By Ron Cowen
By measuring tiny dips in the intensity of X rays from a distant star, astronomers say that they have detected more than 50 of the tiniest chunks of ice ever found in the outer solar system.
Since 1992, researchers have discovered nearly 1,000 frozen bodies beyond Pluto. All are at least several tens of kilometers in diameter. In contrast, the newfound objects are only 10 to 100 meters in diameter.
These small fry can’t be seen with even the world’s largest telescopes. Instead, a team led by Hsiang-Kuang Chang at the National Tsing Hua University in Hsinchu, Taiwan, used an indirect technique. The group looked for random, brief drops in brightness of the star Scorpius X-1, the brightest X-ray source in the sky.
Because Scorpius X-1 lies close to the plane in which Earth orbits the sun, millisecond dips in its brightness could be caused by tiny objects at the edge of the solar system passing between it and Earth. The star itself probably doesn’t produce such dips, the team notes.
Using data recorded by NASA’s Rossi X-ray Timing Explorer over 7 years, the Taiwanese researchers identified 58 short-lived dips in the light from Scorpius X-1. Those dips probably represent 58 previously unknown bodies in the outer solar system, the researchers say in the Aug. 10 Nature.
Extrapolated from the findings, the total number of objects between 10 and 100 m in diameter in the outer solar system could be as high as a quadrillion, notes astrophysicist Asantha Cooray of the University of California, Irvine, in a commentary accompanying the report. That’s a thousand to a million times as many objects in that size range as computer simulations have produced.
The simulations have assumed that small bodies—debris left over from the planet-formation process—initially stuck together to create big objects. Collisions between small and large bodies, which have different speeds, eventually created dust. That dust then either drifted out into space or spiraled into the sun, reducing the population in the outer solar system.
Collisions in the outer solar system may have been less frequent than modelers had assumed, says Cooray. With fewer collisions, a greater number of objects would have remained in that neighborhood.
However, the mismatch between simulations and the new data could simply result from uncertainties in interpreting the X-ray data, the Taiwanese team notes. A small body closer to Earth could produce a dip similar to that of a large body that lies farther away. Although changes in the shape of the dip could, in theory, pin down the distance of the object from Earth, the X rays from Scorpius X-1, though intense, weren’t bright enough to reveal those changes.
The evidence for the small, icy objects is convincing, comments planetary scientist David Jewitt of the University of Hawaii in Honolulu. However, given the uncertainties, “no revision [of the structure] of the solar system is needed at this time,” he says.
