Astronomers discover the seven Earth-size TRAPPIST-1 planets may be made of similar stuff

The study by the University of Washington and the University of Liege measured the densities of the seven Earth-sized planets in the Trappist-1 system. These planets, located 40 light-years away in the constellation Aquarius, are similar in size to Earth but orbit closer to their ultra-cool red dwarf star. The study found that the planets are about 8% less dense than Earth, suggesting different compositions. Three hypotheses were proposed: the planets may have a rust-covered surface, a smaller iron-rich core, or a deep ocean layer. The outer four planets might have water accounting for 5% of their mass, while the inner three could have water-heavy clouds if they possess a dense atmosphere like Venus.

How do the compositions of the planets in the Trappist-1 system compare to that of Earth?

What hypotheses were proposed to explain the density differences of the Trappist-1 planets?