Most stars orbit in the Milky Way's galactic disc. But is it possible for one to orbit perpendicular to it?

Here on Earth since we're inside the galactic plane we can't get a good view of what the Milky Way looks like. But would the whole Milky Way be visible from a planet orbiting such a star?


2 Answers 2


The Sun and most of the other stars are in the bulging disk of the Milky Way galaxy, but about 1% of the galaxy's stellar mass is in the galactic halo. The halo also includes 50 globular clusters and about 20 satellite galaxies according to Helmi 2008: The stellar halo of the Galaxy. Here is a nice graphic:

enter image description here

Note that the Sagittarius Stream of stars is extremely close to passing the galactic poles. If you were on a planet next to one of these stars as it passed a pole, you would probably have a glorious view of the Milky Way stellar disk.

Bodies in the galactic halo don't necessarily follow the elliptical paths predicted by Kepler, so their orbits may not be consistently perpendicular to the galactic plane. Some stellar streams have extremely odd orbital paths like the Phlegethon stellar stream, referenced in this answer:

enter image description here

There is some speculation that the Halo stellar streams are the remnants of dwarf galaxies the Milky Way has absorbed. Indeed, Liang et al. say that:

the Galactic halo has complicated assembly history and it not only interacts but also [is] strongly mixed with other components of the Galaxy and satellite dwarf galaxies

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    $\begingroup$ I'm already jealous of people living close to the poles because they get to watch aurorae, of people living far away from light pollution, and of people living in the southern hemisphere because they can see wonderful celestial objects. Now I'm really jealous of aliens living in the Sagittarius stream. $\endgroup$ Jan 15, 2021 at 9:03
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    $\begingroup$ Well, you can think they are quite lonely. What is the average separation between stars in the galactic halo? I guess the closest neighbor to most of them is hundreds of ly away. $\endgroup$
    – ksousa
    Jan 15, 2021 at 16:27
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    $\begingroup$ Do we have a good sense of how big in the sky the Milky way would be from a hypothetical Earth-like planet in the Sagittarius Stream (at "typical" or furthest extent)? Andromeda sized? Moon sized? Filling-up-half-the-sky sized? $\endgroup$
    – R.M.
    Jan 15, 2021 at 17:00
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    $\begingroup$ @ksousa The galactic halo has a lot of substructure and star clumps that astronomers refer to as "overdensities". So while the average distance between stars in the halo is large, a star picked at random is likely to be in a star-dense region of space. This would be another great questions for Astronomy Stack Exchange. $\endgroup$
    – Connor Garcia
    Jan 15, 2021 at 17:47
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    $\begingroup$ @R.M. The above graphic is somewhat misleading about the distribution of stars in the Halo as it depicts them in a thin "shell" around the galactic disk. The orbits among stars in the Galactic Halo vary enormously. Some pass quite near the center of the galaxy and then zoom way out again, similar to a comet in our Solar System. Since their position in the shell varies, we would expect the apparent size of the Milky Way from their vantage point to vary accordingly. These stars are often called Population II stars. $\endgroup$
    – Connor Garcia
    Jan 15, 2021 at 17:57

Most stars in the galaxy are in the disc, but there is also a population in the galactic halo, these are in orbits at essentially random inclinations to the disc. There will be some that orbit perpendicular to the galactic plane.

Most of these stars are red dwarfs and very old (more than 12 billion years old) and formed early on in the evolution of the galaxy. They are also "metal poor" so have very little of any elements except hydrogen and helium. As such they are unlikely locations for Earth-like planets or for life.

There are also some stars in the galactic halo that seem to have come from small galaxies that collided with the milky way. These tend to be more like disc stars.

Given the huge numbers of stars, I'm sure that there are some in the galactic halo that have planets from which the whole milky way can be seen.


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