A Dead Planet is a world unable to sustain life in any form, which is one of many planet types and, one type of many planets in Galactic Civilizations III.
How to Identify?
Dead worlds can be identified by hovering your mouse over the planet, it will indicate it's a dead planet. This planet will be a class 0 world and will never have the ability to be colonized or mined.
Dead planets are worlds which are unable to sustain any form of life known to exist in the galaxy today. A few examples of a dead planet would be a gas giant or a planet with little or no known atmosphere. Because of this there isn't any use for these planets at this time and, only time will tell if they will be used in the future.
One may use dead worlds as obstacles to keep an enemy at bay when being attacked. One may also use this as part of a line of ships to keep the enemy outside of a specific section of space.