Kerbal Star Systems Wiki

Jool is the 5th planet and nearest gas giant to Kerbol. It is the second largest planet in the Kerbol system, and is host to 4 of the largest moons nearby.

In-game Description

"Jool is particularly known for being a rather large, predominantly green planet. Kerbalkind has longed to visit it since it was first spotted in the sky. Philosophers reason that the swirling green planet must be a really nice place to visit, on account of its wholesome coloration.
If you look at Jool through a telescope, it is fuzzy."

Physical Characteristics

Jool is widely known as being a green planet, this is an unique color for the non-living planets, only Telim is known to possess a similar colour. It is a gas giant, meaning it has a large shell of Hydrogen and Helium surrounding a small possibly rocky core.


The green color of Jool is likely the result of certain compounds in its atmosphere. It is believed Jool gets its green color mostly from toxic potassium and chlorine gas in its upper atmosphere. However, these gases only makes up a very small part of its overall composition, which is hydrogen and helium.

Great Green Spot

Jool's Great Green Spot is one of the most prominent and famous features of the gas giant, and has been present on the surface since its discovery. The spot is a high-pressure storm driven by pressure differences between different gaseous bands in Jool's atmosphere. These bands are in turn driven by Jool's internal heating, and so these storms tend last for a very long time. The Great Green Spot itself has lasted for centuries, though some Kerbal scientists state that it's shrinking.

GreenSpot 0.8


Jool is known to have faint rings just barely visible to the naked eye. However, Kerbal scientists ensure that the rings do exist. They are probably leftover debris from the formation of the major Joolian moons. The rings are kept mostly by Alnym, a small red moon close to the rings.

Orbit - Rotation

Jool's orbit marks the boundary between the inner and outer Kerbol System. It is beyond the habitable zone for water, but tidal forces from Jool could allow the moons to host liquid water. Jool's rotation is also fast for a planet, a day on Jool lasting about 9 or 10 hours. As a result, Jool is slightly flattened out at the equator.

History of Observation

Jool is one of the 6 planets visible to Kerbals without a telescope. Jool's discovery is unknown, but the first records date back to ancient Kerbal mythology, where they named Jool after the ancient god of thunder, which seems a fitting name. In the year 3138, astronomer Kalileo Kerman pointed the first telescope in the skies and observed the 4 major moons of Jool, now known as the Kalilean moons. A few centuries later, astronomers observed the massive storms in Jool's cloud tops, storms that have been raging for centuries.


Jool is known for having 4 of the largest moons in the Kerbol System and 1 small moon. The 4 major moons are known as the Kalilean moons, named after Kalileo Kerman. The five moons are:


  • Equatorial Belt
  • Polar Region
  • Storms
  • Great Green Spot
  • Temperate Belts
  • Sub Equatorial Belts
  • Irregular Belt
  • Sub Polar Belts
    Jool Biome.png



  • Jool and Telim are the only planets that are green, but don't possess plant life.