That's pretty wild.
A lot of computer have simulated the creation of our solar system and shown that we once had another gas giant that was ejected shortly after its formation.
You are correct. Apparently rogue planets, especially those gas giants, are very common.
I've always found the concept of rogue planets fascinating, maybe because it originally came about in science fiction, unless I'm mistaken. Though in a way, I suppose a rogue planet is nothing more than an asteroid or comet with adequate mass to assume a spherical shape.
On that note, I wonder what it is about rogue bodies that prevents them from being captured by the gravity of another celestial body. For instance, the majority of the moons in the solar system are just captured asteroids, but how can asteroids of similar size zip right by a planet in very close proximity without becoming trapped by its gravity? Maybe it's the super slow moving asteroids that become moons?