This representation of the Trappist-1 system shows which planets could potentially have temperature conditions which would allow for the presence of liquid water, seen generally as essential for life. The inner three planets are likely too hot, and the outer planet is probably too cold, but the middle three planets might be just right. (NASA / JPL-Caltech)
Our solar system has but one planet orbiting in what is commonly known as the habitable zone — at a distance from the host star where water could be liquid at times rather than always ice or gas. That planet, of course, is Earth.
But from a theoretical, dynamical perspective, does this always have to be the case? The answer to that question is no because a number of stars are known to have more than one habitable zone planet.
Now a team from the University of California, Riverside has published a study that concludes as many as seven Earth-sized, habitable zone planets could orbit a single star — if there were no large Jupiter-sized planets in the system and if the star was of a particular type.
The article, published in the Astronomical Journal, concluded that seven habitable zone planets was the ...