A galactic halo is a roughly spherical structure surrounding most galaxies. It has a lower density of stars.
Structure[]
A galactic halo is composed of the following:
- Free stars
- Groups of stars associated in Globular Cluster structures
- Galactic corona
- Dark matter.
Free stars most often have less luminosity then stars in the main galactic disk. Also, star formation is far more rare. It appears that most stars in a halo were formed long ago or are ejected from the galactic disk. These stars have strange trajectories. Retrograde or highly elliptical orbits had been observed, as well as stars having hyperbolic trajectories.
The galactic corona is a halo of gas and dust that surrounds the galaxy. It is more rarefied then gas in nebulas found inside the galaxy. Because of its low density, it can hardly form new stars.
Most galaxies are known to also host a halo of dark matter. The structure of this exotic matter is yet unknown. It is not known if and how exactly it interacts with conventional matter.
Stars and surrounding space[]
On one hand, we can compare the galactic halo with the far reaches of a stellar system, like the Kuiper Belt or the Oort Cloud. With less gravitational influence from the galactic center, stars can have much larger Hill spheres. With less flybys from other stars, planetary systems are less disturbed. This could imply that stars in the galactic halo might have more planets then stars in the galactic disk.
On the other hand, we don't know what dark matter is made of and how exactly it interacts with conventional matter. Galactic halos host large amounts of dark matter, which could influence stars in the region in unknown ways.
The galactic halo is far enough from the galactic core. It is known that most galaxies host hypermassive black holes in their core, which can act like Quasars. A quasar is bright enough to heat planets located at many light years away. Planets and asteroids in the galactic halo are far enough from a quasar, so they can retain volatiles more easily.
Another interesting aspect is that the galactic halo appears to be made mostly of hydrogen, with lass amounts of heavy elements. This could influence the structure of planets within the area. On the other hand, the galactic halo might contain many planets ejected from the galactic disk.
Habitability[]
The galactic halo is too far for telescopes to spot planets, but there is a high chance that such planets exist.
Most stars in a galactic halo are small, M-Type Stars or White Dwarfs. They can have large Hill spheres, but their habitable zones are very small. Also, there should be many free-floating planets, ejected from the galactic disk and from the globular clusters. They are Rogue Planets.
Planets formed in the halo should have a different chemical composition then planets formed in the galactic disk. With a big abundance of hydrogen and helium and low concentrations of heavier elements, we expect there to be many giant planets. Icy planets, similar to a Pluto-Class Planet, should be the second most often category. Rocky planets should be rare, since heavier elements are not found in large amounts.
Most rocky planets could come from the inner galactic disk and therefore be free-floating within the halo. The few rocky planets orbiting a star might be orbiting stars ejected from the disk or (in more rare situations) formed around native stars.
Based on these arguments, we can see that rocky planets suitable for terraforming should be in less numbers then in a galactic disk.
Environment[]
In a galactic halo, interstellar environment is different then in the disk of a galaxy. Galactic winds are slower, so that a solar wind can create a larger protective helioshell. On the other hand, the galactic corona can interact with the solar winds in yet unknown ways. Concentration of cosmic rays might be a bit smaller then in a galaxy, but still high enough to threaten life.
It is yet unknown what is the composition of dark matter. So, we don't know how it interacts with planets in a galactic halo.
Terraforming[]
As Galactic Colonization will occur, humans will visit and settle in other galaxies. Before that or during that phase, mankind will also settle on habitable planets located in the galactic halo.
We don't know yet what dark matter is made of. Theories, more or less accepted for the moment, vary from an exotic state of matter, nebulas, dead stars and black holes, to anomalies in the space-time continuum. It could be possible that we find clumps of antimatter or exotic types of matter that can be used to generate energy more easy or alter the space-time structure to travel through space faster. Probably the best way to find out what dark matter is, is, when technology will allow us, to go to the galactic halo and sample it directly.