Big enough for gravity to squeeze it to round. Most asteroids are odd shaped rocks. They don't count. Get enough rocks to generate enough gravity to pull the rocks into a sphere, then you've got a planet.

I agree about the clearing. But they'll invent some new rule. "But it's smaller than a moon in the same solar system!"

I have a simpler rule. If it's big and round, it's a planet. Unless it's burning with the flames of a nuclear fire, in which case it's a star.

Some planets go around stars. Some planets go around other planets.

Let the planets proliferate.

Don't let the astronomers know. They'll redefine it as a dwarf-moon planet or something.

I was just trying to be all sciency. Radio may well be better.

So that's where the first interstellar civilizations will form. It's still a long trip, but imagine being able to exchange laser messages in a month, round trip. You can imagine a buzzing web of communication, and much more incentive to try to mount a trip. Wow.