I am building a planetary system, but I always thought how many planets should I have in this system. When you look at our solar system there are 8 planets. Other exoplanetary systems can have up to the same amount. I am here wondering whether I should have the same amount or have more than what our solar system has. Eight seems relatively good but I think ten or more would be a better number. The definition of planet i am using is the same definition most people use and my system won't include planets on the same orbit neither binary planets.

How many more planets can a star have naturally on average?

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    $\begingroup$ Keep in mind: The definition of what a planet is is completely arbitrary. If you are designing a new star system, assuming you have someone living in there, their definition is 100% going to differ. What I'm saying is that you can have the exact same solar system as our solar system and still have 10 planets in it. $\endgroup$ – Raditz_35 Jun 23 '18 at 1:10
  • $\begingroup$ Actually the first few asteroids that were discovered were considered to be planets and counted as such, so there were once over 10 planets listed. And of course it is now estimated that there might be hundreds of dwarf planets in the outer solar system, which is one reason why Pluto was declared to not be a planet. It would be perfectly possible for some people to claim that Star System X has 5 planets and people with other definitions to claim that it has 457 planets. Some astronomers believe all spherical bodies - planets, dwarf planets, & moons - should be counted as planets. $\endgroup$ – M. A. Golding Jun 23 '18 at 1:28
  • $\begingroup$ You might try this blog about planetary systems. It may not directly answer your question, but it will give plenty to think about planets. planetplanet.net $\endgroup$ – a4android Jun 23 '18 at 2:21
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    $\begingroup$ @Raditz_35 The definition of a planet is not arbitrary. There is a formal definition by the International Astronomical Union. $\endgroup$ – StephenG Jun 23 '18 at 5:01
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    $\begingroup$ @StephenG The formal definition by the International Astronomical Union is not legally binding on anyone, is not totally logical, is different from some earlier definitions of planets, and is challenged by some astronomers who advocate other definitions of planets. Thus it is quite possible for different characters in a story to have different definitions of planets and to count radically different numbers of planets in the same star system. $\endgroup$ – M. A. Golding Jun 23 '18 at 15:34

The number of total planets doesn't really matter so much as how many of each type there are, and the moons can come in handy too.

You have:

  • Rocks that are too close to the star (e.g. Mercury)

  • The "habitable zone" (e.g. Venus, Earth, Mars) - this is where you'll have your civilizations

  • The "gas giants" (e.g. Jupiter & Saturn) - these are your asteroid vacuum cleaners that make it possible for the planets in the habitable zone to evolve without constantly being bombarded. Some the moons here are usable with substantial protective terraforming (Europa, Ganymede, Titan)

  • The outer planets (Uranus & Neptune) - these don't really matter

Bear in mind that living in space on a rotating space station is probably more feasible than living on most planets.


Hmm... In general, all known star systems with planets have fewer than eight known.

Now for those with habitable worlds, that number on average will probably be higher (having outer-system gas giants helps with deflecting meteors/comets from coming to the inner worlds). In addition, many systems probably do have several more, but we just haven't detected them.

I think eight is then a reasonable number (one can effectively add more "planets" by having planet-sized moons of a super-Jupiter type world -- even one in the habitable zone).

Also what type of star is this system's central sun? This could affect the number and type of planets.

  • $\begingroup$ Welcome to Worldbuilding, Majestas 32! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ – FoxElemental Jun 23 '18 at 21:41
  • $\begingroup$ I already took the tour! $\endgroup$ – Majestas 32 Jun 24 '18 at 3:22
  • $\begingroup$ What's the lower limit on exoplanets we can currently detect? I looked it up, and the Kepler telescope detected some moon-sized, but that's the Kepler, not common exoplanet detection? $\endgroup$ – Malandy Jun 24 '18 at 3:44

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