Pebbles in comets Available to Purchase

Comet Hartley 2’s orbit extends from Earth’s orbit to just beyond Jupiter’s. In November 2010, when Hartley 2 neared perihelion, NASA’s EPOXI spacecraft flew within 700 km of the 2-km-long comet, shown here. Among the mission’s findings was a surprise: The gas and dust spewing from Hartley 2 contained gravitationally bound pebbles. Erosion could conceivably account for the pebbles, but Katherine Kretke and Harold Levison of the Southwest Research Institute in Boulder, Colorado, have proposed a different explanation. Although comets are thought to form through the stepwise agglomeration of ever-bigger pieces, a competing hypothesis has emerged: The collapse of a cloud of dust and larger particles through gravitational instability. Kretke and Levison asked themselves whether the pebbles that EPOXI detected could be the uncoalesced leftovers of a putative collapse. To answer that question, they created a family of models of the Sun’s protoplanetary disk. They then identified the zones...