I knew it, I knew it

"Not hundreds of years, or hundreds of thousands. He had been at the bottom of the sea while the solar system captured a new planet, and lost a good third of its asteroid belt, while oceans of food yeast mutated and went bad, and mutated again, and again…At the bottom of the sea he had waited while yeast became grass and fish and now walked on two legs like a thrint.

A billion years wouldn't be long enough. Two billion might do it."
The World of Ptavvs. Larry Niven

Some one and a half billion years ago there was a telepathic race of intelligent beings who ruled the galaxy. They enslaved all intelligent life. They were the thrintun.

One thrint had a mishap that disabled his ship. He put himself and his greatest possessions into a stasis field, and his ship crashed into a solar system that the thrintun were developing. His spare suit crashed into a satellite of the eighth planet of the system's sun and knocked it out of orbit.

It became a planet, orbiting the sun at odds with the rest of the system, it's orbit taking it far inside the eighth planet's orbit, and then flinging it out again.

Life and sentience happened. Space travel began, too. Tentatively at first, but colonisation of habitable worlds became the next big thing. Of course, they knew about the strangeness of the ninth planet, they thought it might be an interloper from another star system, they thought they need to study the skies more.

No matter that Pluto has been booted out of an elitist club. They should have asked Larry Niven. He told us science fiction buffs exactly how Pluto became a planet, way back in the late Sixties.

He knew how it all happened. Ha.

Cheers!

Update: Chandru and I are hopelessly under-informed. Raj knows more about this.