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| Artists impression of the late heavy bombardment of our solar system |
4.5 billion years ago, long before Homo
habilis first picked up a pointed stone and used it as a tool, something truly
spectacular occurred that would change the space around it forever. The Sun was
born!
Imbedded in a fluffy cloud of gas and dust
compressed under its own gravitational pull the core of the Sun burst into
existence igniting a fusion reaction that would and will continue to fuel it for over 10 billion
years.
At the start of its life a star rotates
very quickly, while there is no way to really know how fast the Sun's early rotation rate was the impact that it would have had on the surrounding disk is also hard to determine. We do, however, know that over time a star will loose its angular momentum through outflows and winds reducing its angular rotation over time, or 'spinning-down'. Using a technique called Gyrochronology we can estimate that at the age of 100 million years the Sun would have been rotating over 10 times its current ~25 day rotation
period, so we can only assume that in the lifetime of solar system formation or disk dissipation the Sun had a much faster rotation rate.
This large rapidly rotating mass at the
center of the cloud spins the material surrounding it causing it to flatten
into a disk – like spinning out a pizza base from a ball of dough. By observing
other pre-main-sequence stars and measuring the dust emission in the infrared
and mm wavelengths we can estimate what the Sun might have looked like shrouded
by its protoplanetary disk.
From observations it is estimated that it
takes around 7-10 million years for the protoplanetary disk to dissipate
potential forming a planetary system invisible to our current instruments.
The evolution of solids in the
protoplanetary disk is a multi-stage process:
First the gas and dust of the disk condense
to form micron-sized particles, 100 times smaller than the thickness of a sheet
of paper, to cm sized oxide and silicate grains. Over the next few million
years evaporation and recondensation will be the dominant process in the disk.
From studies of meteorites and asteroids it
is estimated that this high-temperature nebula process lasted between 3-5
million years before larger asteroid like bodies formed.
These asteroid-like bodies would have then
later formed bodies capable of retaining their own heat or substantial
radioactive material. Over the next 2-3 million years through collisions and
gravitational interactions planetesimals emerged. Followed by a chaotic period
of ‘shock processing’ or ‘heavy bombardment’ where the material fought its way
into stable orbits or was chucked out of the solar system entirely –
potentially forming some of the comets that come back to visit their original
home every few hundred years.
Theoretical models of protoplanetary disks
to early solar systems help us understand how material is likely to behave
within the disk and the likelihood of forming planetary systems that are stable
like our own. With the discovery of such systems over the last few decades an
increased effort has been applied to such simulations to determine if we really
are the exception to the rule, which thus far appears very different to our
own.
The nature of the very early solar
environment is still largely a mystery but scientists are working with renewed
vigor from analysis of meteorite to observations and simulations of young
protoplanetary disks. In an effort to answer the question; where did it start
and how did we get here?
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| A time line of the protoplanetary disk and its different stages |
What’s next?
Ian Czekala from the astrobites team has a
good review article on Protoplanetary disks and their evolution - http://astrobites.org/2011/03/11/review-article-protoplanetary-disks-and-their-evolution/
If you want to know a bit more about
gyrochronology and the methods used here is the paper written by Sydney Barnes
explaining in detail the technique used
Gyrochronology: S. Barnes 2007 - http://arxiv.org/abs/0704.3068
