A composite showing Neptune, its fuzzy rings, and some moons. via
Though we’ve been exploring space for over a half century, there’s still plenty to find in our own backyard. Case in point: last week, Mark Showalter, a keen-eyed researching with the SETI institute, found a previously unseen moon orbiting Neptune in archival data from the Hubble Space Telescope.
The moon, for the time being, is called S/2004 N 1. Preliminary estimates suggest it’s no more than 12 miles across, so small that from our Earthly vantage point i’s about 100 times as dim as the faintest star we can see. Even Voyager 2—the planet-hopping probe that flew past Neptune in 1989 and caught a brief glimpse of the planet’s moons and rings—didn’t see this moon. It is currently the smallest known moon in the Neptunian system and the 14th one we’ve found. It also moves fast; it orbits around Neptune once every 23 hours.
Showalter noticed the moon as a tiny white dot about 65,500 miles away from Neptune. He tracked that dot, watching it move against the starry background through 150 Hubble pictures of Neptune taken between 2004 and 2009. After careful tracking, he was able to plot the moon’s orbit as sitting between the those of Larissa and Proteus.
Neptune‘s largest moon, Triton, was discovered by astronomer William Lassell in 1846, just seventeen days after the discovery of the planet itself. Lassell, like Showalter, noticed the moon as a dot near the planet and tracked its progress along the sky.
Neptune, as seen by Voyager 2. via
It was over a century before astronomers found another Neptunian satellite. In 1949, Gerard Kuiper, for whom the Kuiper belt of distant small bodies is named, found the second moon Nereid. Nereid, interestingly, isn’t the next largest moon; Proteus is. But it’s too dark and close to its host planet to be seen easily from Earth.
It would take Voyager 2’s close pass by Neptune in 1989 to reveal more moons. Six, in fact: Naiad, Thalassa, Despina, Galetea, Larissa, and Proteus. In 2003, using improved ground-based telescopes, astronomers added five more moons to Neptune’s total: Halimede, Sao, Psamathe, Laomedeia, and Neso. S/2004 N 1 brings the total to 14. For now.
That we’ve just discovered a previously unknown moon, particularly one as small as S/2004 N 1 raises an interesting question: when is a moon a moon and not just a rock in space?
Until the 17th century, we only knew about one moon: ours. Then Galileo invented the telescope, turned it skyward, found Jupiter’s four largest moons, and opened up a world where any planet could have moons.
Astronomically speaking, a moon is any natural celestial body that orbits around a planet. And for the sake of the “what is a moon” question, a planet includes (in our solar system) the four rocky planets, the four gas giants, the dwarf planets, and a number of minor bodies. So a moon can’t orbit a star—though perhaps the exception would be if some cosmic collision were to knock a moon out of its planetary orbit and into a stellar one.
But that doesn’t mean a moon has to be geologically dead like our Moon. There are a lot of moons some astronomers would happily call planets in light of their surface activity. And there are some moons astronomers think might have been small, independent planets before they were captured by a larger body. Neptune’s Triton, for example.
Triton, in false colour, as seen by Voyager 2 in 1989. via
Voyager 2 returned a wealth of information about Triton, from its cantaloupe rind-like surface to its icy volcanoes that likely spout a mixture of liquid nitrogen, methane, and dust that freezes and falls back onto the surface as snow. The moon is extremely icy, reflecting almost all of the little sunlight that hits it making it one of the coldest objects in the solar system, a frigid -400 degrees Fahrenheit.
Triton is also the only moon in the solar system that circles its planet in the direction opposite that planet’s rotation. It’s a bizarre arrangement that suggests it might have passed so close to Neptune that it was captured by the planet’s much stronger gravitational pull. And the moon is getting closer to its host planet all the time. Neptune's gravity drags on the counter-orbiting Triton, slowing it down and bringing it closer to itself. It will take millions of years, but eventually Triton will fall close enough to Neptune that it will break apart.
But in the mean time, there’s a lot to learn about Triton. And Neptune’s other moons. And probably a lot more moons to find.