Brief Overview of TRAPPIST-1 Solar System and Why its Discovery by NASA is Significant

On 22 February 2017, astronomers announced four additional exoplanets around TRAPPIST-1. Besides TRAPPIST, this work utilized the very large telescope at Paranal and the NASA Spitzen Space Telescope , among others, and brought the total number of planets to seven, of which three are considered to be in its habitable zone. The others could also conceivably be habitable (possess liquid water somewhere on their surface)

TRAPPIST-1 is an ultra cool dwarf star about the size of Jupiter.
The planetary system of this star is composed of seven temperate terrestrial planets, of which five (B, C,E, F and G) are similar in size to Earth, and two (D and H) are intermediate in size between Mars (with a diameter about half that of Earth) and Earth. Three (e, f and g) orbit in the habitable zone. The total mass of the six inner planets is about 2% the mass of TRAPPIST-1, a fraction similar to that for the Galilean satellites of Jupiter suggesting perhaps a similar formation history.