Earth is only just within the Sun's habitable zone

If our planet were just a little closer to the Sun, a runaway greenhouse effect would render it unliveable, a climate model suggests. The simulation, which helps to define the inner edge of a star system’s 'habitable zone', drastically reduces the fraction of Sun-like stars that might harbour a rocky planet suitable for life, some scientists suggest. But others note that the model, although detailed, might be too restrictive because it applies only to Earth-like planets on which water is abundant.