Seeing Red in the Outer Solar System

Large, cometlike objects that wander in the dark chill beyond Neptune may come in two colors, an unexpected dichotomy that may provide clues to the forces that shaped the outer solar system. A new analysis of the elusive bodies, reported in tomorrow's issue of Nature, suggests that some have a neutral color, while others are among the reddest objects in the solar system.

Astronomers found the first of these "Kuiper-belt objects" (KBOs) in 1992. So far, researchers have tracked the dim trails of 60 KBOs, out of a suspected population of perhaps 100,000 that orbit in Pluto's neighborhood. The icy bodies range up to several hundred kilometers across, but their extreme faintness--more than 1 million times fainter than the dimmest star visible to the naked eye--makes their properties difficult to study. However, powerful telescopes can glean the colors of KBOs, which hint at their compositions.

The new study of 16 KBOs spanned 2 years at the Steward Observatory's 2.3-meter telescope at Kitt Peak, Arizona. Stephen Tegler of Northern Arizona University in Flagstaff and William Romanishin of the University of Oklahoma, Norman, found that about half of the KBOs reflected sunlight without changing the light's color too much, suggesting that their surfaces are composed of cometlike dirty ices. But other KBOs absorbed blue wavelengths, making them appear markedly red. "Nature tends to like a continuum of colors, so this caught us by surprise," Tegler says. The finding may mean that processes at the solar system's birth or throughout its 4.5-billion-year history--such as heating, ultraviolet bombardment, or impacts--have acted on some KBOs but not others. Cosmic rays could burn off some hydrogen, leaving a dark red carbon-rich crust. Impacts, on the other hand, could cover the surfaces with fresh, color-neutral debris. But why there would be two distinct colors, Tegler admits, is a "total mystery."

Astronomer David Jewitt of the University of Hawaii in Honolulu, who co-discovered the first KBO, isn't convinced by the results. His own analysis of a larger number of KBOs--including data taken at the 10-meter Keck Telescope in Hawaii--shows a range of colors, not two distinct populations. "They may be correct, but it's very hard to achieve the precision they claim," Jewitt says.