Evidence for Nebulae About Other Stars Regardless of the means of starting the formation process, planetarysystems are believed to grow naturally from physical events thatdevelop after an interstellar cloud has begun to contract.
(A supernova is the explosion of a star in the last stages ofits life.) Possibly the concussion from the explosion triggeredthe collapse of the interstellar cloud to form the solar nebula.
That is, as the central portion of the solar nebula contracted,the temperature rose to around 2000 K, hot enough to vaporizeall compounds in the dust except the "high-temperature"metallic and silicate minerals in the inner portion of the disk,while the outer disk remained relatively cool.
Repeated encounters among them resulted in the accretion of literallybillions of still larger asteroid-sized aggregates called planetesimals,which orbited the center of the solar nebula.
Those comets brought water, allowing oceans and rivers to form.
In the Beginning...
About 5 billion years ago, a star exploded with a supernova, creating a nebula that would eventually form the solar system.
About 4.6 billion years ago, the first static electricity and magnetic forces caused the first rocks to form, eventually the rocks got big enough to start affecting each other gravitationally.
About 4.8 billion years ago, self-gravity caused the nebula to contract, creating a protoplanetary disc.
(The existence of dust particlesin the interstellar gas clouds out of which stars are formed wasaccepted in the 1930s.) This modern version of the nebular hypothesisis called the protoplanet hypothesis, and it owes muchof its recent revival to the power and scope of computer analysis.
A new factorwas introduced in the form of the existence in the cool gaseousnebula of a small amount of dust, providing nuclei for the condensationof gas particles into larger aggregates that could accrete andsolidify into the embryo planets.
Protoplanet Theory, the Solar Nebula By mid-century, astronomers once more turned their attentionto possible improvements in the nebular hypothesis.
Albedo; when ice and snow reflect solar radiation into space, in absents of greenhouse gases, which don't exist within the atmosphere, heat therefore escapes the planet....
In either version,however, this theory has serious failings in that solar matter,whether pulled or ejected, could not have acquired sufficientangular momentum nor could hot gas have condensed into planets.
Solar wind pushed lighter materials out, separating the material that would make the Terrestrial planets and the Jovian planets.
by Jacob Hobson
Earth's early atmosphere was believed to be made up of mostly carbon dioxide and methane.
Encounter Theories At the beginning of this century, attempts to reconcile the nebularhypothesis with physical principles were temporarily abandoned.
We might guess intuitively that the distributionof angular momentum in the forming Solar System ought to roughlymatch the distribution of mass; the central mass could not havetransferred this much momentum to the planets.
In the asteroid belt between Mars and Jupiter, the temperatureof the solar nebula was lower so that carbon- and water-rich mineralscould coalesce in the forming planetesimals.
First, whereas 99 percentof the Solar System mass resides in the Sun, 99 percent of theangular momentum of the system resides in the planets' orbitaland rotational motions.