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Space Tourist
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Density of Planemos

06 Sep 2017 06:46

I have a question - insofar as planemos are concerned, real estimates of their density varies A LOT. There are estimates ranging from 1 trillion to 100 quadrillion planemos in The Milky Way. So what does Space Engine make of their density? How many are there, more or less, in a galaxy like the Milky Way? High estimate numbers, or low estimate numbers?

I presume that it is somewhere in between, seeing as they are very numerous, but it would be nice to know a concrete(ish) figure as I intend to make a video around them.
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Space Pilot
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Density of Planemos

06 Sep 2017 13:17

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 ly3). 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 ly3. 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.

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 :)
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Density of Planemos

07 Sep 2017 01:38

From the 'SolarSystemGenerator' section in the 'main-user.cfg'.
    ProbPlanemo             0.45        // probability of planemo (rogue planet) among all systems at the last octree level

If I understand this correctly, it means that about 45% of all star systems are planemos.
This agrees approximately with my counting.

It also means that you can change the probability.
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Author of SpaceEngine
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Density of Planemos

07 Sep 2017 01:57

JackDole is right, this parameter is a probability of the planemos (on the last level of the star octree, but it roughly equal to the total number of stars). This value is based on some scientific work where they estimated number of planemos per star based on observations. Maybe more recent work increased this number, but you should avoid to change ProbPlanemo to hight values (close to 1), because you will end up with red dwarfs replaced by planemos. To support quadrillions of planemos, change in the SE code is needed (increasing of the depth of the star octree, it also should change the traverse algorithm, because now it is based on the star system luminosity, but all planemos have zero luminosity).

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