The giant planets of the outer solar system are like mini-solar systems on their own, because each has a large entourage of moons. Consider Neptune, the solar system’s fourth-largest major planet, which is putting in its best showing of the year. It rises at sunset and remains in the sky all night. It’s also at its brightest for the year. But it’s still so faint that you need a telescope to see it, in the constellation Pisces.

Until 1989, astronomers knew of only two moons orbiting Neptune. But 35 years ago, the Voyager 2 spacecraft discovered a half-dozen more moons as it flew past the planet. And since then, astronomers have used telescopes on the ground to find even more, bringing the total to 16.

Some of the moons probably formed with Neptune, four and a half billion years ago. But some of the more recently discovered moons may have been asteroids that were captured by Neptune’s gravity. And others may be fractured remains of a larger moon that split apart when it was hit by an asteroid or comet.

Neptune’s biggest moon may be a newcomer as well. Triton orbits in the opposite direction from most of Neptune’s moons, suggesting that the planet captured it in the distant past. And billions of years in the future, Triton will move so close to Neptune that the planet’s gravity will rip it apart. That will create a giant but short-lived ring around this giant planet.

More about Neptune tomorrow.

Script by Damond Benningfield