# Do solar systems have to evolve in a galaxy?

Is it possible for a solar system like ours to exist and evolve apart from all other star systems and galaxies. Sort of like a stand alone solar system or rogue solar system for lack of a better name. Or is there some requirement that a solar system capable of managing and growing planets especially ones that will occur in the habitable zone originate in a galaxy?

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Probably not. You'd need a could of gas dense enough to contract under its own gravity. In the space between galaxies, matter is spread too thin for a solar-system sized cloud to exist. A much larger cloud could be more diffuse and still collapse, but being much larger, it would collapse into a galaxy.

Even within a galaxy, stars seem to form when shock waves from nova or some other event initiates the gravitational collapse.

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Marc, thank you for your answer, but please see our help center for guidance on how to write a good answer for this site. Answers need better backing as explained in the link. – called2voyage Apr 11 '14 at 14:44

It may be possible for a solar system like ours to exist outside of a galaxy. This topic was discussed briefly in this episode of CBC's Quirks and Quarks where they were talking about extremely fast moving stars.

These stars are believed to have originally come from the centre of the galaxy but have achieved such huge speeds that they were flung out, and some that have been observed are going in such bizarre angles that they may actually have come from other galaxies.

The conclusion was basically, if such a solar system does exist, the view from the planet is probably pretty damn good.

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It would be interesting to know if a planet in a star's habitable zone could survive in the zone through the gravitational chaos of such a galactic ejection. My guess is no. – Marc Feb 14 '14 at 19:22
I think it could because it is really the sun of the planetary system(sun just means star with planets orbiting it) that makes the planet survive, not the galaxy. – Caters Oct 8 '14 at 16:01

Stars form in star-formation regions, which itself develop from molecular clouds. The paper "The Minimum Mass of Molecular Cloud Cores" [1] claims that there is a minimum mass and density for a molecular cloud to turn into a star-formation region and they claim it's $10 \times M_{sol}$ with the cloud having a core diameter of $0.1\; pc$.

So if there were regions satisfying these properties elsewhere you could say that star formations is possible, though I can't quote a source which has found such a region yet.

Edit: Apparently such regions were found [2] between M81 and M82, although their mass is quite higher that the theoretical low limit; the mass is supposed to be $10^{4.3} - 10^{6.5}\;M_{sol}$.

1. Kamaya, Hideyuki. "The Minimum Mass of Molecular Cloud Cores." The Astrophysical Journal Letters 466.2 (1996): L99.; http://iopscience.iop.org/1538-4357/466/2/L99/fulltext/5258.text.html
2. de Mello, D. F. "Blue Blobs: Star-Forming Regions Outside Galaxies." Revista Mexicana de Astronomía y Astrofísica 35 (2009): 211-212.; http://www.astroscu.unam.mx/rmaa/RMxAC..35/PDF/RMxAC..35_ddemello.pdf
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Evidence for stars evolving alone outside galaxies is very hard to get. And for a good reason: those stars are very hard to detect.

Galaxies are quite easy to observe: one of the reasons for that, is that the surface brightness of a galaxy does not change with the distance (a good explanation here: http://mysite.verizon.net/vze55p46/id18.html, 5th paragraph) (Note: this is not true anymore for cosmological distances because the expansion plays a role, but this demonstration is usefull for the nearby universe in which we focus now).

However, observing stars is very hard as they do not have a surface brightness: the light we receive evolves in 1/radius2. Do stars outside galaxies exist? Yes: we have evidence for intergalactic stars. See here for example: http://hubblesite.org/newscenter/archive/releases/1997/02/text/ which I think was the first discovery (anyone can back me up?). Or see this more recent paper about "rogue stars" evaded from our Galaxy: http://lanl.arxiv.org/abs/1202.2152. Those findings are not surprising as we were expecting that some stars are sometimes ejected from galaxies by gravitational forces. Can these stars carry on their planetary system with them? On this question I am not an expert, so I won't give an answer.

That was for stars created inside galaxies. What about star formation outside galaxies? This is a recent subject of discussion but we have growing evidence that this could be possible. Here is a paper about the discovery of intergalactic HII regions, which are regions of star formation outside galaxies: http://arxiv.org/abs/astro-ph/0310674.

Therefore we showed that: 1/ there are stars outside galaxies 2/ there is evidence for star formation outside galaxies.

In the first case, it may be possible that these stars carry their planetary systems with them. In the second case, it sounds plausible that the forming stars will build their torque of material which might give birth to planetary systems. There is however no evidence yet for this (to my mind).