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In many science fiction stories, we hear the term star system that mostly refers to a star and its planets. This is often used interchangeably with the term solar system. But after some research I understand this:

Planetary system refers specifically to anything orbiting a star that is not a star itself. According to Wikipedia, this generally refers to the planets, but may include other objects like asteroids and dwarf planets. It does not refer to the star itself.

Star system refers to any system with two (binary) or more (multiple) stars, but only the stars themselves, not planets or anything else.

Solar system generally refers to our own Solar system but there are situations where "solar system" (lowercase) refers to a star and the planets or other objects orbiting it (as long as something orbits the star).

So, is it incorrect to use the term solar system/star system to refer to a star and its planets?

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I think it's largely a matter of personal preference. Some people will want you to be technically correct and only use "solar system" to refer to the Sun and the objects that are under its gravitational influence. Wikipedia defines it as such:

The Solar System comprises the Sun and the objects that orbit it, whether they orbit it directly or by orbiting other objects that orbit it directly.

NASA says that the term is more general:

A solar system refers to a star and all the objects that travel in orbit around it. Our solar system consists of the sun - our star - eight planets and their natural satellites (such as our moon); dwarf planets; asteroids and comets.

As an astronomy enthusiast, I do differentiate between "stellar system"/"star system" and "planetary system". I'd use "planetary system" to refer solely to the planets in a system, as well as their moons, while I'd use "stellar system" to refer to the whole thing. That's just my opinion, though, and I'm unsure what professionals use. In casual conversation (especially with a non-expert), you can probably use whatever term you want. If you're using it formally, you should probably do some more research. And while you're at it, check whether or not you should capitalize "Solar" and "System" in "Solar/solar System/system". If people nag you about terminology, they'll nag you about capitalization.

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  • $\begingroup$ Of course, I always capitalize "Solar system", just like Earth, the Moon and the Sun when referring to ours, because they're names. Normally I'd use star system or solar system to refer to the star and its planetary system. But I've also read that the term "single star system" exist which refers to a system with a single star, so our Solar system would be a single star system as far as I understand. $\endgroup$ – N van Oosten Nov 13 '14 at 19:34
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"we have detailed knowledge of only one planetary system, the Solar System. Data from other planetary systems around both main sequence stars and pulsars ..."[1]

In the above quote, one can see that the authors say the Solar System is a planetary system and in the very next sentence they say "planetary systems around...stars" so the question of whether the term planetary system includes the central star or just the planets seems to be a linguistic issue and it changes, even by the same authors, from one sentence to the next depending on context.

Linguistics aside, from an astronomy perspective it is clear that one cannot sensibly discuss a planetary system without reference to its central star. Everything about a system depends on the star. The concept of habitable zone doesn't even make sense without the star.

Consider a red giant star that has engulfed its inner planets. If the planets are massive enough they will survive. e.g. Kepler-70b. How can one exclude the star from the planetary system if there are planets inside the star, except by linguistic hair-splitting which as the opening quote shows would be contrary to actual usage which is context-dependent.

[1] Planetary Sciences by Imke de Pater and Jack J. Lissauer

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  • $\begingroup$ Don't forget Orphan planets i.e. planets not orbiting around stars. $\endgroup$ – Niccolò Nov 14 '14 at 2:04
  • $\begingroup$ This is another definition that confuses me, because I automatically assume planetary systems are just the planets and other objects orbiting the star. But of course we only know of one planetary system, (in) the Solar system, because there are very few exoplanets that are confirmed. $\endgroup$ – N van Oosten Nov 14 '14 at 8:21

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