My 2 cents on some of the cosmology:
Gas giants that orbit close to their host star are called "Hot Jupiters", and can have a very fast orbit. Many such worlds end up tidally locked [eccentric] with their host star, exaggerated by a bright red/orange/yellow spot on the near side, and often a massive comet-like tail on the far side. Such a planet may be a source of metals, as due to the temperature and turbulence, metals can condense in the upper atmosphere like rain drops. In addition, any such system that has one or more short-term orbital gas giants is unlikely to have some, if any, rocky (terrestrial) planets. The end result of these hot jupiters is believed to be a world like Mercury, which is in essence, the core of a dead gas giant... and very rich in metallic resources.
Red Dwarves have at least two very distinct stages. While young, a Red Dwarf is highly magnetic and has a very unstable field. In short, a young Red Dwarf is prone to tantrums. Such as enormous coronal mass ejections (solar flares) capable of baking the atmosphere off nearby planets. Thus, a young red dwarf should be considered as highly dangerous, rendering habitation within the system difficult if not outright foolhardy.
Habitable terrestrial planets may be unlikely around a Red Giant star. However around such a star, habitable moons orbiting gas giants are far, far more likely.
Asteroid belts will likely contain at least one body that holds a significant fraction of the mass of the entire belt. In our own solar system, Ceres is an example, which contains roughly a third of the belt's mass. Such bodies could be referred to as minor, or dwarf, planets, and are likely to accompany an asteroid belt.
Gas giants with multiple large moons can generate tidal heat within the moons, leading to the nearest-orbiting moons to being highly volcanic. In theory, this could act as a bastion for sub-surface habitability even when starlight is negligible or non-existent.
Magnetars are basically young neutron stars, but are quite rare. Equally so, they are exceptionally terrifying and dangerous. These stars have ungodly magnetic fields, which based on distance, will most likely tear spaceships apart. Neutron stars and Magnetars are both able to emit a mass ejection like a solar flare, but the mechanics behind are very different. In essence, the crust suddenly moves a few millimeters, but because of the intense gravity and intense density, the material ejected is enormous and incredibly volatile to life in nearby solar systems. Thus, any world within many lightyears of a neutron star that would naturally be habitable would be significantly less likely to be so.
o7