Sun, planets, moons, asteroids, comets.
All actual size with proper textures and surface deformations when available, with atmospheric modeling to the extent of sound propagation, density, gravity, and possibly pseudo-weather effects. Each (except the sun) has a semi-transparent icon superimposed over it whenever the player is oriented in the right direction (e.g. from the surface of another planet).
There is no way for the user to reach the asteroids, so they are only displayed for the end-game cutscene. Comets may be shown with their orbit lines.
An interstellar surveyor who needs to collect samples of human activity in the solar system. A luminous centaur-like being, once known as Sagittarius, the Archer is fond of nectar and ambrosia (volcanic magma), has an Earthlike sense of hearing and is forbidden to visit Earth. The Archer glows when energized but gradually darkens (loses energy) over time. The Archer can replenished energy by spending time in planetary cores or on the sun; but absorbing too much energy from a planet destroys it and ends the game. He has a satchel for collecting samples and a bow that he uses to travel to other planets.
Artificial objects/materials in the solar system that the user must collect. Examples: Teflon(tm) on the moon from the Apollo missions, optical lenses in probes, tear-resistant inflatable plastic on Mars (rover landings), copper foil, solar panels, etc.
Where the Archer keeps the samples he collects.
The bow is a tool for targeting planets; when the user presses the button to aim it, some of the Archer's energy is transformed into a luminous arrow. The user holds down the button to pull back the bowstring, moves the trackball to ail, and releases the button to fire. Striking a planet forges a link between it and the planet from which the Archer fired.
Turn into comets if they don't hit anything. If they hit the planet the user is on, they return energy to the planet and the user can go to the core to eat it back.
Created when the user strikes another planet using the Bow & Arrows, the link is a permanent beam of light that connects the cores of the two planets through space. When first created, the user is automatically taken across the link, but afterwards the user must touch the link to use it again. Once the user touches to the link, it works like an elevator: the user presses the up or down buttons to move along in either direction between four stages: orbit, rings (if there are any), the surface, and the core. Each time the user moves between stages, he is released from the link upon arrival. The user always arrives in orbit when traveling along a link to another planet.
When the user acquires all of the samples or runs out of energy, the character is shown turning into a sphere of energy that launches off into space, and the view switches to show the solar system gradually shrinking, the orbits proceeding at high speed and all of the connections visible. If the user found everything, it displays "Good Job!" and if the user ran out of energy it displays "You Failed". Also, if the user completely consumes the core of a planet, the planet explodes and then the view fades to grey and it displays "You Failed".
Gravity
Weather