What Is the Habitable Zone (Goldilocks Zone)?

The habitable zone is the range of distances around a star where a planet's surface could be just the right temperature for water to stay liquid. Because it's neither too hot nor too cold, it's also called the Goldilocks zone.

What is the habitable zone?

All life as we know it needs liquid water. So when astronomers look for places beyond Earth where life might exist, they first check whether a planet is in a spot where water on its surface would neither freeze nor boil away. Too close to the star and water evaporates; too far and it freezes solid. The comfortable stretch in between is the habitable zone.

The nickname Goldilocks zone comes from the fairy tale in which a girl named Goldilocks picks the porridge that's neither too hot nor too cold. Being in the habitable zone doesn't mean a planet has life. It only means the planet meets one of the conditions for having liquid water.

Brighter stars push it farther out

The amount of light a planet receives drops with the square of its distance from the star. So to get as much light as Earth does, a planet has to be farther from a brighter star and closer to a dimmer one. Work it out and the distance to the habitable zone scales with the square root of the star's brightness.

• For a star 4 times brighter than the Sun, the habitable zone is about 2 times farther out.
• For a star 1/100 as bright as the Sun, it moves in to about 1/10 the distance.

For the Sun, the habitable zone is estimated at roughly 0.95–1.67 AU. 1 AU is the average distance between Earth and the Sun. The boundaries shift quite a bit depending on which climate model is used, so different studies give slightly different values.

The frost line and giant planets

Beyond the habitable zone lies another important boundary: the frost line. While the planets were forming, the region beyond this line was cold enough for water to exist as grains of ice. In the Solar System it sits at about 2.7 AU, near the asteroid belt between Mars and Jupiter.

Beyond the frost line, ice as well as rock becomes solid building material, so the seeds of planets grow bigger, much faster. Once a core gets heavy enough, it can pull in the surrounding hydrogen and helium gas and become a gas giant like Jupiter or Saturn. That's why all of the Solar System's giant planets lie beyond the frost line.

Distance isn't enough

Venus is often considered to sit near the inner edge of the habitable zone and Mars within it, yet neither has liquid water on its surface today. That's because many other conditions have to line up besides location.

• Atmosphere and greenhouse effect: Too thin an atmosphere can't hold in heat, while too thick a one can trigger a runaway greenhouse effect like Venus's.
• Magnetic field: A planet's magnetic field blocks particles streaming from its star, protecting the atmosphere from being stripped away.
• Plate tectonics: On Earth, moving plates cycle carbon, helping keep the climate stable over long periods.
• Tidal locking: Dim stars have habitable zones so close in that a planet may end up with one side always facing the star, which can create extreme temperature differences between the day and night sides.
• Planet size: If a planet is too small, its weak gravity can't hold on to an atmosphere for long.

The habitable zone moves, too

A habitable zone isn't fixed forever. Stars like the Sun slowly brighten as they age, so their habitable zones gradually drift outward. The Sun is thought to have been about 30% dimmer when it was born than it is today. That's why scientists also study the continuously habitable zone, the region that stays inside the habitable zone for a long time. After all, life may need hundreds of millions or even billions of years to appear and develop. In the distant future, as the Sun grows brighter, Earth will one day cross the inner edge of the habitable zone.

Exoplanets in the habitable zone

• TRAPPIST-1 e, f and g: The red dwarf TRAPPIST-1, about 40 light-years away, has seven Earth-sized planets. The star is only about 0.05% as bright as the Sun, so its habitable zone is very close in, and planets e, f and g are considered likely to fall within it.
• Proxima Centauri b: A planet that orbits the nearest star to the Sun about every 11 days. It lies within the habitable zone, but because its star flares so often, it's still unknown whether it has kept an atmosphere.
• Kepler-186f: One of the first Earth-sized planets confirmed in another star's habitable zone. It's been suggested that it could have water, but its atmosphere is still unknown.

All of these planets simply lie within habitable zones; whether they actually have water or life has not been confirmed.

In Planetica

When Planetica creates a system, it calculates that star's own habitable zone and frost line from the central star's brightness. Each planet's temperature comes from the star's brightness and its orbital distance, and Planetica also shows whether a planet is inside the habitable zone. Compare a system with a bright star to one with a dim star and you'll see firsthand how much the habitable zone shifts. Try changing temperature or atmosphere in the Adjust tab to explore which conditions besides distance change how a planet looks.

Keep an eye on the frost line, too. Rocky planets tend to sit inside it, while ice worlds and giant planets tend to settle beyond it, so it's a great clue for understanding how your system is laid out.

Read more

• 9 Types of Planets
• Star Types and Colors: Spectral Classes O, B, A, F, G, K, M
• What Is the Habitable Zone (Goldilocks Zone)?