Pluto's Satellites: Strange Companions in the Shadow of the Dwarf Planet

Pluto, Charon, Hydra and Nix — **Pluto**, **Charon**, **Nix** and **Hydra** were photographed by the Hubble Space Telescope on May 15, 2005. **Nix** and **Hydra** are ∼5,000 times fainter than Pluto. Since then, two other natural satellites (**Styx** and **Kerberos**) have been discovered.
Image credit: NASA, ESA, H. Weaver (JHU/APL) (new window)

Scientific Summary

The Pluto system includes a primary satellite, Charon, likely formed by a giant collision, and four small irregular satellites (Nix, Hydra, Kerberos and Styx) discovered between 1978 and 2012. Their near-coplanar orbit and near-resonance suggest a common origin, linked to a major impact event in the early solar system. The diversity of their sizes (from ~10 to 1,212 km) and compositions, with Charon being mainly composed of water ice mixed with rock, while Nix and Hydra show significantly lower densities (~1.0 to 1.2 g/cm³) suggesting an ice-rich interior, offers a unique window into accretion processes and orbital dynamics in the Kuiper Belt.

How did Pluto's satellites form and what do they reveal about the dwarf planet's history?

The central question of the article is the origin and evolution of Pluto's satellite system, a dwarf planet located at the edge of the solar system. Data from space observations (Hubble) and the New Horizons mission indicate that this system is not the result of simple gravitational capture. The current configuration, dominated by the Pluto-Charon pair (a binary system where the barycenter is located outside Pluto), and the presence of four small moons in nearly circular orbits close to the equatorial plane, support a giant collision scenario. A massive impact between Pluto and another Kuiper Belt body would have stripped an icy mantle, giving birth to Charon, and then the resulting debris disk would have agglomerated to form the smaller satellites. This hypothesis is supported by Charon's predominantly icy composition, contrasting with Pluto's rockier nature, as well as the very low densities of Nix and Hydra, revealed by the most recent dynamical measurements. The study of these moons, with their varied sizes and shapes (some potato-shaped), allows us to trace the violent history of the dwarf planet in the early solar system.

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From Charon to Styx: Exploring the Hidden Worlds Around Pluto

Charon

Charon, discovered in 1978 on magnified images of Pluto by James Walter Christy (born 1938), in collaboration with Robert Harrington, is a large satellite relative to Pluto. Charon's diameter is 1,212 ± 2 km (mean radius of 606.0 ± 1.0 km), which is a little less than 3 times the diameter of our Moon. Charon's mass is about one-eighth that of Pluto (mass ratio of about 0.117), making it, proportionally, the largest known natural satellite relative to its planet.

This infernal pair orbits its common center of gravity (barycenter) like two objects connected by a rigid central bar, like a dumbbell. Charon is located on average about 19,600 km from Pluto and moves around the dwarf planet with a period of 6.387221 days, equal to Pluto's rotation period: the two bodies are mutually synchronous, each permanently showing the same face to the other. Charon contains proportionally more ice than Pluto: the dwarf planet is composed of about two-thirds rock by mass, compared to about 60% for Charon, which thus appears as a significantly more water-ice-rich object than its companion.

Hydra and Nix

Hydra and Nix are two small satellites discovered orbiting Pluto using the Hubble Space Telescope in May and June 2005 by an American team led by Hal Weaver (born 1953) and Alan Stern (born 1957). The two moons (P2 and P3) orbit at approximately 49,300 km and 65,200 km from the system's barycenter, respectively, much farther than Charon.

Kerberos

A fourth moon was discovered on June 28, 2011, by a team led by Mark Showalter (born 1957) using the Hubble Space Telescope; it has been named Kerberos since 2013. Before the New Horizons flyby, Kerberos's gravitational influence on its neighbors Nix and Hydra seemed disproportionate to its faintness: astronomers deduced a size between 13 and 34 km and assumed a dark surface to explain this discretion. Close-up images obtained by the probe in July 2015 completely contradicted this prediction. Kerberos turned out to be much smaller than expected (about 19 × 10 × 9 km) and, above all, much brighter, with an albedo of about 50%, comparable to the other small moons and revealing a surface of relatively pure water ice. Its bilobed shape, composed of two distinct masses about 8 and 5 km in diameter attached to each other, suggests it results from the merger of two smaller bodies.

Styx

As for P5, named Styx, it was discovered on June 26, 2012, by the same team led by Mark Showalter, while astronomers were preparing for the New Horizons mission flyby. It is the smallest of the four small moons and the closest to Pluto, with an equivalent diameter of about 10.5 km for an elongated and irregular shape, close to a potato shape.

N.B.:
The New Horizons probe, launched on January 19, 2006, was the first spacecraft to visit Pluto. Placed directly on a solar escape trajectory at a speed of about 16.26 km/s (58,500 km/h) relative to Earth, it holds the record for the highest launch speed ever achieved by a spacecraft launched from Earth. It subsequently benefited, in February 2007, from a gravitational assist from Jupiter which allowed it to gain about 4 km/s more and shorten its journey to Pluto by several years.

Table: From the Giant Charon to Pluto's Small Moons

All moons orbit in a near-coplanar plane, which argues for a common origin (giant collision). Pluto's small moons, Nix, Hydra, Kerberos, and Styx, have orbits close to resonances with the Pluto-Charon pair, without being perfectly synchronized. Their orbital periods are organized around 3.3:4:5.1:6.1. The densities of Kerberos and Styx remain poorly constrained due to their very low mass, barely dynamically measurable.

Physical, orbital characteristics and peculiarities of Pluto's satellites
Moon	Dimensions (km)	Distance to barycenter (km)	Orbital period (days)	Density (g/cm³)	Peculiarity
Charon	⌀ 1,212 ± 2	≈19,596	6.387221	1.702 ± 0.021	Largest satellite proportionally to its planet — unique binary system
Nix	48.4 × 33.8 × 31.4	≈49,338	25.5113	1.03 ± 0.20	Very bright surface, rich in water ice — density close to Hydra's
Hydra	52.0 × 36.5 × 29.3	≈65,186	38.7428	1.22 ± 0.15	Largest of the small moons — low internal cohesion, complex shape
Kerberos	≈19 × 10 × 9	≈58,280	32.7517	<2.0 (poorly constrained)	Bilobed shape — much smaller and brighter than expected before New Horizons
Styx	⌀ ≈10.5 ± 3	≈43,172	20.8825	<2.1 (poorly constrained)	Smallest and closest to Pluto — least well-constrained orbit

Masses and Densities: What the Dynamics of the Small Moons Reveal

Masses refined by Hubble and New Horizons

For a long time, the masses of Pluto's four small moons could only be estimated indirectly, from their mutual gravitational influence on series of images from the Hubble Space Telescope. In 2023, a new analysis combining all Hubble observations between 2005 and 2019 with images from the New Horizons probe allowed these estimates to be significantly refined. It shows that Nix and Hydra are actually less massive than previous solutions suggested, with respective masses of about 2.6 × 10¹⁶ kg and 3.0 × 10¹⁶ kg.

Densities revealing an icy composition

Combined with volumes deduced from New Horizons' close-up images, these masses give densities close to 1.0 to 1.2 g/cm³ for Nix and Hydra, much lower than that of Charon (1.70 g/cm³) and even more so than that of Pluto (1.86 g/cm³). Such a density, associated with their very bright surface dominated by water ice, indicates that the interior of these two moons is significantly richer in ice, with relatively low internal porosity compared to other small bodies of similar size like Phobos, Deimos, or the Kuiper Belt object Arrokoth. The masses of Kerberos and Styx, on the other hand, remain too close to the measurement noise to be determined precisely: only upper density limits (respectively <2.0 and <2.1 g/cm³) could be established.

A predominantly icy debris disk confirmed

This result strengthens the scenario of a predominantly icy debris disk, originating from the outer layers of the two precursor bodies during the giant collision that created Charon, whose agglomeration later gave rise to the four small moons.

Nix and Hydra, Fragments of Charon's Interior? Clues from JWST

Unique spectral signatures observed by JWST

A more recent, still preliminary, line of evidence completes this picture. In July 2025, at a scientific conference dedicated to the tenth anniversary of New Horizons' Pluto flyby, a team from the Space Telescope Science Institute presented unprecedented observations from the James Webb Space Telescope concerning Nix and Hydra. These data, obtained fortuitously while the instrument was primarily observing Pluto and Charon, suggest that the colors and spectrum of the two moons do not match any of the major families of Trans-Neptunian Objects classified so far: their surface is dominated by a reddish material, likely carbonaceous, which appears unique in the trans-Neptunian region.

Fragments of Charon's crust?

According to the authors of this presentation, this spectral signature would bring Nix and Hydra closer to Charon's deep interior than to typical objects in the surrounding Kuiper Belt, supporting the hypothesis of a common origin by giant collision: the two moons could be fragments of the crust and upper mantle of a proto-Charon, torn off during the impact. The authors even put forward the idea that dust from micrometeorite impacts on Nix and Hydra, captured by Charon's gravity, could today fall back onto its surface — a mechanism that would partly explain certain characteristics observed on Charon's reddish north pole (Mordor Macula).

N.B.:
These conclusions currently rest on a conference communication and have not yet been subject to peer-reviewed scientific publication; they should therefore be considered preliminary, pending further spectroscopic observations.

References

NASA NSSDCA, Pluto Fact Sheet (new window)
F. Nimmo et al., « Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto », Nature 540, 94–96 (2016), DOI: 10.1038/nature20148 (new window)
F. Nimmo et al., « Mean radius and shape of Pluto and Charon from New Horizons images », Icarus 287, 12–29 (2017), DOI: 10.1016/j.icarus.2016.06.027 (new window)
H. A. Weaver et al., « The small satellites of Pluto as observed by New Horizons », Science 351, aae0030 (2016), DOI: 10.1126/science.aae0030 (new window)
S. B. Porter & R. M. Canup, « Orbits and Masses of the Small Satellites of Pluto », The Planetary Science Journal 4, 120 (2023), DOI: 10.3847/PSJ/acde77 (new window)
N. Taylor Tillman, « Pluto's dwarf planet partner Charon may have spilled its guts to create 2 of the pair's moons », Space.com, September 1, 2025 (report on a communication by B. Holler, conference « Progress in Understanding Pluto: 10 Years After Flyby », July 2025) — link to article (new window)

FAQ: Everything you need to know about Pluto's satellites

Who discovered Charon and when?

Charon, Pluto's largest satellite, was discovered in 1978 by astronomers James Christy and Robert Harrington. They identified it by observing magnified images of Pluto, on which a bulge was visible.

Why is the Pluto-Charon system said to be a "dumbbell" system?

Because their common center of gravity, called the barycenter, lies outside Pluto. They both orbit around this point, like the two ends of a rigid bar, which is unique in the solar system for a planet and its moon.

What are Pluto's other satellites and when were they discovered?

The four other moons are Nix and Hydra (discovered in 2005 by the Hubble Telescope), Kerberos (discovered in 2011), and Styx (discovered in 2012). They are small irregularly shaped bodies, measuring between about 10 and 55 km in their largest dimension.

Why was Kerberos thought to be dark, when it is actually bright?

Before the New Horizons flyby, Kerberos's gravitational influence on its neighbors seemed too strong for its faintness, leading astronomers to imagine a rather large and dark moon. Images from the probe showed, in 2015, that it is actually a small bilobed body about 19 km long, covered in relatively pure water ice and as bright as the other small moons.

Which space mission allowed close study of Pluto?

This is the New Horizons probe, launched in 2006. It flew past Pluto in 2015, providing the first detailed images of the dwarf planet and its satellites, and greatly enriched our knowledge of this distant system.