Pluto and its five moons: A miniature solar system that defies conventional wisdom

Scientific Summary

Pluto is a dwarf planet in the Kuiper belt possessing a complex system of five moons (Charon, Nix, Hydra, Styx and Kerberos). Its main satellite, Charon, is so large (1212 km) that it forms with Pluto a binary system with synchronous rotation, whose barycenter is external to Pluto. The four other moons, Nix, Hydra, Kerberos and Styx, are much smaller (equivalent diameters of about 10 km for Styx and Kerberos, and 40 km for Nix and Hydra) and orbit further out according to orbital resonances which contribute to their dynamic stability. The New Horizons mission (2015) revolutionized our understanding by revealing active geology, a tenuous atmosphere composed of nitrogen, methane and CO, indications of a possible internal ocean, and evidence of cryovolcanism, making Pluto a natural laboratory for the study of icy bodies.

Why do Pluto and its moons form a complex system and what did the New Horizons mission reveal about this system?

Unlike most planets and their moons, Pluto and its largest satellite, Charon, are in synchronous rotation, always showing the same face to each other. Furthermore, the barycenter of their orbit lies outside Pluto, making them a true binary system. This unique configuration, coupled with the presence of four small moons (Nix, Hydra, Kerberos and Styx) whose orbits are in resonance, testifies to a rich dynamic and gravitational history, probably resulting from a giant impact.

The New Horizons mission, in 2015, was fundamental in deciphering this complexity. Before its flyby, Pluto was just a distant point of light. The probe provided unprecedented data that transformed our view of this dwarf planet and its moons. It revealed surprising geological diversity on Pluto, with water ice mountains, the vast icy plain Sputnik Planitia, and evidence of cryovolcanism, suggesting recent activity. It also characterized the small moons for the first time, refining their sizes and shapes, and detected a tenuous but complex atmosphere, composed of nitrogen, methane and carbon monoxide, with photochemical hazes.

Physical and Orbital Characteristics

General Data and Composition

Pluto has a mean diameter of about 2376 km, and a mass of about \(1.303 \times 10^{22}\) kg. Its mean density (\(\approx 1.85\) g/cm³) indicates a composition primarily of water ice mixed with a rocky core.

The Pluto-Charon Binary System

Its main satellite, Charon, is relatively large (1212 km in diameter), with an exceptionally high size ratio to Pluto (about 0.51), making the Pluto-Charon system a synchronous binary system: the two bodies perpetually face each other.

The Other Moons and Their Orbital Dynamics

The four other moons are much smaller and elongated in shape, with equivalent diameters of about 40 km for Nix and Hydra, and about 10 km for Kerberos and Styx. They orbit around the common barycenter of the system, beyond Charon's orbit. Styx, Nix, and Hydra are linked by a three-body orbital resonance (close to the 3:4:6 ratio), analogous to the Laplace resonance that links the Jovian moons Io, Europa, and Ganymede, which contributes to the long-term dynamic stability of the system. Kerberos, however, does not fully participate in this resonance. A remarkable fact revealed by Hubble Space Telescope observations and then by the New Horizons probe: unlike most satellites in the Solar System, Nix and Hydra are not locked in synchronous rotation but rotate chaotically, under the effect of tidal forces exerted by the Pluto-Charon pair.

Internal Structure: a Subsurface Ocean beneath Sputnik Planitia?

The Enigma of Sputnik Planitia's Alignment

One of the most intriguing results from the New Horizons mission concerns Pluto's internal structure. The position of the vast icy plain Sputnik Planitia, aligned very precisely with the tidal axis of the Pluto-Charon system, led several teams to propose that this alignment results from a reorientation of the entire dwarf planet (a phenomenon of "true polar wander"), caused by an excess of mass localized beneath this basin.

The Liquid Ocean Hypothesis

However, such a deep impact basin should present a mass deficit, not an excess. To explain this positive gravitational anomaly despite a negative topography, F. Nimmo and his colleagues proposed that a liquid water ocean persists beneath Pluto's icy crust: the thinning of the crust beneath the basin, combined with a slight deposit of solid nitrogen on the surface, would suffice to generate the observed mass excess. Such an ocean would require a rigid and poorly conductive ice shell to remain liquid for billions of years despite Pluto's extreme cold.

Competing Hypotheses and Scientific Debate

This is, however, an indirect clue and not a direct detection: a competing hypothesis, put forward by J. Keane and I. Matsuyama, attributes the reorientation to the cyclic accumulation of volatile ices rather than an internal ocean. The two mechanisms are not necessarily mutually exclusive, and the question remains actively debated by the planetary science community.

Table: Pluto, Charon and their moons, a gravitational dance

Table of orbital and physical characteristics of the Pluto system
Body	Dimensions (km)	Mean distance from Pluto (km)	Orbital period (days)	Density (g/cm³)	Ratio to Charon's period
Pluto	⌀ 2377 ± 3	—	—	≈1.85	—
Charon	⌀ 1212 ± 2	≈19,596	6.387221	≈1.70	1 (reference)
Styx	≈16 × 9 × 8	≈42,400	20.16155 ± 0.00027	poorly constrained	≈3.2
Nix	≈50 × 35 × 33	≈48,690 ± 120	24.85463 ± 0.00003	poorly constrained (≈1.3–1.5?)	≈3.9
Kerberos	≈19 × 10 × 9	≈57,750	32.16756 ± 0.00014	poorly constrained (≤2.8, 1σ)	≈5.0
Hydra	≈65 × 45 × 25	≈64,721 ± 90	38.20177 ± 0.00003	poorly constrained (≈1.3?)	≈6.0

N.B.:
The period ratios with Charon are close to, but distinct from, the integers 3:4:5:6; they therefore do not constitute a strict mean-motion resonance with Charon. However, Styx, Nix, and Hydra are indeed linked to each other by a three-body resonance close to the 3:4:6 ratio (Showalter & Hamilton, 2015).

Detailed Physical Analysis

Dynamics of the Pluto-Charon Binary System

The large relative size of Charon induces a common center of gravity located outside Pluto, a characteristic of a binary system. This configuration influences rotation and mutual tidal forces, leading to total synchronization: Pluto and Charon always present the same face to each other.

From an energetic point of view, the dissipation of tidal forces via internal friction has allowed this mutual gravitational locking, with notable effects on the tectonics and geology of both bodies.

Orbital Resonances of the Small Moons

The small moons, with their resonant orbits, suggest a stable gravitational balance resulting from multi-body interactions. Their composition is assumed to be predominantly icy, with densities lower than Charon's, but precise data remain to be refined: the masses of Styx, Nix, Kerberos, and Hydra are still too imprecise to deduce reliable densities.

Their resonant orbital relationships ensure remarkable dynamic stability despite their small size and low mass.

Origin and Formation of the Plutonian System

The discovery of these minor moons has also allowed the study of debris dynamics and the potential formation of the system, hypothetically resulting from a giant impact on Pluto, similar to the hypothetical formation of our own Moon.

This scenario explains the composition and orbital arrangement of the satellites, while highlighting the complexity of gravitational interactions in this outer solar system.

Why have the observations made by New Horizons revolutionized our understanding?

Unprecedented Direct Exploration

The New Horizons mission, launched in 2006 and flying by Pluto in July 2015, marked a major advance in the study of trans-Neptunian objects through the collection of in situ data of unprecedented precision. Before this mission, our knowledge relied primarily on telescopic observations limited by the distance, size, and faint luminosity of Pluto and its moons.

Unexpected Geology on Pluto

New Horizons provided high-resolution images showing Pluto's geological diversity, revealing complex terrains such as water ice mountains, nitrogen ice plains (notably Sputnik Planitia), fractures and faults caused by tectonic stresses, as well as evidence of cryovolcanism. This diversity indicates recent, even current, geological activity, which was unexpected for a body of this size and distance from the Sun.

Characterization of the Secondary Moons

From a dynamic perspective, New Horizons allowed precise measurement of the orbital and physical parameters of Pluto's moons, including their sizes, shapes, compositions, and albedos. Images and spectra confirmed the predominantly icy composition of the secondary moons, with possible variations in ice type and contamination by organic materials or tholins.

A Tenuous but Active Atmosphere

The discovery of a tenuous but complex atmosphere around Pluto, mainly composed of nitrogen (\(N_2\)), methane (\(CH_4\)), and carbon monoxide (\(CO\)), has modified our understanding of sublimation, volatile cycles, and climate feedbacks in extreme cold environments. Layered atmospheric haze layers were also detected, testifying to active atmospheric photochemistry.

A Natural Laboratory for Icy Bodies

New Horizons transformed Pluto from a distant and poorly known dwarf planet into a natural laboratory for studying the geophysics of small icy bodies, multi-body orbital dynamics, and chemical evolution in the outer regions of the solar system. These observations have profoundly influenced theoretical models of satellite formation, low-pressure atmospheres, and differentiated internal structures in the Kuiper belt.

References

NASA NSSDCA, Pluto Fact Sheet
S. A. Stern et al., "The Pluto system: Initial results from its exploration by New Horizons", Science 350, aad1815 (2015), DOI: 10.1126/science.aad1815
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
H. A. Weaver et al., "The small satellites of Pluto as observed by New Horizons", Science 351, aae0030 (2016), DOI: 10.1126/science.aae0030
M. R. Showalter & D. P. Hamilton, "Resonant interactions and chaotic rotation of Pluto's small moons", Nature 522, 45–49 (2015), DOI: 10.1038/nature14469
M. Brozović, M. R. Showalter, R. A. Jacobson & M. W. Buie, "The orbits and masses of satellites of Pluto", Icarus 246, 317–329 (2015), DOI: 10.1016/j.icarus.2014.03.015
F. Nimmo et al., "Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto", Nature 540, 94–96 (2016), DOI: 10.1038/nature20148

FAQ: Everything you need to know about Pluto and its complex system

Why is Pluto no longer considered a planet and what has it become?

In 2006, the International Astronomical Union (IAU) redefined the definition of a planet. Pluto does not meet all the criteria, notably that of having "cleared its orbital neighborhood" of other debris. It was therefore reclassified as a dwarf planet, becoming the prototype of this new category of celestial objects.

What makes the Pluto-Charon system unique in our solar system?

The uniqueness of the Pluto-Charon system lies in their size ratio. Charon is so large (about half of Pluto's diameter) that the barycenter (the common center of gravity) of their orbit lies outside Pluto. This makes them a binary system, a unique characteristic among the major and dwarf planets of the solar system.

How did the New Horizons mission change our view of Pluto?

The New Horizons probe was the first to fly by Pluto in 2015. Before that, Pluto was perceived as a frozen and geologically dead world. The mission revolutionized this view by revealing an incredibly diverse surface with ice mountains, nitrogen ice plains, and evidence of recent geological activity (cryovolcanism), and even hints of a possible internal ocean beneath Sputnik Planitia. It also showed that its atmosphere is complex and dynamic.

What are the characteristics of Pluto's small moons (Nix, Hydra, Styx, Kerberos)?

These four moons are much smaller than Charon, with equivalent diameters of about 10 km for Styx and Kerberos, and 40 km for Nix and Hydra. Their orbits are further from Pluto. Data from New Horizons indicate they have a predominantly icy composition and their orbits are linked by gravitational resonances that contribute to the long-term stability of the entire system.

What is the presumed origin of Pluto's moon system?

The scenario favored by scientists is that of a giant impact. A very long time ago, a large celestial body collided with Pluto. The debris ejected by this impact then aggregated to form the five moons we see today, which would explain their orbital arrangement and composition.