Our invented planet is unlikely to be the only one orbiting its sun. This section discusses others within a solar system.
In SF, interstellar travel is virtually a given, whether that’s within a solar system or between systems and galaxies. In fantasy, other planets are seldom mentioned unless there’s an event of some significance like a conjunction of moons, stars, the sun, and planets. This might be an oversight, for magic or other supernatural means could allow characters to move between worlds (and moons). If a magic portal can get people from one place to another, the difference between traversing two sides of a planet and between two planets might be negligible for all we know. It could take tremendous power from a wizard or we can decide the ability has become commonplace. Perhaps magical doorways have been created, the magic imbued within them so that ordinary people can use them, just like a technology. We could similarly have magic-powered spacecraft.
When inventing other planets in a system, we should have rocky planets like Mercury, Venus, Earth, and Mars closer to the sun, and gas giants farther away; temperatures are too high for gas planets to form close to the sun. Saturn isn’t the only planet in our system that has rings; they are just more pronounced, so don’t be afraid to do this.
There’s a habitable zone that’s not too far from or too close to the sun, where Earth-like life can develop. Only so many planets can fit within this zone, limiting the number of habitable planets. With a yellow star like ours, this is two to three planets. With a red dwarf sun, scientists concluded that a maximum of five could occur, and closer to the sun, because a red star is cooler. With trillions, not billions, of years for life to develop, life might flourish on more than one world. Life would likely be quite different from Earth, however. Decide if you need more than one habitable world.
Determine how many planets are in your solar system, their names, order, type (rock, gas, ice), and impact, if any, on your world. Some will be visible to the naked eye, though this depends partly on luminosity, a measure of reflectivity of the sun’s light. These planets can be mentioned for added realism, making our main world seem less like an island. Planets are rarely visible; significance is often added to their appearance, particularly in less advanced worlds.