How a terraformed Neptune could look like, made in Universe Sandbox 2 by Jeansowaty.

Neptune is an icey gas giant. That doesn't mean it's impossible to terraform. Neptune has a rich abundance of hydrogen and helium in its atmosphere. It has a rocky core with a mass of 1.2 Earth masses, that would require terraforming. Terraforming Neptune would be extremely difficult. Blasting off much of its layers and leaving the core would currently be impractical. But that step would only be first. As the atmospheric layers are slowly removed, (and potentially used for fusion with any excess hydrogen and helium added to Saturn or Jupiter) Neptune's mantle of methane, ammonia and water would be collected and stored.  Since this layer is between 10 and 15 Earths in mass, the bulk of which could be used to create multiple moons around Neptune each of which more massive than Earth with the potential for life there with their oceans. The orbits of the moons would need to be close to each other and Neptune to take advantage of the increased heat from tidal forces. And once the core is left, it would be given a new atmosphere (using the Oxygen from the Water collected, Nitrogen from the Ammonia, and Carbon from Methane); then a heat source to sustain stable temperatures. Since its magnetic field would be weakened once the water-ammonia layer is gone, some extra protection may be needed (like an ozone layer). The low elevations of Neptune's core would be filled with water. The new atmosphere would need to be composed of mainly nitrogen and carbon dioxide. The addition of breathable oxygen will also be needed. The Neptunian core is only slightly larger than Earth, so holding on to the atmosphere wouldn't be a problem. If the  core of the rocky core melts, Neptune's core can be active and have plate tectonics. Life can exist here for billions of years.

Terraformed Neptune (far right) with Titan as a moon to restart the dynamo.

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