How Many Moons Does Neptune Have?

QUICK ANSWER

Neptune has 16 known moons. The largest by far is Triton, which is nearly the size of Earth's Moon. Triton is unusual because it orbits Neptune backwards (retrograde), suggesting it was captured from the Kuiper Belt rather than forming alongside Neptune. Neptune's moons are named after sea gods and water nymphs from Greek mythology.

Neptune has 16 known moons, but Triton is the star of the show. It's the seventh-largest moon in the solar system, similar in size and composition to Pluto, and famously orbits Neptune the wrong way. The retrograde orbit is one of the strongest clues that Triton wasn't born alongside Neptune but captured from somewhere else.

How many moons does Neptune have?

Sixteen confirmed moons. According to NASA, Neptune's 16 known moons include one giant (Triton), one mid-sized moon (Proteus), and a collection of smaller ones, mostly under 200 miles across. Two new moons were added in February 2024 from telescope observations dating back to 2021. The moons fall into three groups based on their orbits: inner regular moons close to Neptune, irregular outer moons, and Triton, which doesn't fit either category.

What is Triton like?

Strange and impressive. Triton has a diameter of about 1,680 miles, making it the seventh-largest moon in the solar system and nearly the size of Earth's Moon. Triton has nitrogen geysers actively spewing material into space, a thin atmosphere of nitrogen and methane, and a surface temperature around -391°F, making it one of the coldest known objects. Triton is similar enough to Pluto in size, density, and composition that the two are widely thought to share an origin in the Kuiper Belt.

Why does Triton orbit backwards?

Because it was probably captured. Triton orbits Neptune in the opposite direction of Neptune's rotation, a configuration called retrograde. This is unique among large moons in our solar system; every other major moon orbits in the same direction as its parent planet's spin. The retrograde orbit strongly suggests Triton didn't form alongside Neptune but was instead a Kuiper Belt object that wandered too close and got captured by Neptune's gravity. The capture probably disrupted Neptune's original moon system, scattering or destroying earlier satellites.

What happens to Triton long-term?

Triton is doomed. Like Mars's moon Phobos, Triton is slowly spiraling inward toward Neptune due to tidal forces. In roughly 3.6 billion years, Triton will get close enough to Neptune to be torn apart by tidal forces. The shattered debris is expected to form a ring system around Neptune, similar to what may eventually happen at Mars. Triton's eventual destruction will be one of the more dramatic events in the late solar system, though we obviously won't be around to see it.

Neptune has 16 known moons, with Triton standing out as one of the most unusual moons in our solar system. Triton's retrograde orbit, captured-from-the-Kuiper-Belt origin, and inevitable destruction billions of years from now make it scientifically fascinating. The other 15 moons are smaller and less well-known, but Triton alone makes Neptune's moon system memorable.

How Many Moons Does Neptune Have?