprised by the findings," Iizuka said, adding he plans to investigate whether similar water activity happened on other asteroids.

His team also hopes to trace how water was stored, mobilised and finally delivered to Earth.

NASA and other institutes are also studying samples brought back from the 4.5-billion-year-old asteroid Bennu in 2023.

A better understanding of asteroid history "will help us to unravel the evolution of the solar system", Iizuka said.