Have a look at
this thread from the old forum. Here we talked about rogue planets before those where included in Space Engine. I think there is a lot of usefull information there
Making it short:
Lower bound: 1,8 Jupiter-mass planemos per main sequence star of the galaxy (that means around
450 billion giant planemos in the Milky Way at least).
Upper bound: 100.000 planemos (ranging from the mass of Mercury to 10 Jovian masses) per main sequence star of the galaxy (that means around
25 quadrillions of planemos in the Milky Way).
I don't really know the numbers that SpaceEngineer used in the end for the program but you can easily make an estimate. I located myself in 15 different points of the galaxy and made a search for 10 ly radius (that is a sphere containing a volume of 4189 ly[sup]3[/sup]). The mean number of planemos in those searches is 40 planemos. To calculate the volume of the galaxy we consider a low estimate; imagene the thickness of the disk is just of 2.000 ly and that there is no bulge, no halo, no clusters, with the disk extending 150.000 ly across (it probably gets further), you have that the Milky Way galaxy has a volume of around 35 trillion ly[sup]3[/sup]. So adding up my estimate is that
Space Engine has at least 340 billion planemos in the Milky Way. So I think that probably Space Engineer choosed the lower estimate for the procedural generation of planemos (I'm also curious about the reason, also because this estimate is for jupiter-mass planemos).
You should check the journals where the articles I have linked were published because a rigorous talk about this includes the reputation of the journal and if it's a peer-review one.
By the way, if you are interested for your video, check the links I provided in the old forum thread. You can estimate the number of interstellar asteroids as 500 quintillion and interstellar comets as 500 sextillion in our galaxy (a crazy number). Interstellar space is full of objects that travel acros the vast void.
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Sorry for asking but woudn't be better to have a thread for planemos instead of creating one specifically for this very precise question? I mean, threads should be more than specific questions so the forum gets enriched and it doesn't fill with death threads. Maybe next time