Scientists examining samples taken from the exterior of the International Space Station (ISS) have made a rather unexpected discovery- traces of marine plankton and other microbes growing on the surface of the illuminators. What’s more, it seems they could have been living there for years.

The intriguing discovery was made after ISS cosmonauts took surface samples during a routine spacewalk around the satellite. The samples were later analyzed by high-precision equipment as part of a so-called “Test” experiment, ITAR-TASS revealed. Scientists were then able to confirm that these organisms are capable of living in space despite the hostile conditions experienced. Furthermore, some of the studies demonstrated that the organisms could even develop in the vacuum of space.

“Results of the experiment are absolutely unique,” chief of the Russian ISS orbital mission Vladimir Solovyev told ITAR-TASS. He believes that further study is warranted, but admits he is puzzled as to how the organisms arrived on the ISS surface as they're not native to the launch site. 

“[Plankton in] such phases of development is found on the surface of the ocean. It isn’t characteristic to Baikonur,” Solovyev explained, referring to the Baikonur Cosmodrome in Kazakhstan where crew and cargo are launched for the ISS. “It turns out that there are some rising air currents, which settle on the surface of the station.”

According to Solovyev, the surface of the ISS is heavily polluted due to engines from delivery spacecrafts and other factors. Crew members have therefore initiated a cleanup operation to put the illuminators in order.