“This thing has always been cold,” said S. Alan Stern, the principal investigator of the mission, “and it’s not large enough to have a geological engine like Pluto. It should be a real window into the earliest days of the solar system.”
Objects in the outer solar system tend to be red, caused by the chemical reactions induced by radiation, but the cold classicals are even redder — ultrared, astronomers say.
A surprising number of the cold classicals, about a third, are binaries — two objects about the same size orbiting each other.
For Dr. Levison, this abundance of pairs could say something about the gentle nature of how the building blocks of planets came together. It’s possible that collisions out there are rare enough that such pairs have survived over the eons.
Dr. Malhotra is among the scientists who assert that the outer part of the Kuiper belt appears slightly warped, and that this could be a sign of gravity being exerted by something larger. Thus, not only might planets comparable in size to Mars and Earth have existed in this region, but they might also still be there today.
Dr. Gladman, for one, agrees. “I would bet my career there’s something of the scale of Mars- to Earth-size objects in the outer solar system,” he said.
Even farther out, beyond the Kuiper belt, are another group of objects that are even more puzzling. The first, Sedna, was discovered in 2003. At its closest approach to the inner solar system, it is seven billion miles from the sun, or more than twice as far out as Neptune, and too far away to ever have been gravitationally kicked by any of the giant planets. At its farthest orbit, Sedna is some 87 billion miles from the sun.