One is their limited scope. Currently, habitable zones are defined solely by distance from a star, but there are other factors that could determine whether or not a planet could hold liquid water. Venus, the hottest planet in the solar system, actually falls within the Sun’s habitable zone; Mars, which falls well outside the habitable zone, has substantial evidence (in the form of dry riverbeds and floodplains) that there was once liquid water on its surface (Tyson, 210). The difference between the two is atmosphere. Venus has a thick carbon dioxide atmosphere that prevents almost all of the incoming solar radiation from leaving again (Tyson, 210). It’s the same greenhouse effect Earth has, only turned up to eleven. Meanwhile Mars’s atmosphere, which is also mostly carbon dioxide, is extremely thin: 100 times thinner than Earth’s atmosphere (Sharp). Researchers from Caltech believe that, in the past, Mars had a thicker atmosphere with a better greenhouse effect, allowing for liquid surface water despite its distance from the sun (Redd). This atmosphere was lost over time, leaving Mars’s surface a frozen, inhospitable wasteland where once there could’ve been the possibility of water, and therefore, of…