Astronomers from the University of Amsterdam have offered an explanation for the formation of the Trappist-1 planetary system. The system has seven planets as big as the Earth that orbit close to their star. The crux, according to the researchers from the Netherlands, is the line where ice changes in water. Near that ice line, pebbles that drifted from outer regions to the star receive an additional portion of water and clot together to form proto-planets. The article with the model has been accepted for publication in the journal Astronomy & Astrophysics.