Russia has been testing samples of plankton and other microorganisms from outside of the International Space Station to find any traces that could point to the existence of alien or extra-terrestrial life forms, the Russian space agency Roscosmos said in a press release on May 26.

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"The micrometeorites and comet dust that settle on the ISS surface may contain biogenic substance of extra-terrestrial origin in its natural form," Roscosmos officials said, noting that the surface of the ISS is a "possibly unique and easily available collector and keeper of comet substance and, possibly, of biomaterial of extra-terrestrial origin."

The ISS surface could constantly catch dust and microparticles like bacteria, fungal spores, and sea plankton even if it travels at an altitude of 400 kilometers. It is also usually targeted by a flow of grain-sized comet particles, making it a versatile tool for exobiological research.

At least 19 dust samples have been collected from the ISS surface during space walks since 2010 under the experiment called "Test." However, analyzing the small comet materials is complicated, and standard methods will not yield quick results.

On the other hand, a test result from 2013 revealed that some microorganisms indeed thrive on the outside of the ISS and even on hostile space conditions like vacuums, extreme temperatures, and harsh cosmic radiation.

Just recently, a team of scientists from NASA revealed that they found a new bacterium on filters installed on the ISS. The new bacterium called Solibacillus kalamii, named after former Indian president A.P.J. Abdul Kalam, has still not been found on Earth, although experts ruled out the extraterrestrial life form possibility, Sputnik News reported.

This is not the first time Russia made claims about possible extraterrestrial life on the ISS. Vladimir Solovyev, chief of the Russian ISS orbital mission, said in 2014 that traces of sea plankton and microscopic particles had been found on the illuminator surface.

However, the Russian's American and German colleagues expressed doubt of the claim. NASA spokesperson Dan Huot told Space.com that the alleged sea plankton are residues that can accumulate on sensitive areas like the window and will build up whenever thruster firings are performed.

"I don't know where all the sea plankton is coming from," he added.

German's Aerospace Center spokesperson Alisa Wilken also said that although "bacterial DNA" was discovered, the specifics of the claims were questionable "as it cannot detect all kinds of bacteria and it also cannot test whether the bacteria are living and thriving or not."

The ISS, which was launched in 1998, is a joint initiative of different space agencies including Canada, Europe, Japan, Russia, and the United States.