In 1954, 1975, and 1978[11] Swedish astrophysicist Hannes Alfvén included electromagnetic effects in equations of particle motions, and angular momentum distribution and compositional differences were explained. In 1954 he first proposed the band structure in which he distinguished an A-cloud, containing mostly helium, but with some solid- particle impurities ("meteor rain"), a B-cloud, with mostly hydrogen, a C-cloud, having mainly carbon, and a D-cloud, made mainly of silicon and iron. Impurities in the A-cloud form Mars and the Moon (later captured by Earth), in the B-cloud they condense into Mercury, Venus, and Earth, in the C-cloud they condense into the outer planets, and Pluto and Triton may have formed from the D-cloud.