In the 1950s, he enabled his physics students to work at CERN, who were officially unable to participate in this project due to political reasons at that time.

The discovery was made with the use of a nuclear emulsion plate exposed to cosmic rays, based on their energetic but delayed decay.

[3] This achievement proved to be fundamental in the development of nuclear and elementary particle physics as for the first time, researchers observed atomic nuclei in which at least one proton or neutron was replaced by a slightly heavier hyperon (containing a strange quark).

In 1969, he was awarded the Marian Smoluchowski Medal by the Polish Physical Society.

[7] According to physicist Andrzej Kajetan Wróblewski, Pniewski is among the greatest Polish physicists of the 20th century alongside Marie Curie, Marian Smoluchowski, Karol Olszewski, and Marian Danysz.