What I don’t understand is how you define a single point (even if the point spans a million years) of “when the solar system formed.” They say the chemical composition of the Earth solidified “only 3 million years after the solar system formed” — isn’t the formation of the planets itself part of the formation of the solar system? How does one define the moment of formation? Or does this mean that we know with certainty that there was no physically consistent body one could identify as “Earth” 3 million years prior, and then within those 3 million years, it coalesced and solidified?
We don't. It's usually within a range. My illustrated book shows the timeline with more context and detail. Note that the events are provided on a timeline with some uncertainty (e.g., ± 1 million years):
I was under the vague impression that there is a single moment when the gravitational force in the core of the proto solar system outweighs (ha!) the electromagnetic repulsion of deuterium/tritium nuclei and fusion is achieved.
The resulting radiation pressure clears out the debris from the solar system’s accretion disk, and this process surely is not instantaneous but is relatively fast.
