Discovery Hints at a Quadrillion Space Rocks Beyond Neptune

Dozens of rocky bodies that are part of a sea of small rocky fragments never observed before have been spotted in the suburbs of our solar system beyond planet Neptune, thanks to a novel technique.

These newly detected chunks of dust and rock coined Trans-Neptunian Objects (TNO) are smaller than 330 feet (100 meters) across. They are leftovers from the formation of planets.

Scientists had previously detected TNOs larger than 31 miles (50 kilometer) across such as the Kuiper Belt Objects (KBO), a subset of TNOs. They suspected that there may be distant objects beyond Neptune since the 1940's, but it wasn't until 1992 that the first KBO was discovered.

Since then, they've found so many large objects in the outskirts of the solar system that they had to come up with crazy names, like Plutinos, Centaurs, and Cubewanos, to keep them in order. And although researchers suspected the presence of smaller objects, they didn't have a way to detect the sea of debris.

"The searches for Kuiper Belt Objects usually look for reflected light from the Sun and the small motion relative to fixed background stars," said Asantha Cooray, assistant professor of Physics and Astronomy at the University of California, Irvine. The amount of reflected light from a small body, however, is so extremely dim that not even the largest telescopes, or much larger telescopes one could imagine building either on Earth or space, could see it.

But scientists didn't look for the reflected light this time. Examining data from NASA's Rossi X-ray Timing Explorer, they monitored the light from a background star, Scorpius X-1, as small objects moved in front of it in what are called occultations. They found obvious dips in the light.

Other than the Sun, Scorpius X-1 is the brightest X-ray source in the sky, said study leader Hsiang-Kuang Chang, Associate Professor of Physics & Institute of Astronomy at the National Tsing Hua University, Taiwan.

"We discussed various possibilities for causing these dips and concluded that occultation by small TNOs are the most likely one," he told SPACE.com in an email interview.

Alltogether, Chang and colleagues identified 58 definite dips. Their findings are detailed in the Aug. 10 issue of the journal Nature.

Observing occultations is a widely known method for studying foreground objects by monitoring the light of background stars. The rings of Uranus were first discovered during an occultation of a star by Uranus. But never have such small objects been detected this way.

"The interesting thing here is that instead of monitoring optical stars, these authors monitor light from an X-ray source since X-ray detectors can record light at small time intervals compared to optical detectors," Cooray told SPACE.com. "A 100-meter body only occults a background source for about 10 milliseconds and optical detectors cannot record light continuously at such small time intervals."

Based on this finding, the researchers estimate that the number of TNOs reaches around a quadrillion, rather than the mere billions to a trillion as previously thought.

This shows an extremely dense disk of material at the outer edges of the solar system mostly populated by smaller bodies, Cooray said. "Since these are leftover material from the solar system formation process, it says that the original disk from which the planets formed was more massive at distances around Neptune than previously suggested and in strong conflict with some of the early models for the formation of Kuiper Belt Objects."

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