A globular cluster is a group of many stars (up to millions), bound together by their gravity and having a roughly spherical structure. Star density is far higher in center then at the border.
Their interstellar medium contains only small amounts of gas and no nebulas. So, formation of new stars is impossible. Also, stars with high luminosity, like O - type stars and B - type stars do not exist and the risk of a supernova is equal to zero. Existing stars seem to have stable orbits.
A globular cluster is kept together by its own gravity. Stellar density is very low at the border (up to the density seen around the Solar System). At an average distance from the center, stellar density rises to a star at every cubic light year. In the center, stars are found at 30 to 150 AU one to another. In some clusters, central black holes have been detected, but they are not massive enough to keep the system bound together. This automatically implies that stars in the core are not orbiting something, their movement is complex and influenced by their combined gravity. The very small radius of a star, compared to the high distances between stars, explain why stellar collisions are rare.
Distant stars seem to follow orbits around the core. However, it their orbits are highly elliptical, as they return to the core, since the many gravity interactions they will face, the future of their orbits are completely unknown.
It is questionable if planets can orbit stars in a globular cluster. Suppose the Sun were a star inside a cluster. Given the fact that stars are usually at 30 to 150 AU away one from the other, the Kuiper Belt will be instantly removed from orbit. All gas giants will be perturbed and in the end will be ejected. However, the probability that a star will pass at only 1 AU away, is high and it is a certain thing if we wait a million years. Such an event can send all rocky planets on hyperbolic orbits.
What planets might still be orbiting stars in the core of a globular cluster, are those orbiting very close: Hot planets and in case of M - type stars there should be safe orbits for a Habitable Zone Planet. Also, if a star is in a stable orbit at the outer part of the cluster, it can host planets easily.
Stars in globular clusters show low metalicity, This automatically means that there are low amounts of heavier elements. While there might be enough hydrogen and helium to form gas giants, there will be moderate deposits of oxygen, nitrogen or carbon, to form a Carbon Planet, to create water for an Oceanic Planet or to resemble a Pluto - Class Planet. Heavier elements, needed for the majority of rocky planets, will be scarce. So, there will not be many rocky planets.
Rough planets Edit
A Rogue planet is a planet that does not orbit a star. Inside globular clusters, there might be a lot of rough planets, ejected from their previous orbits. However, during a flyby, usually the energy is transferred from the larger to the smaller object. Stellar flybys could eject many of these planets from the cluster. Still, there should be many of them wandering inside the cluster, far more then the planet that would orbit stars.
If the planet is moving through the core, it will be illuminated by passing stars. Usually, it will be at over 30 AU from a star, but from time to time it can come much closer, op to less then 1 AU. Then, as it moves away from the core, where distance between stars increases to over 1 AU, the planet will receive far less light and heat and will be sent in an era of cold and darkness. Given the relatively small speed of celestial objects, compared to the size of a globular cluster, it will take many years, even millennia, for a planet to dramatically change its position up to a point where its climate will change dramatically.
Terraforming and colonization Edit
Since the vast majority of planets would had been ejected from orbit, rough planets and rough asteroids should be the best destination for settlers.
Large scale Industrial colonization would not be so feasible, since globular clusters show to have a low density of heavy elements. Still, it would be better to extract and transport raw materials from nearby then to bring everything from the galactic core.
Each rough planet is moving on a complex path, influenced by the presence of nearby stars. The planet might come very close to a passing star for a short period of time, but most of the time will stay at high distance. Since distances change, so does the amount of light and heat received. The planet will move away from a star, but will get closer to another one. On a short timescale, changes in temperature and luminosity will be manageable. However, on a longer timescale, everything will change.
Terraforming might be possible, if there is enough light for plants to survive. However, the planet will not be habitable forever. We might never find a rough planet habitable for a million years, but for 1000 years, we can find one that will only need little adjustments.
Life in a globular cluster Edit
Once planets are terraformed and settlers move to their new homes, they will be surprised to see some major differences compared to their parents who live in the galactic disk. The first thing that will be amazing, is the sky. The planet will be illuminated by many suns, that will lurk fade in the sky. There will not be a day and a night, but depending on how many stars can be seen and how bright they will be, there might be a shiny day and a darker day. At every moment, there will be a few suns visible in the sky and the sky will be always blue.
The second major difference is about the solar wind. Since here are many stars and each one has its own wind, all winds are interacting. Without the gravity of a single star, the planet will certainly have its internal dynamo active. As solar winds interact with the many magnetic fields, huge auroras might form.
The third major difference is about the way you define a solar system. Inside the Solar System, once the planets and moons are terraformed and colonized, Trade Routes will form between them, following the best planetary alignments. Spaceships will travel after a schedule. People will be connected one with each other. Connections to other solar systems will be much more difficult. Not the same will happen in the core of a globular cluster. There might be hundreds of rough planets, maybe 30% of them terraformed. From one planet, you will have other planets located throughout the space. As they move, some might come closer, facilitating trade, while others might move away forever.
In a solar system, planets are usually orbiting in the same plane. It is somehow a 2D world. However, for many of us, it is a 1D world. If you ask someone to draw the Solar System, you will see the Sun in one point, then all planets placed in a row, from Mercury to Pluto, even if in fact they orbit and are never found in this alignment. In a globular cluster, it will be impossible to have even a 2D representation of nearby planets, it must be a 3D scheme and it will change over time.
Another difference is that any terraformed planet will be a Short Lived Earth, unless major maintenance work is done. It will come closer or further to stars. For a short time, the planet might get very close to a star, but then, it can get far to a point where light is too dim for plant life. Maintaining a terrafomed world will be an expensive task. The planet will require sometimes to change its trajectory, an action that might be very expensive.
Nevertheless, planets in a globular cluster can be terraformed and can offer very interesting and unique conditions for future settlers.