Pluto leads the way in planet formation

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Images from the Hubble Space Telescope cast new light on the orbits, shapes and sizes of Pluto's small satellites. The analysis comes just before a planned reconnaissance by the first spacecraft to visit them. See Article p.45

Pluto and its large moon Charon together make up the only 'binary planet' in the Solar System. With a mass roughly 11% that of Pluto, Charon orbits the binary system's centre of mass at a distance of 17,500 kilometres every 6.4 days. Over the past decade, images from the Hubble Space Telescope (HST) have revealed four circumbinary satellites with orbital periods of 20–40 days and masses roughly 0.001% (or less) of Pluto's (Fig. 1). Before the discovery of the innermost and least massive of these moons, Styx, dynamical studies1 had suggested that the other three, Nix, Kerberos and Hydra, are packed as closely together as possible, with no room for other stable satellites between their orbits.

Figure 1: Pluto and its satellites.

NASA, ESA, M. Showalter (SETI Inst.)

This optical image, taken by the Hubble Space Telescope, depicts Pluto, its large moon Charon and four smaller moons Styx, Nix, Kerberos and Hydra. The image was taken in July 2012 when Styx was discovered. Showalter and Hamilton2 have used such images to derive several properties of Styx, Nix, Kerberos and Hydra. The ellipses shown are illustrative paths of the moons around the centre of mass of the system.

On page 45 of this issue, Showalter and Hamilton2 present an analysis of all available HST images of the system, and derive new orbits and masses for the moons. They also derive limits on the moons' previously unknown shapes and reflectivities. As well as confirming that the moons are in extremely tight orbits, the authors infer new relationships between the orbital periods of satellite pairs. These results may help us to understand how planets and satellites form and remain on stable orbits for billions of years.

The architecture of Pluto's small satellites closely resembles that of several planetary systems discovered by the Kepler space observatory3 (Fig. 2). In these systems, every object has a gravitational sphere of influence that prevents other objects from orbiting nearby. The more massive the object, the larger its sphere of influence. When the gravitational spheres of neighbouring objects nearly overlap, it is impossible to place other bodies on stable orbits between them. In tightly packed systems, the spheres of several (perhaps all) of the objects almost overlap. Small particles, such as interplanetary dust, might orbit in these intermediate regions, but large objects cannot.

Figure 2: Orbital architecture.

The satellite system of Pluto–Charon resembles some of the exoplanet systems discovered by the Kepler space observatory. Pluto's small moons orbit the system's centre of mass clockwise; the exoplanets orbit their respective stars (Kepler 730 and Kepler 2169). For each system, the scale is set relative to the orbit of the innermost moon or planet (the relative scales vary across systems; the gap between Pluto and Charon is not on the same scale as the orbits of the moons). The dots indicate the relative positions of the moons or planets; the circles show their respective gravitational spheres of influence. Similarly to the exoplanets, the spheres of influence of Pluto's moons leave little space for other potential (as yet undiscovered) objects in intermediate orbits.

These tightly packed systems place severe constraints on theories of planetary-system formation. According to current thinking, planets (and satellites) start as small seeds in a disk or ring surrounding the star (or planet) at the centre. These seeds grow by agglomerating other small solid objects along their orbits. Eventually, growing bodies feel the gravitational tugs of others in the system. Continued growth results in 'overpacking', whereby the spheres of influence of many growing objects in orbit overlap. As the gravitational forces between these objects build, their orbital motions become chaotic, and further growth is promoted through mergers of objects. When only a few planets (or satellites) remain, they settle into nearly circular orbits and their spheres of influence do not overlap. How some systems end up with objects in closely packed orbits is an open question.

Current hypotheses4,5 on the formation of the Pluto–Charon system focus on a giant impact in which a proto-Charon collided with a proto-Pluto to form a binary planet surrounded by an expanding ring of debris. Pre-existing moons might have survived the impact and new moons may have grown out of small particles in the debris. As well as having ended up in tightly packed orbits, the four moons that are the end product of this process (Styx, Nix, Kerberos and Hydra) exist in orbits with orbital periods in an observed ratio of roughly 3 : 4 : 5 : 6 times that of Charon6, respectively. High-quality measurements of the orbits and masses of all the moons in the system are needed to understand how this process works.

To constrain these properties, Showalter and Hamilton measure precise positions of the moons on the HST images. Assuming that the four moons follow elliptical orbits around Pluto–Charon, the authors present detailed modelled fits to their positions that yield the period, orientation (the inclination of the orbital plane with respect to the orbital plane of Pluto–Charon) and ellipticity of each orbit. Variations in the brightness of the moons at different times along their orbits allowed the authors to derive estimates of their sizes, shapes, reflectivities and masses. They conclude that the moons have orbital-period ratios of 3.16 : 3.89 : 5.03 : 5.98 — close to, but not quite, integers. Curiously, the synodic period of Styx and Nix (the time interval between orbital phases when two moons line up on the same side of their planet) is almost exactly 1.5 times the synodic period of Nix and Hydra. How this 'three-body resonance' developed during the growth of the moons is unclear6.

The shapes and compositions of Pluto–Charon's four moons provide crucial tests of models of planet and satellite formation5,7. Large fragments that survived the giant impact, thought to have led to the creation of the system, might have irregular shapes; satellites grown from much smaller particles might be more rounded. The authors find that the ellipsoidal shapes of the two larger moons, Hydra and Nix, seem more consistent with grown satellites than with impact fragments. Their optical reflectivity, at 40%, is similar to Charon's (36–39%), but lower than Pluto's (50–65%, which is comparable to the reflectivity of sea ice). With a reflectivity of only 4–6%, Kerberos is as dark as coal and seems out of place with such bright companions. Perhaps it is a dark fragment that was ejected during the giant impact.

It is hoped that NASA's New Horizons8 spacecraft, due to fly by Pluto in July, will throw yet more light on these questions. Close-up images taken by the spacecraft will further constrain the sizes, shapes and reflectivities of Nix, Kerberos and Hydra, but not of Styx — it is too small to be resolved in the images. The mission's spectroscopic measurements of the relative abundances of various ices will probably yield a reflectivity for Styx, and allow comparison of the compositions of the satellites. If new satellites or rings of small particles are found, and their bulk properties established, this will provide additional information on the extent of the system. These much-anticipated observations will lead to improved theories of the formation and evolution of planets and their satellites. Linking all these results to ongoing observations of the growing population of known exoplanets will extend tiny Pluto's reach far beyond the Solar System.Footnote 1


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Correspondence to Scott J. Kenyon.

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Kenyon, S. Pluto leads the way in planet formation. Nature 522, 40–41 (2015) doi:10.1038/522040a

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