A deserted outer Solar System
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 06, 2008

After clocking up over 200 hours of observations as part of the Taiwanese-American Occultation Survey (TAOS) to map the distribution of small Kuiper Belt objects in the outer Solar System, astronomers have come up empty-handed.

TAOS dedicated two years to periodically photograph portions of the sky to look for small chunks of rock and ice orbiting beyond Neptune, in a region of the Solar System called the Kuiper Belt. The survey targeted Kuiper Belt objects (KBOs) with sizes between 3 and 28 kilometres and watched for stars to dim as these KBOs passed in front of them, since such objects are too small to see directly. Out of 200 hours of observations, TAOS did not spot any such occultations.

The New Horizons mission is currently en route to the Kuiper Belt system and will reveal more about the types of bodies that reside there. The Kuiper Belt extends outwards beyond the orbit of Neptune. The map above shows the dates of various landmarks in the New Horizons mission. Image: NASA.

The Kuiper Belt contains objects in a range of sizes from a handful of large dwarf planets Pluto, Eris, Makemake and Haumea, to many more smaller ones. The commonness of a given size of KBO gives information about the history of planet formation and dynamics, in terms of the history of colliding and planetary building events as well as destructive collisions to shatter rocky bodies.

Astronomers questioned whether they would find more and more KBOs as sizes decreased further, or whether the distribution levelled out. The fact that no occultations were seen has set a stringent upper limit on the number density of KBOs between 3 and 28 kilometres in diameter, revealing the outer Solar System as less crowded as some theories suggest. Astronomers speculate that this is either because small KBOs have already stuck together to form larger bodies, or frequent collisions have ground down small KBOs into even smaller fragments below the threshold of the survey.

The paper announcing this result appears in the 1 October issue of the Astrophysical Journal Letters.

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