 One, it had to orbit the Sun. Two, it had to be big enough to assume hydrostatic equilibrium, which is a fancy way to say it needed to be round. And three, it had to, quote, clear the neighborhood around itself. Basically, to have a strong enough gravitational pull that there wasn't anything left immediately around it, like no asteroids or other small bits of rock or ice. And that third requirement, that's what disqualified Pluto.