7 planets orbiting TRAPPIST-1 may be made of similar stuff

Washington, Jan 23 : The seven rocky planets orbiting the the red dwarf star TRAPPIST-1 have remarkably similar densities, which provides clues about their composition.
Located about 40 light-years away, TRAPPIST-1 is home to the largest group of roughly Earth-size planets ever found in a single stellar system.
Previous calculations determined that the planets are roughly the size and mass of Earth and thus must also be rocky, or terrestrial — as opposed to gas-dominated, like Jupiter and Saturn.
The new paper, published in the Planetary Science Journal, offers the most precise density measurements yet for any group of exoplanets — planets beyond our solar system.
The more precisely scientists know a planet’s density, the more limits they can place on its composition.
Consider that a paperweight might be about the same size as a baseball yet is usually much heavier.