The planets were all found in super-tight formation around an ultra-cool dwarf star called Trappist-1. Estimates of their mass also indicate that they are rocky planets, rather than being gaseous like Jupiter. Three of them are perfectly sitting in the habitable zone of the star, known as Trappist-1e, f and g, and may even have oceans on the surface. The researchers believe that Trappist-1f in particular is the best candidate for supporting life. It's a bit cooler than Earth, but could be suitable with the right atmosphere and enough greenhouse gases.
The planets are so close to each other and the star that there are seven of them within a space five times smaller than the distance from Mercury to our sun. This proximity allows the researchers to study the planets in depth as well, gaining insight about planetary systems other than our own.