Astronomers have re-examined the possibilities for "habitable zones," or "Goldilocks zones," surrounding alien stars. Researchers found that habitable planets can exist in orbits closer to their parent stars than previously believed, because the solar energy required to start a runaway greenhouse effect is higher than was thought.
Too close to a star, an otherwise pleasant planet develops a runaway greenhouse effect, a feedback loop that leads to extremely high surface temperatures. The oceans boil, becoming thick clouds of vapor in the atmosphere. The thick atmosphere traps solar heat on the planet’s surface. [9 Exoplanets That Could Host Alien Life]
The greenhouse effect traps infrared rays (heat) from the sun in the atmosphere, raising surface temperatures. Despite having a similar size to Earth, the planet Venus is entirely uninhabitable due to its runaway greenhouse effect.
Over time, the sun’s brightness increases, pushing back the inner boundary of the habitable zone. Currently, the inner boundary is 95 percent of the distance from the sun to the Earth. In a billion years, the Earth may develop a runaway greenhouse effect like Venus'.