Sen—On July 14 NASA's New Horizons spacecraft made its closest encounter with dwarf planet Pluto. As the probe makes its way deeper into the Kuiper belt I thought I would take a moment to look at the rest of the known dwarf planets lurking in the outer Solar System. Some might consider Pluto to be the king of the Kuiper belt at 2,370 kilometers in diameter, but it is easy to forget that there are a slew of other Pluto-sized or slightly smaller bodies beyond 30 AU. The incredibly detailed but brief view of the Pluto system with New Horizons will also profoundly transform our understanding of these other large bodies.
The early and mid 2000s were really the gold rush for distant icy dwarf planet discovery. Recent results suggest that the era of finding new icy Pluto-sized bodies has come to an end or at least a lull until much bigger telescopes equipped with wide-field cameras come online in the next decade. But until then we have a handful of roughly Pluto-sized objects residing in the Kuiper belt that stand out amongst the sea of the planetesimals residing beyond Neptune. These large planetoids have strange names and designations like: Eris, Quaoar, Makemake, Orcus, 2007 OR10, Haumea, 2012 VP113, and Sedna.
These icy worlds really are the lighthouses of the Kuiper belt. They're large enough and bright enough for the ensemble to be studied by ground-based and space-based telescopes. Much has been learned remotely about their surfaces and their formation during the early times of the Solar System. The orbits of the discovered moons about many of these bodies have helped probe their internal composition, mass, and bulk density. Unlike most smaller Kuiper belt objects, these Pluto-sized bodies are mostly round, likely internally differentiated with the ice and rock separated, and contain frozen volatile ices, like nitrogen, carbon monoxide and methane, in varying amounts on their evolving surfaces.
The dwarf planets in the Kuiper belt are truly unique worlds each with their own story to tell. Some interesting factoids about these distant Solar System residents that you might not know:
Haumea is now a American-football shaped body once likely spherical. It is thought that it suffered a mantle shattering collision. It is the progenitor of the first and only identified collisional family in the Kuiper belt. The collision stripped the differentiated body, leaving only a rapidly rotating rocky core with a thin vineer of remaining water ice in place. The rest of the icy mantle was flung into the Kuiper belt and formed the two moons (Hiʻiaka and Namaka) orbiting Haumea. Hi'iaka has the deepest water ice absorption measured in the entire Kuiper belt.
Makemake is very much a Pluto analog—it has both methane and nitrogen detected on the surface, but unlike Pluto there is less nitrogen, so more methane atoms are in contact with each other. Sunlight makes Makemake a photochemical kitchen. When the methane is irradiated it is churned into ethane and other more complex organics (the molecule loses a hydrogen bond and more carbon atoms become chained together). There has been evidence to support this with the spectroscopic discovery of ethane and later the presence of ethylene and acetylene solids on Makemake’s surface.
Eris, the body whose discovery lead to Pluto's demotion from full planet status, is about the same size as Pluto but nearly 25 percent more massive. It currently reigns as the most distant dwarf planet with a nearly uniform icy surface containing methane and nitrogen ice. Eris might potentially have an atmosphere like Pluto that is currently frozen out on the surface due to its far distance from the Sun.
Pluto is the only dwarf planet with a confirmed atmosphere. It suffered a collision that is thought to have had a large impact that formed its moon Charon and potentially the myriad of smaller satellites about Pluto. The reigning impact model indicates that Charon is made mostly of impactor material.
Sedna’s highly eccentric orbit with its closest approach to the Sun at 76 AU is dynamically detached from Neptune and with its furthest point in its orbit located at roughly 1000 AU, Sedna is too far away from the edge of the Solar System to feel the effects of passing stars in the current solar environment. Its orbit implicates some other mechanism no longer active in the Solar System occured to emplace this world on its orbit. There are many theories for Sedna's origin including a stellar encounter or scattering from an additional planet-sized body.
2007 OR10 has one of the reddest surfaces in the Solar System. There is suggestive evidence for methane on its surface with the red color thought to be a result of an organic crust made from solar irradiated methane.
What we are learning about Pluto
What's exciting to me about the Pluto mission is that it brings the ground truth for all the observations taken from Earth. We have broad-brush strokes about Pluto and its siblings. For example, when I look at the images of Pluto, I was not surprised by the molted surface and Pluto’s shades of reddish brown compared to the more cream colored Charon. Previous Pluto observations have detected a surface of nitrogen ice with dissolved methane ice dissolved in nitrogen ice. The patty dark areas would then be a reddish organic gunk crust made from irradiated methane. Lighter areas are likely fresher deposits of methane ice condensed out from Pluto's tenuous atmosphere. How these substances are exactly distributed on Pluto's surface is an open question. New Horizons is providing the fine brush strokes that will make the painting come into view. Just like what the European Space Agency's Rosetta mission is doing for comets, New Horizons data will challenge and validate the hypothesis and inferences made from the nearly decades worth of remote telescope observations of Pluto and the other icy dwarf planets.
These rich worlds beyond Neptune I think are each worthy of a dedicated robotic mission. Perhaps in the next 50 years, a spacecraft will be on its final approach to Eris or Makemake. For now, Pluto will be the only of these large bodies that New Horizons will visit. None of the known dwarf planets are close enough for New Horizons to veer off with the available fuel aboard the spacecraft. New Horizons, if it gets an extended mission, will likely visit one of the smaller post-Pluto encounter targets in the cold classical Kuiper belt that was recently discovered in Hubble Space Telescope observations. We might have to wait a long time for a mission to Haumea or Quaoar, but in the meantime the wealth of information gleamed from New Horizon's Pluto flyby will be applied to better understand Eris, Quaoar, Makemake, Orcus, 2007 OR10, Haumea, 2012 VP113, Sedna, and their yet to be discovered friends.