Tiny new moons have been spotted orbiting Neptune and Uranus

Astronomers have spotted new moons around Uranus and Neptune for the first time in a decade. These are the faintest moons ever spotted orbiting any planet, and they prove a long-standing idea about satellites in the outer solar system.

Scott Sheppard at the Carnegie Institution for Science in Washington DC and his colleagues found these moons using the Magellan telescope in Chile and confirmed them using several other large telescopes around the world. “We looked about four times deeper than anyone has before,” says Sheppard. “These moons are on the edge of our ability – they’re just faint, faint points of light.”

Generally, when looking for moons, you can only take a picture with a maximum exposure of about 5 minutes before the image becomes overexposed and the movement of the moons makes it useless. Sheppard and his team got around this by taking many of these 5-minute images in a row, observing for hours and then combining the dim parts of the images. That enabled them to spot the dim points of light shining from the faintest moons ever discovered – and the smallest moons found to date around their respective planets.

The new moon around Uranus is provisionally named S/2023 U1, but it will eventually be given the name of a character in a Shakespeare play, to match the planet’s other moons. It is only about 8 kilometres across, and it completes an orbit once every 680 Earth days.

One of the new moons around Neptune is called S/2021 N1, and it awaits an official name from Greek mythology. It is about 14 kilometres across and takes about 27 Earth years to orbit the planet, making it the most distant moon from its host planet ever found. It is also the faintest moon ever found.

The brighter, larger moon found orbiting Neptune is called S/2002 N5 – as its name suggests, it was first spotted more than 20 years ago, but it was lost before astronomers could confirm its orbit. “You can lose a moon really easily,” says Sheppard. “We basically need really, really good weather, we need the telescope to be working perfectly, we need everything to go right to detect these moons.” If anything goes wrong and a night of planned observations is lost, moons move in their orbits and become extremely difficult to find again, as happened with S/2002 N5.

Each of the three new moons has a similar orbit to two other satellites in its planetary system, and these fellow travellers form small groups that orbit together. This means that each of these groups probably formed together when a larger moon broke up in the chaos of the early solar system.

“Until now it was unclear whether Uranus and Neptune had these groups of outer moons like Jupiter and Saturn do,” says Sheppard. “We believe these are fragments of once bigger moons, and there are probably many more smaller ones to find.” Unfortunately, we are at the limits of what we can discover with current technology, he says, so it might be another long wait before any smaller moons than these are spotted around Uranus and Neptune.

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