Habitable Zone

The habitable zone of a star is the region around the star where the surface temperature of an orbiting planet or exoplanet would be in the right range of temperatures for water to exist as a liquid. This region is called "habitable" because all known life on Earth requires liquid water to exist. Universe Sandbox can estimate and display the size of the habitable zone for any simulated star.

The habitable zone displayed by Universe Sandbox is indicated by a green color. The region interior to the habitable zone (indicated in red) is too hot to support liquid surface water, while the region exterior to the habitable zone (indicated in blue) is too cold.

Related Properties & Settings as the Related Props and Settings

Props

• The size of a star's habitable zone depends on its Luminosity, which is located in the Overview tab of the star's properties panel.
• The habitable zone is measured from the surface of the star rather than its center, so the Radius of the star can also affect the location of the edges of the habitable zone.
• If an object is selected, the size of the habitable zone depends on the object's Average Albedo and Infrared Emissivity.

Settings

• The habitable zone for a star can be displayed using the Habitable button in the View menu in the bottom bar, or by using the Habitable Zones setting in the full View Settings Menu. The habitable zone can also be toggled on and off using the H hotkey.

Models

As of Update 30, if an object in the simulation is selected, the size of each star's habitable zone will be calculated based on that object's properties. The calculation determines the distance required between the star and the object such that the object's Equilibrium Temperature equals either the freezing point or boiling point of water (on Earth under standard conditions). The equation for calculating this distance is:

$d = \sqrt{\frac{L_*}{4 \pi \sigma T^4} \frac{1-a}{1-\epsilon}},$

where $L_*$ is the Luminosity of the star, $\sigma$ is the Stefan-Boltzmann constant, $T$ is the freezing or boiling point of water, $a$ is the object's Albedo, and $\epsilon$ is the object's Infrared Emissivity. This means that an object with a smaller albedo or a larger emissivity will have a larger habitable zone.

If no object is selected, the size of the habitable zone is estimated using the Kopparapu model Habitable Zone Calculator algorithm, which calculates a conservative and optimistic distance for the inner and outer edges of the habitable zone. Universe Sandbox averages these two distances to simulate the habitable zone of a star.