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The largest piece of real estate in the solar system is also the least explored. The solar system's outer rim, the Kuiper Belt, extends from just beyond Neptune's orbit to 5 billion miles from the Sun. It is a debris field of presumably millions of icy bodies left over from the formation of the planets.

First hypothesized 60 years ago by planetary expert Gerard Kuiper, the existence of such an outer comet belt wasn't proven to exist until astronomers began discovering objects in the Kuiper Belt (other than Pluto) in the early 1990s. Now Hubble Space Telescope has stumbled across the smallest thing ever seen in the Kuiper belt.

The previous record holder is 30 miles across. But the interloper Hubble spotted is merely one half-mile across. It would just stretch across the width of New York City's Central Park. In terms of angular size, the object would be the apparent width of a dime located 1/3rd of the way between here and the moon! It is 100 times fainter that Hubble's detection limit.

So how did the Hubble ever uncover such a puny object?

In a paper published in Thursday's science journal Nature, Hilke Schlichting of the California Institute of Technology is reporting that the telltale signature of the small vagabond was extracted from Hubble's engineering data, not by direct observations.

Hubble has three optical instruments called Fine Guidance Sensors (FGS). The FGS provide high-precision pointing information, which guides the observatory's attitude control systems. The sensors exploit the wavelike nature of light to make precise measurement of the location of stars.

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Schlichting determined that the sensors were so good that they could detect the effects of a small object passing in front of a star. She and her team had to sort through a total of 50,000 guide stars observed over 4.5 years.

This needle-in-a-haystack search yielded evidence of one transit lasting merely 0.3 second. The duration of the occultation was brief largely because of the Earth's orbital motion around the Sun. This was only possible to capture because the FGS instruments sample changes in starlight 40 times a second. The KBO's distance was estimated from the duration of the occultation, and the amount of dimming was used to calculate the size of the object.

At 1,400 miles across, the much maligned dwarf planet Pluto is the second largest known member of the Kuiper Belt, but there are many more fragmentary bodies hundreds or tens of miles across. Now Hubble has provided the first observational evidence for a population of comet-sized bodies in the Kuiper Belt that is made up of the ground-down fragments of collisions among larger bodies.

The team plans to go hunting for more comet-sized fragments by reviewing more FGS data from nearly the full duration of Hubble operations since its launch in 1990.