The Enduring Mysteries of the Outer Solar System

By Charles Q. Choi
Special to SPACE.com
posted: 31 December 2007
7:00 a.m. ET

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."

Secrets in the Oort cloud?

A distant reservoir of trillions of comets known as the Oort cloud theoretically lies up to 100,000 astronomical units from the sun — an astronomical unit or AU being about 93 million miles (150 million kilometers). This means the Oort cloud is a fifth of the way to the nearest star, so far away that objects within it have never been seen directly, only inferred — but it must exist, given all the comets seen over the years.

The Oort cloud is the conjectured source of comets that require centuries or millennia to complete their long journeys around the sun. Since these "long-period comets" come from all directions, the Oort cloud is often thought to be spherical. However, while comets such as Halley's do not come from the Kuiper belt, their orbits also do not jibe with a spherical Oort cloud, Jewitt explained. This suggests there may be an "inner Oort cloud" shaped kind of like a doughnut.

Astrophysicists think the Oort cloud is a remnant of the protoplanetary disk that formed around the sun roughly 4.6 billion years ago. Learning more about the Oort cloud could shed light on how our solar system — and Earth — were born, Jewitt said.

Are there more dwarf planets?

So far, three dwarf planets are recognized — Ceres, Pluto and Eris. The Kuiper belt, which lies about 50 AU from the sun, could hold some 200 more. Beyond that there could be scores of dwarf-planet-sized bodies beyond roughly 100 AU from the sun "that nobody had seen before due to their faintness and slow motion," said astronomer Chad Trujillo at Gemini Observatory in Hawaii. "Even a body as big as Mars could be missed in our current surveys if it were moved beyond a couple hundred AU."

Trujillo noted projects such as Pan-STARRS (Panoramic Survey Telescope And Rapid Response System) and the LSST (Large Synoptic Survey Telescope) "should fill this gap in our knowledge in the coming decade."

Where do the dwarf planets come from?

There are theories that the dwarf planets of the outer solar system may have dwelt in the inner solar system billions of years ago, based on their current orbital trajectories. If so, "why are there so many ices on their surfaces?" Trujillo asked. Bodies in the inner solar system are generally expected to lose their ice due to sunlight.

Trujillo and his colleagues suspect the ice now seen on these dwarf planets is relatively new, with such replacement ice coming perhaps from within these worlds, erupting out during "cryovolcanism." Of course, further research is needed to see if such ice renewal would be enough to cover the dwarf planet after they voyaged from the inner to the outer solar system, he added.

Do cosmic rays come from a bubble around the solar system?

When the supersonic wind of charged particles that flows from our sun collides with the thin gas found between the stars, the solar wind essentially blows a bubble in this interstellar medium — a ball known as the heliosphere.

Scientists have thought unusually weak cosmic rays — energetic particles that zip from space at Earth — come from the heliosphere. Specifically, these rays are thought to come from the "termination shock" — a shock wave of compressed, hot particles that results when the solar wind abruptly brakes against interstellar gas. (The termination shock appears to be about 75 to 85 AU from the sun.)

However, Voyager 1 saw no sign these anomalous cosmic rays were produced at the termination shock. "Perhaps it crossed the shock at the wrong time or place," said MIT astrophysicist John Richardson, or perhaps the standard view on how these anomalous cosmic rays are generated is wrong. Voyager 2 crossed the termination shock in 2007 about 10 billion miles away from where Voyager 1 crossed it in 2004, and its data, which is still being analyzed, "may help us understand where these particle are produced," he explained.

"Cosmic rays have been reported to affect Earth's weather so understanding their source is important," Richardson added. Moreover, high-energy particles from shock waves triggered by huge eruptions from the sun known as coronal mass ejections can damage spacecraft and astronauts, and better understanding the termination shock could help understand these other, potentially dangerous particles.