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7 Мarch 2006

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 mission^{4, 5} is now successfully launched (Fig. 1) and on its way to a flying visit to Pluto and its companions in 2015

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 surveys^{6, 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

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 impact^{3, 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

Source: Nature