The coming months promise to shed new light on the Solar System’s underworld.
Pluto has always had something of a raw deal. In classical mythology, while Zeus got sovereignty of heaven and Poseidon mastery of the seas, their brother Pluto (former name, Hades) was lumbered with the underworld and its legions of the dead. Pluto the planet had its discovery delayed by a decade-long legal battle, and then barely made it into the textbooks of the twenty-first century before astronomers decided to strip away its full planetary status. Its classification of dwarf planet is still contested by some. To others, it is the first example of the plutoid category of trans-Neptunian objects.
And then there are its moons. When the fourth of Pluto’s satellites was discovered in 2011, a campaign headed by Star Trek actor William Shatner proposed the name Vulcan. Preferring to maintain the underworld theme, astronomers chose Cerberus, after the dominion’s three-headed guard dog — although because that is already the name of an asteroid, they had to settle on the Greek spelling, Kerberos.
The other minor moons, Styx (gloomy river and one-time plunge pool for the infant Achilles), Hydra (many-headed serpentine sentinel) and Nix (a variant spelling of Nyx, goddess of the night) are joined by a large moon about half the size of Pluto called Charon (ferryman of the Styx and son of Nyx). Charon, some astronomers say, forms with Pluto the Solar System’s only binary planet. Or perhaps that should be double plutoid system.
In other words, nothing about this corner of our Solar System has been straightforward. And as the NASA spacecraft New Horizons hurtles towards it for the first close-up look at these bodies, astronomers this week pose new questions about the heavenly body formerly known as the planet Pluto. The answers, some of which might come when New Horizons flies past the dwarf planet in mid-July, could help researchers to understand how planets and their moons form in the first place.
Little is known about Pluto’s creation, but astronomers had assumed that it formed from the remains of a collision between proto-Pluto and a proto-Charon. The smaller moons may have then come together from bits among the swirling impact debris. The 2012 discovery of Styx was already something of a surprise, because studies had suggested the other three smaller moons were packed so closely together that there was no room for another.
On page 45 of this issue, planetary scientists Mark Showalter and Douglas Hamilton describe how they analysed Hubble Space Telescope images to build up a picture of the orbital configurations and brightnesses of Pluto’s small moons. They find that Styx, Nix and Hydra are locked together in what astronomers call three-body resonance, a phenomenon that links the timing of their orbits and usually makes their movements stable.
They also suggest that Kerberos is a little out of place. Although Nix and Hydra have bright surfaces similar to that of Charon, Kerberos appears as dark as coal, and this raises questions about how this mixed satellite system might have formed. (Pluto is the brightest of the lot, with a reflectivity roughly that of sea ice.) Their findings are discussed in a News & Views article on page 40.
The mythological name of Hades for the god of the underworld was replaced because Pluto has a more positive spin: it associated the ruler with the mineral wealth found underground. The next few weeks promise a revival of interest in Pluto, and a polishing of its image too. It deserves its time in the (dim and distant) Sun.
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To Pluto. Nature 522, 6 (2015). https://doi.org/10.1038/522006b