Earth - The Human/UNSC Headquarters (HQ)/Homeworld.
Luna- Where the Luna OCS Academy is located
Mars - A major site of the Interplanetary War and later during the Human-Covenant war it was the site of the Battle of Mare Erythraeum. Has been terraformed to have an Earth-like climate.
Jupiter - A gas giant, Jupiter is the largest planet in the Sol System.
Io - The site of a UNSC listening post. On October 20, 2552, the listening post on Io detects 15 Covenant ships inbound under the command of the Prophet of Regret.
Europa - It contains a UNSC base and a substantial colony. Spartans Linda and Will were once assigned to a mission on this satellite.
Callisto - The Second Largest Jovian Moon. The site of the signing of the Callisto Treaty and a number of UNSC bases.
Ganymede - The EVA Armor was developed here, home to a number of colonists.
Saturn - A gas giant known for its clearly visible surrounding rings, unlike the other gas giants whose rings are rather dim. Saturn is the second largest planet in the Sol System. Its practical uses to the UNSC are limited, and therefore it is mostly just a tourist destination.
Uranus - An icy gas giant about twice as far away as Saturn from Sol. Its distance from Sol makes it of little use to the UNSC.
Neptune - Furthest away from Sol, it is another icy gas giant similar to Uranus.
Ceres - The largest asteroid to have ever been discovered by Humanity.
Eris - Currently the largest Dwarf Planet known. It is almost 10% larger than Pluto in diameter.
Pluto - The Pluto Slipstream Monitoring Station Democritus is stationed in orbit around it. On November 3, 2552, the Monitoring Station detected Cortana's signal in Slipspace with Dr. Halsey's message to Lord Hood. Pluto has three moons: Nix, Hydra, and Charon.
Haumea - The most notable feature is its highly elliptical shape.
Makemake - The second most distant dwarf planet, its lack of moons mean it and Ceres are the only two known dwarf planets orbiting Sol without moons.