Re: Do stars and solar systems exist outside galaxies?

Until fairly recently, many astronomers would have said "probably not". However, there have been a number of discoveries in the past few years that demonstrate that there are many stars outside of galaxies. This image shows that when galaxies in big clusters interact with each other via gravity, they can produce long streams of stars that stretch far outside the parent galaxies. Fritz Zwicky suggested that these streams existed more than 50 years ago, but many astronomers doubted this claim. Other astronomers have found supernovae and planetary nebulae between galaxies in clusters, providing additional evidence for stars between these galaxies.

If there are stars outside of galaxies, it seems very likely that there are solar systems around some of these stars. Planetary systems around stars are very common, and they usually orbit at distances much smaller than the distance between stars. Even if the stars get pulled out of galaxies, any planets orbiting them will probably continue to orbit them because the planets get pulled in the same direction as their host stars.

There is another type of a star outside of the Milky Way (our) galaxy which almost certainly does not have planets around it. These are hypervelocity stars which orbited too close to the supermassive black hole at the center of the Milky Way and got ejected out of the galaxy at speeds of more than 500 miles per second. The huge gravitational tidal forces which ejected these stars almost certainly would have disrupted the orbits of any planets or protoplanets around these stars.