Or even intelligent life? Is it still vital to have water on a planet like this?
What is a “carbon planet”?
They have high carbon/oxygen ratios (in comparison, Earth would be an “oxygen planet”)
Their atmosphere would have mostly CO or CO2. it would rain gasoline, hail tar, snow asphalt. Volcanos would erupt diamonds. But the features of the planet would be very similar to ours with different compositions (ie rivers of gasoline)
You’d probably need some kind of liquid solvent, though there is some speculation that life started in crystals rather than water. Methane or carbon tetrachloride might do depending on the planet’s temperature. However, both of those liquids are non-polar and would tend to dissolve non-polar molecules better than polar molecules. This wouldn’t work so well for our type of life, but … perhaps.
Is there any reason to think such a planet would be completely devoid of water?
If not, then even if life is impossible without water (as may well be - it is a molecule with quite remarkable properties), it might still be possible on such planets.
Water is not just a solvent in terrestrial life - Biological macromolecules such as DNA, RNA and proteins, as well as membrane lipids are produced by eliminating water molecules from the building blocks by reactions that consume energy, and the resulting molecules can be easily disassembled into their building blocks by reactions that add back the water (hydrolysis). This chemical flexibility is essential for the reactions of life.
Building on what Anaglyph: said, we understand “life” as a series of energy intensive reactions, involving water, carbon dioxide and oxygen, to maintain a compartment (a cell for example) that is significantly different than the environment. The very different chemistry in a carbon plant would require us to study it closely, and for a long time, to see if these are not just repeated non-living chemical reactions.
Basically, its hard enough, at this point, to study Earth’s near neighbors really well. So silicon-based life or carbon-world based life or neutronium-based life are going to stay in sci-fi for some time.