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Is it possible for a sun-like star to hold on to about 16 planets? If so, where could the habitable zone be located at?

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  • $\begingroup$ You might get a better answer at astronomy.stackexchange.com $\endgroup$ – Mawg Jul 4 '17 at 11:57
  • $\begingroup$ The more planets you have the less likely one or two of them are gas giants that are large enough to shepherd your system of debris. A byproduct of that is also tossing out a bunch of the original planets. But without a shepherd or two, your real estate values are going to tank regardless of the location. $\endgroup$ – Mazura Jul 4 '17 at 15:19
  • $\begingroup$ The more planets you have the less likely it is to be naturally stable. It's theoretically possible to have a hundred or more planets, but the chance of that happening naturally is exceedingly slim. One tiny disturbance and the whole thing would crash together and obliterate itself in a chain reaction of debris. $\endgroup$ – tadman Jul 4 '17 at 20:52
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    $\begingroup$ @tadman Or combine into a smaller number of planets, in the manner of Earth and Theia. Which, incidentally, is also likely to cause property values in the vicinity to tank. $\endgroup$ – a CVn Jul 4 '17 at 21:28
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    $\begingroup$ @Mawg yeah, pretty much all stacks dump their "purely-hypothetical-what-if" questions here, and we are pretty much fine with it. $\endgroup$ – Mindwin Jul 5 '17 at 0:37
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TL;DR: Yes, you can have those, if you are willing to use a loose definition of "planet". Or you can keep it under/around 10 and meet the IAU expectations (hard science). As for the habitable zone, depends on your chosen star type.


Long version:

We know very few about planetary formation. What we know so far is that it is very hard for a star not to have planets (a radical change from the XX century).

But it is very hard to actually pinpoint the number of planets in a system.

The system with the most planets we know so far is HD1080 that has between seven and nine planets. We know that Sol has eight planets and a ton of planetoids (sedna, pluto, etc) so that is a good ballpoint number for "hey that's a lot of planets".

Maybe we could have one more planet between Mars and Jupiter if all the mass in the asteroid belt would congeal into a single body. Maybe one more if Pluto gathered the mass of the Kuiper belt. But this is speculation. More on planetary mass

Sixteen may be too much. Who knows. You could get away with that, but the star would have to be way bigger than the sun. Because of Kepler's third law, the orbits of the planets (calculate here) would be influenced by the star, and further planets would need a bigger gravitational pull to remain in orbit. And there are tons of problems with multi-orbit like resonance

The habitable zone depends on the star type. Here is a calculator..

As a P.S: don't throw dwarf planets in the mix (sorry Pluto) or you will get over 100 just in our solar system. (thanks Phiteros for pointing that out). Or do throw those in, just to meet your quota.

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  • $\begingroup$ I would just add one thing - you could have many pluto-sized planets. You might have to relax your definition from the IAU's a little, but such a configuration should be fairly stable. $\endgroup$ – Phiteros Jul 4 '17 at 4:20
  • $\begingroup$ @Phiteros reality-check won't let us call a certain space station "moon" just because we want to. But you start to relax your standards, and suddenly you have 100+ celestial bodies wanting attention $\endgroup$ – Mindwin Jul 4 '17 at 6:17
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    $\begingroup$ not a relevant comment but i will forever believe Pluto is the 9th planet! whatever the scientific community may declare! $\endgroup$ – Dev Jul 4 '17 at 10:23
  • $\begingroup$ Sorry - "the star would have to be way bigger" doesn't make sense. The AIU size minimum for a planet is pretty small - Pluto easily qualifies. The real limit is that each planet must clear its orbit. A small star would have a small accretion disk, which is cleared even by small planets. You'd just not get a Jupiter without sufficient starting mass, but that doesn't restrict the planet count. $\endgroup$ – MSalters Jul 4 '17 at 11:26
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    $\begingroup$ It's worth noting that Kepler (with which almost all of the known exoplanets have been discovered) typically can't detect planets smaller than three times* the Earth's mass (though some smaller ones have been detected). It is possible that some stars with large planet counts could have more planets that have not yet been detected... (*iirc - this is from a side comment in an answer on space.stackexchange that I haven't been able to trivially loacate again) $\endgroup$ – Baldrickk Jul 4 '17 at 15:51
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On 16 planets, yes it is possible, if Jupiter were smaller and didn't interfere with the asteroid belt you'd likely have another planet in that orbit. Though not easy it is possible that 7 more planets could be captured in the kuiper belt and clear their regions, but most would likely need to be captured from outside our solar system which would be a challenge. One or two yes, seven would not be common.

On the habitable zone, if your system were like our solar system then it can be argued we only have Earth as a habitable planet. But I'd argue if Mars were only a little bigger and had a substantive magnetic field it would be habitable too. Now if you really want to maximize the number of habitable planets you have to get creative. So instead of Earth we replace it with a heavy Jupiter, with 4 large moons in its orbit, all now within the habitable zone. Next replace Saturn or Uranus with a red dwarf star, essentially creating a binary star system which is actually quite common. Then the red dwarf can have at least one habitable planet in its orbit, again if you are creative you might squeeze two by having a super Earth with a super moon ie both 2.5 times bigger than our Earth and moon.

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  • $\begingroup$ Mars just needs to be bigger (in size or density). Then it would stay hotter in the middle for longer, giving it a better magnetic field; and have higher gravity, giving it a thicker atmosphere. $\endgroup$ – OrangeDog Jul 4 '17 at 10:25
  • $\begingroup$ @OrangeDog Atmospheric density isn't directly related to gravity (which in turn is related to mass). Look at Venus; pretty much the same mass and gravity as Earth (Venus is about 82% of Earth by mass, and has about 90% the surface gravity of Earth), but over 90 times the atmosphere, differing slightly depending on which exact metric you use. Compare this to Mars' 11% of Earth mass and 38% the gravity but far less than 1% the atmospheric pressure. Other factors, such as for example Mars' lack of a magnetosphere (allowing the solar wind to interact directly with the atmosphere) dominates. $\endgroup$ – a CVn Jul 4 '17 at 21:37
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The habitable zone depends on the star, not on the planets.

Cleared this, in principle it is possible that more than 10 planets orbit a star. The "trick" is in preventing the formation of gas giants like Jupiter and Saturn which, with their mass, account for most of the mass of the system (central star excluded) and also whipe the surrounding from other bodies. Mass-wise you could split the mass of Jupiter in more Mercury-sized planets and easily boost the planet count for your system.

The problem is that, material-wise, rocky planets rely on metals for their formation, and metals are rare in the space, while there is plenty of gases to form gas giants and you cannot have earth sized gas planets.

Since our knowledge of planetary system is pretty limited and surely not yet statistically significative, we cannot rule out such possibility.

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  • $\begingroup$ Note that even if you stripped Jupiter and Saturn of their gasses (and the tricky bits like metallic hydrogen), there'd probably be a left-over rocky solid core. It's not exactly certain, but the guess is the mass of the rocky core of Jupiter might be something like 10-50 Earth masses - easily enough for a couple more rocky planets (though still only ~5-15% of Jupiter's total mass). This sounds like a necessity for any gas giant formed in a solar system like ours - you need to get a lot of gravity before you start reliably capturing the gasses. $\endgroup$ – Luaan Jul 4 '17 at 12:20
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Sixten? Only sixteen?

Theoretically, a star can have up to four hundred and sixteen platens in the habitable zone. See here

enter image description here

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  • $\begingroup$ For all the technical details, see here: planetplanet.net/the-ultimate-solar-system $\endgroup$ – Sean Raymond Aug 19 '17 at 18:44
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    $\begingroup$ I am very curious, how could be such a system stable. I am nearly sure, it is perfectly unstable. $\endgroup$ – Gray Sheep Aug 19 '17 at 22:44
  • $\begingroup$ Sorry, it's not mine. You could contact the author, who is an astrophysicist (and quite impressive, as far as I can understand his abour me page. His website gives his email address as rayray(dot)sean(at)gmail(dot)com. It would be helpful to others if you post any conclusions here. $\endgroup$ – Mawg Aug 20 '17 at 8:58
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What's your time scale? A young star system early in its lifespan could have a few "extra" planets and proto-planets in meta-stable orbits. Relatively soon in geological timeframes, orbital resonances are going to stack up and pull things apart, but for the present everything is behaving itself. So the planets aren't going to crash into one another immediately, but I wouldn't set up shop there for more than a couple thousand years. And asteroid activity is still very high, so mind your flight vectors and keep particle shielding at full power at all times.

Of course, you're probably not going to see native lifeforms in a system like that, but some enterprising settlers might set up shop for research/economic purposes.

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