Uranus, the Sideways Planet

Why We Study Neptune

Neptune, along with its cousin Uranus, is the least-explored planet in our solar system, having been visited by a spacecraft only once. Yet we’ve found more Neptune-sized worlds orbiting other stars than any other type of planet. In order to understand other solar systems and figure out whether our own is unique, we need to learn more about the windy blue world in our own backyard.

Like Jupiter and Saturn, Neptune’s atmosphere consists mostly of hydrogen and helium, but it also has methane that absorbs red light, giving it a deep blue hue. Beneath the atmosphere lies an ocean of water, ammonia, and methane, squeezed by intense pressures into a semi-solid state. For this reason we call Neptune an ice giant, though the ice isn’t anything like what you’d find in your freezer, with temperatures likely reaching thousands of degrees! Electric currents flowing through this icy-hot ocean may be responsible for powering Neptune’s strong and unusually complex magnetic field.

Where Neptune formed and how it got its water isn’t clear. The disk of dust and gas that formed our solar system probably didn’t contain enough material to form Neptune at its current location, 30 times farther from the Sun than Earth. Like the other outer planets, it was probably born closer to the Sun before moving outward, though the Sun would have evaporated its water had Neptune been too close. By figuring out where Neptune was born and how the planet evolved, scientists learn what conditions in the early solar system were like, around the time life arose on Earth.

Neptune has 14 known moons, half of which were likely captured by the planet’s gravity rather than forming in place. Only one moon, Triton, is big enough to be spherical. It has planet-like characteristics similar to Pluto such as a complex icy surface and thin nitrogen atmosphere, and may have started off as a free-roaming dwarf planet. Geysers of nitrogen erupt from Triton’s surface that may be coming from a subsurface water ocean. Future missions would help confirm whether the ocean exists, and whether it might be habitable to life as we know it.

Just like Saturn, Neptune has a set of rings shepherded by small moons and made up of cold, icy particles. But its outermost ring is not complete, comprising five distinct arcs. Scientists think the planet’s moon Galatea prevents particles in these arcs from spreading out to complete the ring but no one knows for sure.