Three planets, TRAPPIST-1e to 1g, exist bang in the middle of the system’s habitable zone. The star is an ultra cool, M-type dwarf star. These kind of stars are not very bright and emit

energy mostly in the infrared. Therefore, their habitable zone lies very close. However, being so close to the star means that the planets are tidally locked: much like our moon, only one

side of the planet faces the star, while the other is in perpetual darkness. Such a scenario would imply that the tidally locked planets are hostile to life. The star-facing side would be

burned and barren, while the darker side would be extremely cold. But the presence of an atmosphere could fix that by redistributing heat.