They detected helium escaping from the upper atmosphere which they believe to be evidence of a retained atmosphere, but haven't been able to fully identify the elements present in the lower atmosphere.

Due to the density of the planet they believe it could be a water world, or a mostly-icy world due to the lack of hydrogen found, and the lower atmosphere could consist of nitrogen, water vapor, and carbon dioxide. Since the host star is very inactive, there's little atmospheric erosion that would strip away a heavier atmosphere.