There’s no reason in principle why life couldn’t evolve on a gas giant, but it would have to be some highly unusual life. Last week, we discussed the possibility of life on Jupiter and found it to be a fairly inhospitable place, at least by Earth standards. Does Neptune fare any better? Yes, actually.
The biggest immediate difference between the two planets is their chemical composition. Once you get past the swirling gas and travel to Jupiter’s center—and given its volatility, you inevitably will—you’ll find yourself surrounded by a core of superpressurized liquid hydrogen. That’s not a very hospitable place to live. What do you find at the core of Neptune?
Methane, ammonia, and water. It’s cold, pressurized, and volatile, but we are basically talking about a planet characterized to a great extent by frozen water and methane. A rocky planet with the same chemical signatures could be harboring extraterrestrial life, as water and methane are the two key biosignatures astronomers are looking for.
But a gas giant with water and methane is still a gas giant, and it remains unlikely that a planet with such a volatile and chemically disparate atmosphere could harbor life. If scientists looked to another solar system and saw an exoplanet with the same characteristics as Neptune, they would most likely reach the same conclusion they reached with respect to the gas giant HD 189733b: it has biosignatures that attract our attention, but it probably doesn’t host any kind of extraterrestrial life.