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By some means, artificial or naturally occuring, a human inhabited star system has been flung from the Milky Way. Prior to being ejected from the galaxy, the system bears at least one Earth-like habitable planet.

Following the ejection, would this system still be stable? Would the force required to eject the system cause the orbits to collapse? Would radiation between galaxies make life on this planet impossible?

And the most important question, would the planet be able to support a civilization until reaching a nearby galaxy?

[Note, this question is not about a rogue planet - ie, a planet without any star - but a rogue star system with a habitable planet around it.]

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    $\begingroup$ HDE's answer is good, but I just wanted to mention that even in the best possible case it would take many millions of years for the star to reach another galaxy (more likely many billions), and that's a very long time for any civilisation to last. (Humanity itself is only a couple of million years old and we've had loads of civilisations in that time.) $\endgroup$
    – N. Virgo
    Sep 6 at 5:59
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    $\begingroup$ You'll note that respondents ignored most of your questions. That's because asking more than one question is literally a reason to vote to close a question (VTC:Needs More Focus) but we're lenient with new users. Good question, BTW, more unique than many. $\endgroup$
    – JBH
    Sep 6 at 5:59
  • $\begingroup$ Regarding radiation outside of galaxies, we don't have the data, so you'd better write this off as a non-factor, anyway heliosphere would be the dominant shield for the planet(s) from interstellar radiation. A civilization does not require the star system to have anything more than a star and a planet where life could be, and should that star be decently stable in itself, life, sentient life and civilization could well emerge on the planet. And yes, too many questions in one post, this is discouraged in here. $\endgroup$
    – Vesper
    Sep 6 at 7:15
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    $\begingroup$ The answerers have pointed out the (at least) somewhat likely mishap that the planet's orbit around its star will be disrupted during the event causing the system to fly away from the Milky Way. Is it necessary for your story that the planet existed and was inhabited before being flung out. I am toying with the idea that what was being ejected was, in fact, a forming (or a freshly formed) solar system. The journey will take a long while. Plenty of time for the protoplanets to become planets, their orbits to stabilize, and for intelligent life to evolve before they reach the next galaxy. $\endgroup$
    – Jyrki Lahtonen
    Sep 7 at 8:55
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    $\begingroup$ @JyrkiLahtonen thank you for the idea of the planetary system being less formed. Gives me some ideas for the story. Think I'm going to go the route of the inhabitants being placed into some kind of stasis. $\endgroup$
    – WasatchWind
    Sep 7 at 21:06

4 Answers 4

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It could certainly be stable; it's possible for a star to be ejected from a galaxy and still have other objects be gravitationally bound to it. Observations of the hypervelocity star HE 0437-5439 indicate that it was likely originally a triple system. During an interaction with Sagittarius A*, two of the stars were flung away at a high speed bound to each other as a binary system. While they eventually merged after tens of millions of years, this was due to normal stellar evolution -- one swelled up into a red giant and engulfed the other.

A similar situation could apply here. Imagine you have an s-type binary system, where the planet is tightly bound to one star (so there are two stars and a planet, as opposed to three stars). During an encounter with Sgr A*, the planet and the star it's bound to are propelled to high speeds, eventually leaving the galaxy. Since hypervelocity stars can reach speeds of ~1000 km/s and the Galactic disk is ~100 kpc wide, it could leave the disk within ~100 million years -- less if the system is ejected at an angle out of the Galactic plane.

Of course, an encounter of this sort could very, very easily lead to disruption of the planet's orbit during the close pass to the black hole -- four-body interactions are, uh, not pretty -- but there's a chance that things could turn out the way you want. The bottom line is that invoking this scenario as a background to explain your rogue system is reasonable; the outcome you want is definitely a plausible one.

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Should be fine

Your safest bet for causing a rogue star would be a galactic collision event in which all the stars that pass through the equilibrium point of the 2 galaxies will shoot off in a more or less straight line. Most will eventually drift far enough one way or the other to fall back into one galaxy or the other, but a few lucky candidates will hit such an equilibrium that they could pretty much keep going for tens of millions of light years. If this is your mechanism for ejection, then the star will not necessarily encounter any spectacularly energetic event to be ejected. It will just go straight instead of slowly curving... so as long as it does not run into the shockwave of any colliding stars on the way out, it should be fine.

As for intergalactic radiation, this is also not an issue. There is plenty of that between stars inside of a galaxy to begin with. Solar wind creates a natural barrier around solar systems that keeps any sort of interstellar medium out.

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If your star travels at ~1000 km/s, you should reach Andromeda in about 600 million years. Give or take.

The sun has an expected lifespan of around 10 billion years, with about 5 billion years left. So it's definitely doable.

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  • $\begingroup$ Before that, Andromeda and Milky Way are expected to collide (and their relative speed is way lower), so your math seems off. $\endgroup$
    – Paŭlo Ebermann
    Sep 6 at 23:10
  • $\begingroup$ @PaŭloEbermann - You are right, I didn't take into account the movement of the galaxies themself. $\endgroup$
    – Magnus Wittstrom
    Sep 7 at 10:34
  • $\begingroup$ Completely messed up my calculations, it should only take about 600 million years. $\endgroup$
    – Magnus Wittstrom
    Sep 7 at 10:45
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Radiation would not be an issue with the star's magnetic field and the planet's magnetic field and atmosphere.

BUT it would be extremely difficult for the planet to remain in a stable orbit around its star whilst the star was being accelerated by a close encounter with another (probably much bigger) star.

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    $\begingroup$ (1) Radiation would not be an issue because there is very little of it in intergallactic space. (2) While it is true that a close encounter with another star might completely disorganize the orbits of planets, the answer would be very much better if the word close would be made more precise. For example, how close to the Sun would a star of the same mass as the Sun have to pass in order to have an appreciable effect on the orbit of Earth? $\endgroup$
    – AlexP
    Sep 5 at 17:28

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