The outer solar system remains mysterious

By Charles Q. Choi

updated 2:02 p.m. ET Dec. 31, 2007

The farthest reaches of our solar system remain the most mysterious areas around the sun. Solving the mysteries of the outer solar system could shed light on how the whole thing emerged — as well as how life on Earth was born.

Why the rainbow of colors in the Kuiper belt?
For instance, the Kuiper belt past Neptune is currently the suspected home of comets that only take a few decades or at most centuries to complete their solar orbits — so-called "short-period comets." Surprisingly, Kuiper belt objects "show a wide range of colors — neutral or even slightly blue all the way to very red," said University of Hawaii astrophysicist David Jewitt.

The color of an object helps reveal details about its surface composition. It remains a mystery why Kuiper belt objects show a much wider range of color — and thus surface composition — than other planetoids, such as the asteroids.

Some researchers had suggested volcanic activity could have led to all these colors — "absurd in the context of 100-kilometer-sized (60-mile) bodies," Jewitt said, as volcanism needs something bigger.

Jewitt and his colleagues had suggested that cosmic rays could have made Kuiper belt objects redder, while impacts with rocks could have dug up more pristine matter that made them less red. Nowadays Jewitt thinks there must be another explanation for this rainbow, but it remains unknown.

What is ultra-red matter?
There appears to be a material dubbed "ultra-red matter" that exists only on about half of all Kuiper belt objects and their immediate progeny, known as centaurs — icy planetoids orbiting between Jupiter and Neptune that very recently escaped from the Kuiper belt.

This ultra-red matter does not exist in the inner solar system, "not even on the comets which come from the Kuiper belt. This suggests that the ultra-red matter is somehow unstable at the higher temperatures close to the sun," Jewitt explained.

The red colors suggest this substance might contain organic molecules. Comets and other planetoids are often thought to have helped bring organic molecules to Earth.

"In the Kuiper belt objects, organics might have been 'cooked' by cosmic ray radiation, giving them dark red surfaces, but there is no proof," Jewitt said. Ideally spacecraft could go out there and find out, he added.

Has the Kuiper belt shrunk?
Theoretical calculations suggest the Kuiper belt was once hundreds or maybe even thousands of times more populated than it is now. "How was 99 percent or 99.9 percent of the mass lost, and when?" Jewitt asked.

One conjecture suggests when Saturn and Jupiter shifted their orbits roughly 4 billion years ago, their gravitational pulls slung Kuiper belt objects out of the solar system. Another says the Kuiper belt objects pulverized themselves to dust, which then was swept away by the sun's radiation. Yet another possibility "is that we are missing something crucial and the conclusion that the belt is heavily depleted is wrong," Jewitt said. "All these possibilities are comparably hard to swallow, but would each be amazing, if true."

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