Goldilocks Planets

Earth is a “Goldilocks” planet. It’s neither too hot nor too cold for liquid water, which is a key ingredient for life. In other words, it’s just right.

Part of what makes Earth just right is its distance from the Sun. It’s in the middle of the habitable zone — the distance where temperatures allow liquid water to exist on the surface.

Astronomers have found several planets in other star systems that are in the habitable zone, and they expect to find many more in the years ahead.

But just because a planet is in the habitable zone doesn’t mean it’s actually habitable. Habitability involves many factors, including the planet’s gravity, the composition of its atmosphere, and the age of its star.

The star’s age is important because as a star gets older, it gets brighter. That changes the distance of its habitable zone. So a planet that started as an iceball could become more comfortable as the star gets older.

A recent study, though, found that a planet can quickly jump from too cold to too hot. As the planet warms, it takes a long time for its ice to start to melt. When it does, much of the water can quickly evaporate. Water vapor traps energy from the star, making the planet warm up in a hurry. It can get so warm that the oceans evaporate, and water vapor at the top of the atmosphere escapes into space.

So it takes a fine balance of temperature, along with the right mixes of gases in the atmosphere, to make a planet just right.

Script by Damond Benningfield