Habitable Zone Research Paper

Earth's orbit within the sun's habitable zone means its temperatures are just right for life. But icy worlds located within their star's habitable zone may abruptly skip from too cold to too hot without going through a habitable stage, a new study finds.

The finding suggests that there may be fewer potentially habitable worlds than scientists previously thought, the researchers said.

Whereas Venus seems too hot for life and Mars seems too cold, Earth lies between Venus and Mars, where temperatures have the potential to be just right for its surface to possess liquid water. This "Goldilocks" zone is also known as the habitable zone, because on Earth, there is life virtually wherever there is liquid water.

Stars like the sun brighten over time. This raises the possibility that a planet or moon that starts out cold and icy

One is how much ice covers its surface. Ice has a high albedo, meaning it reflects much of the light that falls onto it back to space before the light can warm the planet's surface. In this way, the ice cools the planet, which leads to the formation of more ice and makes the planet even colder. The second factor is the amount of water vapor in the planet's atmosphere; water vapor is a greenhouse gas that traps heat, warming a planet, which leads more water to evaporate and makes the planet even warmer.

The albedo of ice plays a dominant role in the climate of icy planets. A 2004 study suggested that Earth escaped a possible "snowball phase" because the sun brightened over time and because volcanic activity on Earth released carbon dioxide and other greenhouse gases that helped the planet retain heat.

In contrast, many icy bodies, such as Jupiter's moon Europa and Saturn's moon Enceladus, do not release large amounts of greenhouse gases via volcanoes. As such, researchers wanted to see how these worlds evolved if they had only the light of their stars to help warm