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The trouble with fictional planets around real stars is that sometimes a real planet gets discovered orbiting that star.

Epsilon Eridani is one well-known example. It's the home of the Babylon 5 station, but the discovery of the real planet around that star would change the Babylon 5 story a bit.

This has happened to me recently. One system that I created a few years ago is a fictional system of planets around a red dwarf star. This system is not especially imaginative - it has a couple of small planets close in (including a habitable world), an asteroid belt, and a couple of gas giants orbiting farther out.

Then on the first of September, some scientists announced the discovery of a planet orbiting that star, Gliese 832, in their paper A Jupiter-like Planet Orbiting the Nearby M Dwarf GJ832.

Now I have some mixed feelings about this. Firstly, I'm going to have to make some changes to the system to accommodate this new planet. But on the other hand, I'm intrigued that the planet in question has some interesting similarities to the main gas giant in my fictional system.

My fictional planet:
Orbital period: 7.8066 years = 2851.4 days
Semimajor axis: 3.123 AU (corresponding to a stellar mass of 0.50 solar masses)
Eccentricity: 0.0052
Mass: 60.93 Earth masses (0.64 Saturn masses, 0.192 Jupiter masses)

The real planet as reported by Jeremy Bailey et alia in the above paper:
Orbital period: 3416 +/- 131 days (9.352 +/- 0.359 years).
Semimajor axis: 3.4 +- 0.4 AU (corresponding to a stellar mass of 0.45 +/- 0.05 solar masses)
Eccentricity: 0.12 +/- 0.11
Mass x sin i: 0.64 Jupiter masses (203 Earth masses)

That's why I only create planetary systems in fictional stars. So I don't have to worry about such thing.

For example: I'm creating the Solaris system, in which is a binary system with 2 dwarf stars: Solaris, K3V and Nemesis, M0V. Solaris system is not a truly extrasolar system, but an alternative version of our Solar System, a better one. It's our system that took a diferent destiny since it's formation to nowadays.

If you do the same thing, you won't have these problems.

I don't see it as a problem as such because the data seems to show that there are no large gas giants farther in. Thus, the main fictional planet is not affected at all.

This system actually looks like a nice place to live. It has a big gas giant at the right distance to keep the habitable zone from being harmed by comets, and appears to have no big gas giants close to the habitable zone to disrupt planets that orbit within it. A planet that close to its sun will be tidally locked to that star. That won't really be a problem for human habitation. Humans have already demonstrated great adaptability in where they choose to live.

For what it's worth, my fictional version of this system is here: Gliese 832

What's the problem with fictional planets existing along real ones around a real star?

Just imagine : you've written a story about fictional characters living in a real place where you've never been, say Berlin. Then one day you discover that there are actually real people living in Berlin.
Does this invalidate your story ?

It becomes an issue for making realistic systems. Let's say you make a planet orbiting the star HD 80606 at 0,5 AU, then outta nowhere, someone finds a planet orbiting that star, a real planet, on a very eccentric orbit (taking it from just a few hundredths of an AU out to ~0.8 AU). Then you've got a real problem. Your imaginary planet crosses orbits of a real planet.

Either boot the imaginary planet from Celestia, or ignore/destroy the real planet.

In his case with Gliese 832, it didn't turn out so badly.

Unfortunately, I started writing about 'real' exo-planets when only Peter van de Kamp was hunting them.

I was okay until a couple turned out to have red/brown dwarf companions instead of gas-giants. Some hasty ret-conning fixed those.

Then, the flood-gates opened and the HotJupiters appeared. They deranged the tidy hotrock(s) // habitable-zone(s) // ice-line giant(s) // Methane giant(s) plan. I down-sized a couple of gas-giants as error-bars shrank, then simply stopped trying to keep up. I've daringly included Epsilon Indi's brown-dwarf outriders in a recent tale, albeit subject to revision...