Thursday, February 23, 2006

Planetary science: Pluto's expanding brood - Nature

Should the "Kuiper Belt Objects" be called "rocky planets"?


and we may start calling them "planetary systems" as well. Many (if not all) of them are likely to have moons.


Richard P. Binzel1


Pluto is no lone ranger in the farthest expanses of the Solar System — its travelling companions now number three. And if Pluto can have so many, why shouldn't other objects in the distant, icy Kuiper belt?

Once thought to be a solitary denizen of the outer reaches of the Solar System, Pluto — which piqued our curiosity in 1978 with the discovery of its large satellite, Charon1 — is becoming ever more intriguing. In fact, the relative sizes of Pluto and Charon (Charon's diameter of around 1,200 kilometres is just over half that of Pluto's) means they are a 'double planet', orbiting a mutual centre of gravity, or barycentre, outside the surface of Pluto. But the story does not stop there. On page 943 of this issue, Weaver et al.2 present Hubble Space Telescope images showing that the Pluto system is at least quadruple. And as Stern et al. indicate in a companion paper3 on page 946, this complexity portends further discoveries: more small satellites may be lurking out there, and cratering impacts on them may have liberated rings or arcs of matter. Propitiously, NASA's New Horizons mission4, 5 is now successfully launched (Fig. 1) and on its way to a flying visit to Pluto and its companions in 2015.

Figure 1: Destination Pluto.
The New Horizons spacecraft took off from Cape Canaveral on 19 January 2006 aboard an Atlas V rocket, bound for the Pluto system. Speedy results are not to be expected: the half-tonne, piano-sized spacecraft must cover a distance of just under five billion kilometres, and will reach a point of closest approach some 10,000 kilometres from Pluto on 14 July 2015.
High resolution image and legend (69K)

Following Clyde Tombaugh's discovery of Pluto in 1930, searching for satellites was an obvious first task. But none was found until Pluto's march towards its point of closest approach to the Sun, coupled with the exquisite optics of a ground-based telescope, finally allowed Charon to be pinpointed1. Since then, ground-based surveys6, 7 have yielded no evidence for other satellites larger than about 160 kilometres in diameter. Motivated by the impending launch of New Horizons, Weaver and colleagues2 secured Hubble Space Telescope time in May 2005 to search for satellites as small as around 25 kilometres across8. Sure enough, two objects were found travelling through space with Pluto that had motions consistent with orbits around the Pluto–Charon barycentre (Fig. 2).

Figure 2: A remote quartet.
Two Hubble Space Telescope images of the Pluto system taken three days apart, revealing the existence of two smaller satellites, P1 and P2, in addition to Charon (discovered in 1978). P1 is the farther of the two newcomers from the system's centre of gravity, which lies just above the surface of Pluto. It completes just one orbit for every six of Charon's; P2 completes about one-and-a-half in the same time.
High resolution image and legend (13K)

This discovery2 prompted a fresh look at Hubble images from 2002 taken to map Pluto's surface. Although these images were not optimized for the identification of satellites, when preliminary orbital calculations gleaned from the 2005 images were added in, the presence of two additional companions was confirmed. The diameters of the satellites, creatively dubbed 'P1' and 'P2' (they will receive their official names later this year), are respectively around 60 and 50 kilometres, assuming surface reflectivities similar to that of Charon. (They are larger if their reflectivities are lower.)

Satisfying as discovery for discovery's sake is, it is the intriguing orbits of the newly spotted satellites that is creating the most scientific excitement. The present, limited data show that P1 and P2 are in circular orbits in the same plane as Charon. Moreover, the radii of their orbits place them in a resonant dance with Charon: for every twelve orbits Charon makes, P1 completes almost exactly two; in the same time, P2 (which is closer in) completes nearly three. Such consonance is not likely if P1 and P2 are captured objects that just happened, once upon a time, to have ventured too close to Pluto: tidal forces from Pluto and Charon are not great enough to coerce captured objects into co-planar resonances over the age of the Solar System3. The most plausible explanation is that Charon, P1 and P2 are all Pluto's progeny, and split off from it through a giant impact3, 9. The disk of material ejected by this collision into orbit around Pluto allowed these satellites — and perhaps others yet unseen — to condense in co-planar, circular orbits3. The resonant niches occupied by P1 and P2 may have been particularly fertile locations for coalescing material, or for maintaining long-term orbital stability.

As Stern and colleagues point out3, implications abound for Pluto's brethren in the Kuiper belt, the disk-shaped region of small, icy bodies found outside the orbit of Neptune. Within current detection limits, up to a fifth of all Kuiper-belt objects seem to have satellites or to be part of a binary system10. Pluto is the first known quadruple system, but multiple companions may be just the tip of the iceberg for the complexities of gravity's play on small bodies far from the perturbative forces of the Sun and giant planets. For example, most ejected debris from cratering impacts on P1 and P2 can easily escape the satellites' surfaces, but not the gravitational hold of the Pluto system. So tenuous rings or ring arcs may be the rule, rather than the exception, for Pluto and other multiple-bodied congregations in the Kuiper belt3. Even quadruple systems may become passé as investigations become increasingly percipient.

Those planning NASA's New Horizons mission, now en route first to a gravity assist from Jupiter in February 2007 and then its July 2015 appointment with Pluto, are now adding to their to-do list highly resolved imaging and spectroscopy of the newly discovered satellites. Refining these satellites' sizes and their orbital positions in nine years' time will also be a priority for observations to follow those currently being reported2. Both on its way in and out of the Pluto system, New Horizons' instruments will canvass the orbit plane for more satellites, rings and other telltale signs that might reveal the origin and evolution of this close-knit family. Pluto is a lonely place no more.

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Christy, J. W. & Harrington, R. S. Astron. J. 83, 1005–1008 (1978). Article ISI
Weaver, H. A. et al. Nature 439, 943–945 (2006). Article
Stern, S. A. et al. Nature 439, 946–948 (2006). Article (2006).
Stern, S. A. et al. Icarus 94, 246–249 (1991). Article ISI
Stern, S. A. et al. Icarus 108, 234–242 (1994). Article ISI
Steffl, A. J. et al. Astron. J. (submitted); preprint available at (2005).
Canup, R. M. Science 307, 546–550 (2005). Article PubMed ISI ChemPort
Stephens, D. C. & Knoll, K. S. Astron. J. 131, 1142–1148 (2006). Article
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