August 18, 2009
How our solar system was formed has fascinated scientists and laymen alike for -- well, for a really, really long time. New research may have answered a piece to the puzzle - how big were the first planetesimals?
For those of you scoring at home," planetesimals" were the first solid objects in our newly minted solar system (also known as the protoplanetary disk). They began life as small grains of dust orbiting an infant sun. These grains would bump into each other, clump together and gradually form larger grains of dust, which eventually became small space rocks.
Now the theory goes that some of these small rock-sized planetesimals aspired for greater things, and continued to gradually grow in size to become asteroids, and that a few of those continued to grow beyond the asteroid stage and become planets.
The problem with this tidy little theory is that when the burgeoning space rocks grew to about one meter (3.3 feet) in size, orbital mechanics tells us the gas comingling with them in the protoplanetary disk should have acted like a brake, slowing their velocity appreciably. Their orbital speed having been cut, these filing cabinet-sized space rocks would have spiraled into the sun. Essentially, the gas would have acted as a celestial "mini-vacuum." The problem is, there are asteroids up there in space. Honest, ask any astronomer. So what happened?
Evidence is now mounting that these small space rocks quickly "jumped" (or grew) in size from below one meter to multi-kilometer in size. Planetesimals that big were big enough to plow through the drag created by the gas in the protoplanetary disk without having their orbits appreciably altered. Hence they did not spiral into the sun.
What data point to a jump in asteroid sizes? Simply, the asteroids available for viewing in the night's sky. Telescopic surveys indicate there is currently a plethora of asteroids less than one kilometer (.62 mile) wide but those over one kilometer drop considerably in number. The authors used computer simulations in an attempt to mimic the impacts and coagulation processes that took place over the millions of years between when the asteroids formed and now. The only way they could arrive at the current asteroid size distribution was to begin these simulations with planetesimals that quickly morphed into asteroids hundreds of kilometers in size. Once their growth spurt was over, these massive celestial bodies began an epoch-sized game of demolition derby as they orbited the sun. Over the eons, and with each extraterrestrial pileup, came fewer and fewer large asteroids - a fragmentation process that continues to this day. Despite the modest sizes of asteroids today, the paper's authors conclude that asteroids must have been born big.
The paper, "Asteroids Were Born Big" is available now online from the ScienceDirect website and will be available in a future edition of the journal Icarus.
For more information about asteroids and other near-Earth objects please visit: www.jpl.nasa.gov/asteroidwatch.