| Watsisname | Date: Thursday, 18.08.2016, 04:54 | Message # 211 |
 Galaxy Architect
Group: Global Moderators
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| A little bit of both. It's subtle and requires a bit of thinking. 
If in Pluto's place, there had formed a Mercury-mass object, then it wouldn't quite be massive enough to clear out its orbit within 10 billion years. There would still be a belt of similar objects hanging around with it.
But the dynamics of accretion growth make it unlikely for objects to reach that mass and then stop -- either they stay well below the orbit clearing mass, or, upon reaching it, they quickly continue to grow to domination. This is why we don't see borderline cases, which we think is true in general from simulations. The process is so rapid, like a runaway snowball, that it's unlikely to stop right at that line.
So probably the most succinct way to put this is this:
Disks either form planets which clear out a path in the process, or they form belts of smaller, similar bodies, like the asteroid and Kuiper belts.
Margot's Pi is a measure of the minimum mass needed to achieve the clearing process within the timescale of the life of the star. Once an object reaches that mass, it tends to go on to dominate its space, accreting or clearing out other material from its orbit so that it's the most massive thing around by far. Soter's discriminant quantifies by how much it is the most massive thing.
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