The planet is comprised primarily of Phazon, and it appears to be the galaxy's main source of the substance. The core of the planet is comprised of pure Phazon, while the atmosphere and landmass possess heavy concentrations of the mutagen. Those who are not resistant to the substance will not last long on the planet's surface.
The landmasses are particularly odd, as they do not actually touch the Phazon core of the planet, but instead float above it. Even odder, it's the larger landmasses that float further away from the core. The largest continents also happens to be the most stable, and some of the smaller subcontinents remain dangerously close to the core.
There is also a ring of Phazon meteors that circles the planet. These are apparently the same kind of meteors that crashed into such planets as Tallon IV and Aether, although the reason why they left the planet's orbit is currently unknown. A large part of Samus's mission hinges upon discovering this reason and preventing the process.
As dangerous as Phazon is, the largest continent still manages to possess a thriving ecosystem. One could find rolling grassy hills, rocky terrain, and harsh deserts on the surface, each playing host to its share of flora and fauna. There is also evidence of interconnected cave systems, some of which are overgrown, and others of which are barren. There is little or no water on the planet, as the groundflow consists largely of Phazon.
At the time of our story, the Space Pirates have already built at least one base on the largest continent and devoted resources to filtering Phazon from the atmosphere and mining for it in the caves. The Pirates have adapted themselves to the high concentration of Phazon, but largely prefer their technological shelter to the harsh wilderness outside.
They have also broken off pieces of the larger continents in such a manner that they float closer to the Phazon core than the continents themselves. These islands are called "floaters", and they are ideal for close examination of the Phazon core itself. However, the terrain of these floaters is mostly rocky and it is uncertain if any natural life could thrive there for long without side effects.
All in all, Nemesis is not an ideal vacation spot, but it would make an excellent backdrop for a Metroid game.