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How can we even know it's a planet when we haven't observed a single orbit, yet?
Maybe it's just a really big comet.We've observed enough of its orbit to know what shape it is.
How can you be so sure it doesn't just decide to leave
And who could blame it?
It's not really big. There are multiple Pluto like planetoids. Also there is a hunk of rock in the asteroid belt youcould name a planet.
Ceres is more than a third of the mass of the asteroid belt. It's at least an order of magnitude bigger than anything else in its orbit. It's hardly a hunk of rock
There's also not that much rock, only 73% of the mass. The rest is ice and mud, with half it's volume being water in some form.
By measuring how fast it's moving and where it is you can find the orbit