article_snippet: This is the belt around a star where temperatures are ideal for liquid water to pool on a planet's surface, and for alien life to exist.
article_snippet: The illustration shows the habitable zone for different types of stars.  
article_snippet: The green area is the habitable zone (HZ), where liquid water can exist on a planet’s surface.
article_snippet: The green area is too hot for liquid water on the planetary surface and The green area is too cold for liquid water on the planetary surface . '
highlights_snippet: Astronomers believe there are one to three planets in the habitable zone of each .
  