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.

Properties

 * 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.
 * The star's Surface Temperature determines which model Universe Sandbox uses to calculate its habitable zone.

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.

Models
If the star's surface temperature is greater than 200 K and less than 72,000 K, 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.

If the star's surface temperature is outside this range, the inner and outer edges of the habitable zone are calculated from the star's radius, R, and luminosity, L, using
 * $$r_{inner} = R + 0.8 \sqrt{L}$$
 * $$r_{outer} = R + 1.2 \sqrt{L}.$$

This corresponds with a Solar System habitable zone from 0.8 to 1.2 AU.

Limitations
The ability of liquid water to exist on the surface of a planet depends on more than just the luminosity of the star. The energy heating a planet's surface can also come from sources like tidal heating, the planet's orbital elements will determine how much energy the planet receives from its star over time, and the temperature of the planet's surface will also be affected by its atmospheric properties.

For these reasons, two planets at the same distance from a star may have significantly different surface temperatures. One may even be habitable while the other is not. For example, the Earth is habitable and the Moon is not, even though they orbit at roughly the same distance from the Sun. A more accurate habitable zone calculation would depend on the planet as well as the star, so the single habitable zone displayed by Universe Sandbox is merely a rough approximation.