Dim and Dimmer
Out in the deep, dark reaches of the solar system beyond Neptune’s orbit lies the Kuiper Belt, a vast ring of icy bodies big and small—including the Pluto formerly known as planet. Now, a Caltech-based team has used the Hubble Space Telescope to spot the smallest object ever seen out there—a chunk of debris only one kilometer in diameter. Over 1,000 Kuiper Belt objects have been discovered to date, but the next smallest one is roughly 30 kilometers across.
This piece of coal-black rubble from the birth of the solar system is some 6.7 billion kilometers away—half again as distant as Neptune—and 100 times dimmer than anything that the Hubble can see directly. Instead, the team used data from Hubble’s fine guidance sensors, which lock onto various guide stars in order to keep the telescope trained on whatever the astronomers happen to be studying. The sensors collect the guide stars’ light 40 times per second in order to measure the slightest changes in their positions, enabling the Hubble to stay rock-steady in its gaze.
Grad student Hilke Schlichting (MS ’07, PhD ’09) sifted through four and a half years of this tracking data, looking for any momentary dimmings of guide stars caused by Kuiper Belt denizens passing between them and us. Specifically, she was searching for a telltale diffraction pattern caused when an interloper intrudes on our line of sight—a new twist on the old high-school optics experiment using a slit and a light bulb, with the Kuiper Belt object acting as the slit.
The data, from 12,000 hours of telescope time, yielded exactly one occultation in the swath of sky extending 20 degrees from the solar system’s ecliptic plane, where the Kuiper Belt is expected to be densest. Based on downward extrapolations from the known population, the team had expected to turn up more than 20 bodies in the 300-meter size range. The whopping deficit supports the notion that these comet-sized objects are continuously colliding and grinding one another down, which would mean that the Kuiper Belt’s dwellers are evolving—they aren’t simply static souvenirs of our solar system’s formation, safely stored in the deep freeze.
Furthermore, says the Nature paper describing the find, this pulverization process “provides the missing link between large Kuiper Belt objects and dust, producing [the] debris disks [observed] around other stars.”
Schlichting’s paper was published on December 17, 2009. The other authors are Caltech postdoc Eran Ofek; Mike Wenz of the Goddard Space Flight Center; Associate Professor of Astrophysics and Planetary Science Re’em Sari; Avishay Gal-Yam, a senior scientist at the Weizmann Institute of Science and a visiting associate in astronomy at Caltech; Mario Livio of the Space Telescope Science Institute; Ed Nelan of the Space Telescope Science Institute; and Shay Zucker of Tel Aviv University. —DS