Ring around the sun.

In 1993, astronomers reported indirect evidence that Earth is embedded in a ring of asteroidal dust orbiting the sun. The finding intrigues scientists because the ring may transport carbon and other elements to our planet (SN: 11/6/93, p.300).

The researchers based their finding on simulations of dust that escapes the asteroid belt and spirals toward the sun. As the dust moves sunward, some of it gets trapped in solar orbits that brush past the inner planets. A small fraction forms a dust ring near Earth, the team calculated.

The simulations indicated that the portion of the ring trailing Earth is closer to our planet and denser than the part ahead of Earth and therefore would appear brighter. A review of 1983 images taken by NASA's Infrared Astronomical Satellite (IRAS) revealed the predicted pattern.

But the IRAS data don't prove the existence of the ring, both because of uncertainties in detector calibration and because the craft didn't map the entire sky at one time, says William T. Reach of the Universities Space Research Association in Greenbelt, Md. The ring moves with Earth, and combining images taken at different times tends to smear its appearance.

In the April 6 Nature, Reach and his colleagues report a new analysis of a sky map taken in 1990 by detectors aboard NASA's Cosmic Background Explorer  satellite. The detectors simultaneously mapped half of the sky each week for 41 weeks. The team says it has confirmed the existence of the ring and predictions of its near-Earth structure.

"Before, you couldn't tell definitively whether it was a ring or not," says Reach. "Now, you can measure it."