A discussion over at the SJ forum, made me dig out some notes on the Mystara Solar System IMC. Although the term Crystal Sphere is used below, this is something I use only in a modified form. More on this later.
The Mystaran Solar System
by Håvard Faanes.The Sun
RW Equivalent: The Sun
Satellites: None
Inhabitants: Fire Elemental Ruler. Fire elemental creatures.
Notes: The Mystaran Sun is a gateway to the Elemental Plane of Fire
1st Planet
RW Equivalent: Mercury
Satellites: None
Inhabitants: Fire elemental races, having escaped the slavery of the Sun
2nd Planet
RW Equivalent: Venus
Satellites: None
Inhabitants: Dinosaurs and Lizardmen
Notes: Will soon be settled by Heldannic Knights
3rd Planet: Mystara
RW Equivalent: Earth
Satellites: 2: Matera (Locations of Pandius, the Immortal City) and Patera (Invisible. Inhabitants: Rakasta, Pachydermions)
Inhabitants: Humans, Demihumans, Humanoids and many others.
Notes: Planet and both moons are Hollow, containing self-sustained civilisations.
4th Planet
RW Equivalent: Mars
Satellites: 2
Inhabitants: Insect Men, Humans (Nithian descended)
Notes: Nithian Ruins. Will soon be colonised by Thyatians and Alphatians (See "Spillworld" from DotE)
5th Planet: Damocles
RW Equivalent: Asteroid Belt
Satellites: None
Inhabitants: Pyrithians (Similar to Emerondians).
Notes: Home of an Advanced Race. Blackmoor Ruins can be found here.
6th Planet: Old Alphatia
RW Equivalent: Jupiter
Inhabitants: Old Alphatians
Satellites: 63
Notes: The "satellites" are actually contained within the breathable atmosphere of the gas giant.
7th Planet
RW Equivalent: Saturn
Satellites: 47
Inhabitants: Air Elementals. Flying Races, Faenare
Notes: The "satellites" are actually contained within the breathable atmosphere of the gas giant.
8th Planet
RW Equivalent: Uranus
Satellites: 27
Inhabitants: Dwarves, Earth Elementals, Giff
Notes: Contains many gates to the Elemental Plane of Earth.
9th Planet
RW Equivalent: Neptune.
Satellites: 13 (Floating Continents)
Inhabitants: Sea Creatures. Water Elementals
Notes: Waterworld. Contains many gates to the Elemental Plane of Water.
10th Planet
RW Equivalent: Pluto
Satellites: None
Inhabitants: Frost World. Ice Demons
11th Planet: Charon
RW Equivalent: N/A
Inhabitants: Demons, Devils
Satellites: None
Notes: Contains gates to the Entropic Planes
Other Worlds
Alpha Centauri: Nearest Civilisation
Epsilon Eridani: Nearest Magical Civilisation
Core Worlds: Space Federation, Planar Spiders, Adapters, Mujina, Oards
Aelos: Another Plane, or Crystal Sphere?
Old Blackmoor: Could be another Planet, I suggest placing it within the Hollow World.
LaTerre: Alternate Prime Plane