Uranus and Neptune have some new moons

Astronomers are adding three newly discovered moons to our solar system’s growing list of known celestial bodies.  A team of international researchers spotted an additional moon circling Uranus’ for the first time in almost two decades and two new moons orbiting the planet Neptune. The discoveries were announced on February 23 by the International Astronomical Union’s Minor Planet Center, a scientific organization who is responsible for designating our solar system’s comets, planets, and moons.

[Related: Neptune’s faint rings glimmer in new James Webb Space Telescope image.]

“The three newly discovered moons are the faintest ever found around these two ice giant planets using ground-based telescopes,” Scott S. Sheppard, an astronomer with the Carnegie Institution for Science who collaborated on the moons’ discovery, said in a statement. “It took special image processing to reveal such faint objects.”

Uranus’ new moon will have a dramatic name

The planet Uranus now has 28 known moons. The new moon is temporarily named S/2023 U1, but it will eventually be named after a character from a Shakespearean play. Uranus moons including Puck, Titania, and Oberon reference A Midsummer Night’s Dream, while the moon Miranda is a reference to The Tempest, both plays written by the English playwright.

At only five miles wide, S/2023 U1 is likely Uranus’ smallest known moon. It takes the tiny satellite 680 days to orbit the planet. Shepherd first spotted S/2023 U1 on November 4, 2023, using the Magellan telescopes at Carnegie Science’s Las Campanas Observatory in Chile. Followup observations were conducted one month later. Marina Brozovic and Bob Jacobson of NASA’s Jet Propulsion Laboratory then helped Shepherd determine a possible moon orbit.

New Neptunian moons–one bright, one faint

With this new discovery, the planet Neptune now has 16 known satellites. The brighter of Neptune’s two newly discovered moons is tentatively named S/2002 N5. It is 14 miles wide and appears to be in a 9-year orbit around Neptune. The fainter moon is named S/2021 N1 and it is about 8.6 miles wide. It circles the planet once every 27 years. Both of these moons will eventually be given names based on sea gods and nymphs in Greek mythology.

The two new Neptunian moons were first observed in September 2021. Shepherd worked with David Tholen of the University of Hawaii, Chad Trujillo of Northern Arizona University, and Patryk Sofia Lykawa of Kindai University, and the Subaru telescope to detect the moons. They confirmed the orbit of the brighter moon (S/2002 N5) over about two years and conducted followup observations with the Magellan telescopes.  

“Once S/2002 N5’s orbit around Neptune was determined using the 2021, 2022, and 2023 observations, it was traced back to an object that was spotted near Neptune in 2003 but lost before it could be confirmed as orbiting the planet,” said Sheppard. 

Detecting the fainter moon (S/2021 N1) required some special observing time under “ultra-pristine conditions” at the European Southern Observatory’s Very Large Telescope and on Gemini Observatory’s 8-meter telescope in order to secure its orbit. 

[Related: Expect NASA to probe Uranus within the next 10 years.]

By using these telescopes, shepherd and colleagues snapped dozens of five-minute exposures over three- or four-hour periods on a series of nights. The short-burst images were then layered so that all three new moons could come into view. 

“Because the moons move in just a few minutes relative to the background stars and galaxies, single long exposures are not ideal for capturing deep images of moving objects,” Sheppard said. “By layering these multiple exposures together, stars and galaxies appear with trails behind them, and objects in motion similar to the host planet will be seen as point sources, bringing the moons out from behind the background noise in the images.” 

More understanding of how these moons were captured can help astronomers learn about the tumultuous early years of our solar system and how the planets at the out edge move. Future missions to Uranus and Neptune are in the preliminary planning stages, and more data on their moons will allow the team to better study these far-flung planets. 

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