So the fact that we can still see these disks around a 10- to 20-million-year-old star means that the dust is being replenished by collisions between planetesimals
It is though Earth was likely formed from a collection of planetesimals
that had already lost their atmospheres.
Whether the four inner planets or a migrating Jupiter nudged the planetesimal
toward the asteroid belt is unclear, the team reports.
Welsh summarized the current theory of planet formation as "interstellar dust under the influence of gravity becomes blobs, and the blobs grow into rocks, the rocks coalesce and become bigger things - planetesimals
and comets - and finally, you get planets.
Ozernoy and Ipatov have estimated the number of planetesimals
that were flung at the early Earth from reservoirs of such bodies following orbits inside Jupiter's path or crossing it.
Johnson and Li found that the formation of planetesimals
can only take place once a minimum metallicity is reached in a protostellar disk.
New observations by Aki Roberge of Johns Hopkins University in Baltimore and her colleagues suggest that a mere stripling of a star, which might be as young as 300,000 years, already has planetesimals
But if the fraction of pebbles is increased slightly, the clumping increases dramatically and quickly results in the accretion of sufficient material to make larger-scale planetesimals
They propose that bits of dust within the disk of material that surrounded the infant sun assembled "into planetesimals
many kilometers across over a time interval of order 1 million years or less," says Jack J.
Renu Malhotra of the Lunar and Planetary Institute in Houston and Norman Murray of the Canadian Institute for Theoretical Astrophysics in Toronto and their colleagues propose that migration begins well after the gas in the protoplanetary disk has dispersed, leaving behind fully fledged planets and a debris field of boulder-size remnants called planetesimals
The results come from new simulations using a theory called the Nice model, which suggests the solar system's giant planets were born closer together and were surrounded by a vast disc of leftovers from the planets' formation called planetesimals
This finding--the first accurate measurement of the density of a carbonaceous asteroid--could prompt a reassessment of the geophysical history of these wandering planetesimals