128 new moons: Advances in technology change our view of the solar system

What Uranus and Saturn reveal about planetary worlds

Driven by cutting-edge technology and advanced data processing, our knowledge of space has changed dramatically over the past decades, and the era of discovery is far from over.

In the early months of 2025, the Solar System’s celestial census underwent a notable update, with a tiny inner moon detected around Uranus and a remarkable swarm of 128 new irregular satellites identified orbiting Saturn.

What is a Moon?

A moon, or a natural satellite, is an astronomical body that orbits a planet, dwarf planet, or minor planet under the influence of its gravity.

Moons vary widely in size and composition, from large, spherical bodies such as Ganymede or Titan to small, irregular fragments only a few kilometres across. They reflect sunlight rather than emit it, and their observation provides key information about planetary formation, orbital dynamics, and the evolution of our Solar System.

There is no strict, size-based definition that separates a moon from a moonlet or a ring particle. In many systems, the transition between these categories is gradual.

A fragment within a planetary ring can accumulate material and become a small satellite or be disrupted and reintegrate into the ring.

Moons are commonly divided into two categories based on their origin and orbital properties. Regular moons formed from the same circumplanetary disk of gas and dust as their parent planet.

They follow prograde (proceeding from west to east), nearly circular orbits close to the equatorial plane, indicating an origin under stable conditions during the early stages of planetary formation.

By contrast, irregular moons are captured objects that originated elsewhere before being trapped by a planet’s gravity. Their orbits are usually eccentric, inclined, and often retrograde, reflecting their external origin. These moons provide valuable clues about past capture events, collisions, and the gravitational evolution of planetary systems. The recently identified small moons around Saturn belong largely to this category, confirming that the Solar System remains an evolving and still partially unexplored environment.

A new moon for Uranus

The James Webb Space Telescope (JWST) was behind the latest discovery in the Uranian system. In February 2025, JWST’s extremely sensitive Near-Infrared Camera (NIRCam) pierced through the planet’s glare, revealing an inner moon provisionally designated S/2025 U1. Measuring only about ten kilometres in diameter, its small size and close orbit explain why it escaped detection by the Voyager 2 flyby in 1986 and by subsequent Hubble Space Telescope observations.

This finding brings Uranus’s confirmed moon count to 29. S/2025 U1 represents a textbook example of a regular satellite, following a nearly circular orbit roughly 56,000km from the planet’s centre, positioned between the known moons Ophelia and Bianca. Its placement offers researchers a precious opportunity to study the ring–moon continuum: the concept that there is no strict dividing line between a ring particle and a true moon.
 
Continuously shaped by the gravitational influence of the surrounding rings and neighbouring moons, this tiny body provides new insight into the dynamics, stability, and long-term evolution of Uranus’s intricate inner system.

© NASA

Saturn’s expanding system

The update concerning Saturn was considerably more striking. In March 2025, astronomers announced the confirmation of 128 new irregular satellites, identified through the shift-and-stack technique applied to extensive datasets from powerful ground-based telescopes. This method aligns and combines multiple exposures along the predicted orbital motion of potential satellites, allowing faint, moving objects to emerge from the surrounding noise.

With this major addition, Saturn’s confirmed moon count now stands at 274, reinforcing its status as the planet with the most satellites in the Solar System. The new bodies are typical irregular moons, and their distribution reveals distinct collisional families, indicating a shared origin in ancient impact events.

Researchers believe that several larger captured bodies were fragmented by collisions with comets or asteroids millions of years ago, creating swarms of smaller remnants that still circle the planet today. These populations preserve a record of Saturn’s violent past, and hide clues about the formation, evolution, and long-term stability of its outer system.

The Solar System’s most famous moons

The ongoing census of natural satellites confirms that the Solar System is a dynamic and continuously evolving environment. While every moon offers its own scientific value, a few stand out as key worlds in our exploration of planetary formation and the potential for life beyond Earth.

© Artist impression

Among Saturn’s satellites, Titan remains one of the most remarkable. It is the largest moon in the Saturnian system and the only one known to possess a dense, nitrogen-rich atmosphere. Unique in the Solar System, Titan hosts stable liquid on its surface, rivers and lakes composed of methane and ethane. This combination of atmospheric chemistry and surface processes makes Titan a compelling analogue for understanding early Earth-like environments.

In the Jovian system, Io and Ganymede stand as two dramatically contrasting worlds. Io is the most volcanically active body in the Solar System, its surface constantly reshaped by powerful tidal forces generated by Jupiter’s immense gravity. Ganymede, on the other hand, is the largest moon we know of, bigger even than Mercury; it uniquely possesses its own magnetic field, suggesting a complex internal structure that includes a liquid metallic core.

Beyond these giants, the icy moons have become the most promising targets in the search for extraterrestrial life. Beneath their frozen crusts, they are believed to harbour vast subsurface oceans of liquid water, maintained by tidal heating. Europa, one of Jupiter’s four Galilean moons, likely conceals a salty global ocean containing twice the volume of Earth’s seas. This ocean is thought to be in direct contact with a rocky seafloor, meaning it may host the conditions for hydrothermal activity and, potentially, life-sustaining chemistry.

Equally intriguing is Enceladus, a small moon of Saturn whose global subsurface ocean vents material into space through towering plumes at its south pole. These geysers release water vapour, salts, organic compounds, and molecular hydrogen, an energy source for microbial life. Because these materials are ejected directly into space, they can be sampled by passing spacecraft, making Enceladus one of the most accessible environments for studying the potential habitability of worlds beyond Earth.

Apart from expanding our scientific inventory, these new discoveries also underscore the remarkable diversity and dynamism of the systems orbiting the giant planets, further blurring the lines between ring particles, moonlets, and fully formed moons. But as we uncover more about their origins and evolution, these discoveries continue to fuel the compelling search for signs of life and the conditions that might support it across the cosmos.