Researchers claim to have found marine plankton on the outside of the International Space Station, but have no idea how the mysterious sea creatures got into space.

The findings suggest some organisms are capable of living on the outside of the station in exposed space, RT reported. Past studies have suggested some of these types of organisms can survive hostile conditions of spaceflight such as radiation and vacuums.

"The results of the so-called 'Test' experiment are unique. On the surface of the [ISS] windows we found traces of marine plankton - the microparticles - that will become the subject of further studies," Solovyev was quoted as saying by Itar-Tass, RT reported.

The researchers are still not sure how the plankton got all the way onto the surface of the space station.

[Plankton in] such phases of development is found on the surface of the ocean. It isn't characteristic to Baikonur [where cargo deliveries to the ISS are launched]," Solovyev said. "It turns out that there are some rising air currents, which settle on the surface of the station."

NASA is yet to confirm the findings, Space.com reported.

"As far as we're concerned, we haven't heard any official reports from our Roscosmos colleagues that they've found sea plankton," NASA spokesman Dan Huot said, Space.com reported.

The current cosmonauts did sample the outside of the windows, but that was not necessarily to test for microbes.

"I'm not sure where all the sea-plankton talk is coming from," Huot told Space.com. "The Russians did take samples from one of the windows on the Russian segment, and what they're actually looking for is residues that can build up on the visually sensitive elements, like windows, as well as just the hull of the ship itself that will build up whenever they do thruster firings for things like re-boosts. That's what they were taking samples for. I don't know where all the sea plankton talk is coming from."