It's not quite extraterrestrial life, but scientists have discovered sea plankton growing in space.

Russian space officials have confirmed traces of plankton and other micro-organisms were found living on the exterior of the International Space Station (ISS), and it appears they've been living there for years.

Russian astronaut Olek Artemyev and Alexander Skvortsov made the discovery after a routine spacewalk around the ISS, during which they took samples from illuminators on its surface, Russian news agency ITAR-TASS reported overnight.

High-precision equipment later revealed the presence of micro-organisms on the wipes.

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Space officials claimed the plankton were not carried there at launch, but are thought to have been blown over by air currents on Earth.

The particles, invisible to the naked eye, were able to survive in the vacuum of space, despite zero gravity, freezing temperatures, lack of oxygen, and cosmic radiation.

Head of the Russian ISS orbital mission Vladimir Solovyev said the results of the experiment "are absolutely unique".

He said the marine micro-organisms were not native to blast-off site Kazakhstan, and may have been "uplifted" to the station at an altitude of 420km.

"Plankton in these stages of development could be found on the surface of the oceans," he said.

"This is not typical for Baikonur [in Kazakhstan]. It means that there are some uplifting air currents which reach the station and settle on its surface."

He said the micro-organisms would be studied further.

Nasa is yet to comment on what this might mean for the American segment of the ISS, and whether similar findings have been made in the past.