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
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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."
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12 Comments
Add CommentIt is always nice when observation bears out theory - but even better when something unexpected turns up. Further inspection of the Kuiper Belt is bound to reveal some surprises.
Reply | Report Abuse | Link to thisNew Horizons has a secondary mission to visit a Kuiper Belt object after Pluto. Since that object is not named on the New Horizons web site, I assume that they are still assessing possible candidates. Does anyone know which object(s) are currently the most probable?
Reply | Report Abuse | Link to thisThere is only one object in the Kuiper Belt larger than Pluto, which is Eris; all others are smaller. The demotion of Pluto by four percent of the IAU is not justified, and the so-called "redefinition" of the term planet should not be accepted as anything but one side in an ongoing debate. The few Kuiper Belt Objects large enough to be rounded by their own gravity are small planets and should be distinguished from their tiny counterparts.
Reply | Report Abuse | Link to thisLaurele - rather than small planets these objects of which you speak are defined as dwarf planets. The demotion of Pluto is simply a part of the process of improving our understanding of the solar system - it is likely that Pluto and the other Kuiper belt objects share a life history that is different than the planets. Its not a problem to me if they feel they need to work on definitions as our understanding improves - it just takes a little getting used to.
Reply | Report Abuse | Link to thisMy issue is with the IAU statement that dwarf planets are not planets. I completely disagree with this, and I do not believe the 2006 decision that demoted Pluto does anything to improve our understanding of the solar system. Jovian planets don't necessarily have the same history as terrestrial planets, but that does not make them not planets. A good definition has to take into account both where an object is and what it is. The IAU definition, adopted by four percent of its members, most of whom are not planetary scientists, and opposed in a formal petition by an equal number of professional astronomers led by New Horizons PI Dr. Alan Stern, does not do this. No one should have to "get used to" a poor definition that represents only one side of an ongoing debate. Our solar system has three, not two subclasses of planets--terrestrials, jovians, and dwarf planets.
Reply | Report Abuse | Link to thisInteresting. In the mature solar system, big objects get ground into small objects. But in the young universe small objects combine into big objects. Why?
Reply | Report Abuse | Link to thisThe problem, laurele, is that although "size" has been the on criterion the general public has focused on, it's only one of the criteria in determining its status. Pluto has a highly eccentric orbit that's outside the ecliptic by 17 degrees, like a Kuiper Belt object. Its orbit is highly elliptical, sometimes even passing inside Neptune's orbit thus becoming our eighth "planet".
Reply | Report Abuse | Link to thisChange is not a bad thing.
It depends on the type of change. Elliptical orbits do not preclude objects from being planets. We have discovered giant exoplanets in orbits far more elliptical than that of Pluto. According to the IAU definition, none of these would be considered planets. One exoplanet system has two giant planets in a 3:2 orbital resonance just like Neptune and Pluto, meaning these two cross each other's orbits. Another system has a giant planet in an extremely comet-like orbit. The more we discover, the more we have to adjust our definitions to accommodate the variety of objects out there. That is why many support keeping the definition of planet broad to encompass any non-self-luminous spheroidal body orbiting a star. We can then distinguish types of planets through use of subcategories. Also, Kuiper Belt Objects themselves have a wide variety of orbits. Plutinos have resonances with Neptune while scattered disk objects have orbits far more eccentric than Pluto's. Again a good definition needs to encompass not just where an object is but what it is. Those objects in hydrostatic equilibrium are different from those that are not. The former are small planets with geology, weather, and even differentiation into core, mantle, and crust.
Reply | Report Abuse | Link to thislaurele - Well said. I'm just a pedestrian, but I agree that anything orbiting a star is a lot like a planet. On more thing: if it's massive enough to be considered generally spherical it ought to count for something.
Reply | Report Abuse | Link to thisAs a geologist I'm at least as annoyed by the artificial distinction between planets and satellites. Large satellites with dynamic histories are at least as much planets in the geologic sense as Mercury. We wouldn't hesitate to compare the Moon and Mercury merely because one is a satellite and the other an independent planet. However, we redefined "planet" after Copernicus and again after minor planets began to proliferate, and I see no reason to think we have the final word on the subject.
Reply | Report Abuse | Link to thisMentions of "demotion" and "relegation" to "dwarf planet" show that the IAU decision was not "simply a part of the process of improving our understanding of the solar system". If it had been, we would be talking of "reclassification to dwarf planet".
Reply | Report Abuse | Link to thisThe commentaries are as interesting as the article itself.
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