Pluto may have a very distant cousin.
Pluto’s long lost relative might be the dwarf planet Ceres. The largest object in the asteroid belt, Ceres was offered up to be reclassified as a planet in the proposed reorganization of the Solar System at the August 2006 International Astronomical Union’s General Assembly.
After the IAU's raucous meeting that debated whether or not the planet count in the Solar System should be increased, Ceres was demoted to dwarf planet status and forgotten by just about everyone.
That is, except for the astronomers who sent NASA’s Dawn spacecraft to rendezvous with Ceres in 2015.
Now, William McKinnon of Washington University in Saint Louis suggests that Ceres may be a runaway from the Kuiper Belt, which forms the vast outer rim of the Solar System where Pluto and its plutoid cousins live. He presented his proposal at the Asteroids, Comets, and Meteors conference in Baltimore on July 15.
Ceres is an unusual inhabitant of the Asteroid Belt. For starters, it is one-third the mass of all the asteroids in the belt. Unlike almost all other asteroids it is spherical, like a planet. Ceres is approximately 580 miles across, or about the size of Texas.
Hubble Space Telescope observations that captured the full rotation of Ceres in 2005 first showed that it is a dynamically relaxed sphere that is slightly oblate. Computer models show that a nearly round object like Ceres has a differentiated interior, with denser material at the core and lighter material near the surface.
Ceres’ unusually low density is very close to that of Pluto’s rather that the other belt asteroids that are mostly rocky and metallic. This offers indirect evidence for a mantle made of liquid water. Based on its density, 30 percent of Ceres may be water.
That’s more fresh water than on all of Earth.
McKinnon’s highly speculative idea is that Ceres migrated from the Kuiper Belt in the early days of the Solar System. The belt’s existence is evidence that the giant outer planets were born much closer together than they are today. They gravitationally perturbed each other's orbits, and in the process a lot of residual icy debris in the young Solar System was shoveled out beyond Neptune’s present orbit to form the Kuiper Belt -- the Solar System's attic.
According to computer simulations, however, some debris was hurtled into the inner Solar System. Ceres may be one of those travelers that migrated across billions of miles from its birthplace.
The problem is that Ceres should have been kicked into an elliptical orbit. The orbit somehow was circularized as it settled into the inner Solar System. This might have happened through gravitational interactions with other primordial bodies that robbed momentum from Ceres. Or, if Ceres’ migration happened very early in the Solar System’s history, a residual gas disk around the sun may have put drag on the mini-planet.
In this scenario, where Ceres swung much closer to the sun, its ices melted and the small world must have been ablaze with ice geysers and cryo-volcanoes. Ammonia and methanol-bearing water-lavas may have flowed across the surface to leave behind the clays observed today. It might have looked like an outgassing comet on steroids.
The decay of radioisotopes in the dwarf planet’s interior, and perhaps heating by the early sun's strong magnetic field, may have kept the interior ocean warm enough to stay liquid to today. What’s more, the ocean could have antifreeze in the form of liquid ammonia mixed with the water.
Whatever Ceres’ origin, it will be an interesting place to visit when Dawn arrives.
Dawn will likely photograph a geologically unique surface that offers clues to Ceres’ past. Especially puzzling in a bright spot in Ceres' northern hemisphere that may be a big impact feature or an ice volcano coating some of the dwarf planet with bright frost.
Unfortunately, Dawn does not have a magnetometer (it was remove from the science payload in 2004) that could have been used to detect an induced magnetic field from a subsurface ocean. NASA’s Galileo orbiter used this same technique to identify liquid-water oceans under the icy surfaces of Jupiter's moons Europa, Ganymede, and Callisto.
Also, proof of Ceres’ pedigree could come from measuring the ratio of hydrogen to its heavier isotope, deuterium, in the ices or water vapor venting from the planet. If the ratio matches the ratio observed in comets, it would offer clear evidence of the Kuiper Belt being Ceres’ birthplace. But Dawn does not have this capability either, and it would have to be reserved for a future follow-on mission.
Nevertheless, my prediction is that Dawn will find that Ceres is so tantalizing, there will be discussion of a manned expedition to the mini planet. At 150 million miles from Earth, Ceres is farther away than Mars. But the dwarf planet has a much smaller gravitational field. So, landing and takeoff would require much less fuel (NASA’s Altair lunar lander on Ceres would weight less that the weight of a typical SUV on Earth). And, it is far less dangerous to land on Ceres than on Mars because there is no need for aerodynamic braking because Ceres is to too small to hold onto an atmosphere.
We would go there if we were convinced that a vast planet-wide ocean was under Ceres’ crust. It would be the nearest planetary ocean to Earth. This dwarf planet could be teeming with subsurface life that is perhaps more robust and diverse that whatever astrobiology might exist on Mars.
In preparation for a human visit heavy-duty drilling equipment, a habitat, chemical analysis laboratory, 
power station, and survey robots would be sent on ahead. A hole could be autonomously drilled deep into the crust.
Once the Ceres ocean was reached, a human survey team would make the long interplanetary trek. They would deploy remotely controlled hydrobots deep below the crust to collect water samples and look for evidence of life.
Imagine, someday before the end of the century astronauts could be doing oceanography on a world over 250 million miles from the sun.
Photos: NASA
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