While some things turn blue when they get really cold, a newly discovered group of frigid solar system objects is decidedly red.

Perhaps they're embarrassed about being so un-neighborly.

The inexplicably red comet-like objects orbit the Sun in the cold outer reaches of the solar system in a region known as the Kuiper Belt. Compared to their gray kin, the red objects have an exclusive hold on the most far-out orbits.

Comets and asteroids take on different colors depending on how much blue sunlight they absorb or reflect, which in turn is tied to their composition. Since the 1950s, astronomers have known that very red asteroids don't mingle with the not-so-red variety in the main Asteroid Belt, between Mars and Jupiter. The red ones dominate the inner regions of this belt.

"It is not particularly surprising that the properties of asteroids should change between the inner and outer parts of the main Asteroid Belt," says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics. After all, this belt marks the transition between the rocky inner planets and the gas giants of the outer solar system, he says. "What is perhaps more unexpected is that a similar transition should occur in the Kuiper Belt."

The Kuiper Belt, a vast region of space beyond the orbit of Neptune, was first proposed in 1951. Some 300 objects have been found in the belt, just in the last decade, and scientists expect the region may be littered with as many as 100,000 objects.

The red Kuiper Belt Objects -- or KBOs, as they are called -- were first noticed three years ago, their color being significantly redder than other KBOs because they absorb more blue light. No one has yet figured out exactly what about their composition causes the color differences.

Scientists can only "speculate on the nature of the blue absorber," said Stephen Tegler of Northern Arizona University, lead author of a paper appearing in the October 26 issue of the journal Nature. "Perhaps, the red objects are rich in complex organic molecules."

Solving the puzzle could help scientists learn more about the formation of the solar system.

KBOs travel in non-circular orbits (much like Pluto, which is considered a KBO by some scientists). These egg-shaped orbits mean a typical KBO's distance from the Sun can range from 38 to more than 50 AU (1 AU, or astronomical unit, is the distance from Earth to the Sun). The point nearest the Sun is called perihelion.

"The objects in our survey are evenly split between red and gray for perihelion distances out to 40 AU," Tegler said. "For these objects, perihelion distance does not seem to affect the colors of the objects. For objects with perihelion distances beyond 40 AU, we only see red objects. The reasons for the patterns remain a mystery."

(Note: The click-to-enlarge image on this page shows the distribution of KBOs. Also shown are Plutinos, objects with orbits similar to Pluto and Centaurs, icy comets thought to have escaped the Kuiper Belt and wandered across the orbits of Uranus, Saturn and Jupiter. The image shows KBOs and planet positions as of October 16.)