A habitable zone (HZ) in astronomy is a region of space where conditions are best for life to form as on Earth. Planets in these areas are the most likely to have extraterrestrial life. The habitable zone may also be called the life zone, comfort zone, green belt or Goldilocks zone. One in five stars have an Earth-sized planet in the habitable zone. However, it is not known what proportion would have conditions suitable for life.