ELUSIVE RUBBLE: Many large Kuiper Belt objects [green] have been found since 1992, but searches for the smallest members of the belt have so far provided only upper limits on their abundance. Image: WIKIMEDIA COMMONS
For decades Pluto, later joined by its moon Charon, had a wide swath to itself on astronomers' plots of the solar system—no other bodies were known to dwell beyond Neptune in the long-hypothesized debris field known as the Kuiper Belt. But in 1992 a pair of astronomers turned up 1992 QB1, a body about 200 kilometers wide circling the sun at a distance of about 6.5 billion kilometers, well beyond Neptune's orbit. The Kuiper Belt, populated by leftovers from the solar system's formation, appeared to be real.
Since that discovery hundreds of large objects, most more than 100 kilometers in diameter, have been spotted in the Kuiper Belt, including some of the roughly Pluto-size bodies that spurred a redefinition of the word "planet" and relegated Pluto to dwarf status. (Pluto is about 2,300 kilometers in diameter.) But the smallest Kuiper Belt objects (KBOs) have been elusive. Although they should be numerous, no body smaller than about 30 kilometers has been directly spotted. Astronomers would like to know the size distribution of all objects in the belt—information that would help describe the composition and collisional history of the objects—but today's telescopes simply cannot pick out the faintest KBOs.
So a number of observational projects have taken a different tack, trying to identify small KBOs by monitoring background stars for sudden dips in brightness that might result from a distant object crossing the line of sight between the star and Earth. These so-called occultation surveys are currently the best shot astronomers have at parsing the smallest constituents of the Kuiper Belt.
But even for those occultation surveys, discoveries are tough to come by. In December 2009 a research team led by Hilke Schlichting of the California Institute of Technology published in Nature an analysis of archival data from the Hubble Space Telescope. (Scientific American is part of Nature Publishing Group.) Schlichting's group found just a single occultation, caused by a KBO approximately one kilometer in diameter, in 4.5 years of observations. And in the April issue of The Astronomical Journal, a group reports finding no occultations at all in 3.75 years of observations at the ground-based Taiwanese–American Occultation Survey, or TAOS. (The latter study, although shorter in duration, had actually collected about 40 times more "star hours" than the Hubble campaign.)
Federica Bianco, a postdoctoral fellow at the Las Cumbres Observatory Global Telescope Network in Santa Barbara, Calif., and the lead author of the new paper, says that TAOS should be sensitive to kilometer-size KBOs. The small haul of the Hubble study and TAOS's empty net, Bianco says, "confirm that there is a lack of objects in that region compared with what one would have naively thought" several years ago, before other searches came up empty or nearly so. TAOS scientist Matthew Lehner, an astronomer at the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei, Taiwan, points out that when the project was designed, the dearth of faint KBOs was not yet well known, and TAOS might have been expected to detect thousands of occultations.
The Hubble and TAOS surveys, along with their predecessors, indicate that some mechanism seems to curb the population of KBOs that have diameters of about a kilometer to tens of kilometers, relative to what one would expect from extrapolating downward from the observed population of large KBOs. "Physically, what that means is that these objects are weak," Bianco explains—collisions in the solar system's evolution have ground them down to even smaller bits that elude detection by the occultation surveys. Scott Kenyon, a theorist at the Harvard–Smithsonian Center for Astrophysics who builds numerical models of planet formation, echoes that assessment. "Both this observation and the Hubble observation confirm the idea that early in the solar system you make big objects and then the smaller objects collided and were destroyed as the system aged," he says. If next-generation observatories such as the James Webb Space Telescope can probe the distribution of KBOs just a fraction of a kilometer across, Kenyon adds, that would indicate just how brittle or mushy the KBOs are.
Astronomer David Jewitt of the University of California, Los Angeles, a co-discoverer of 1992 QB1, the first KBO other than Pluto and Charon, says that TAOS is looking for the proverbial needle in a haystack. "It's not an easy measurement," Jewitt says. "To their credit, they have done this and they report seeing nothing. But with needle-in-haystack projects, I always wonder whether they saw nothing because there was nothing to detect or because they missed the rare and transient brightness dips that small KBOs would cause." The researchers have shielded themselves well against this latter possibility, Jewitt adds, "but you never know."