aliens, planet 9, and an accident

https://en.wikipedia.org/wiki/Planet_Nine

So there might be a planet out there, way beyond pluto, screwing with orbits.

Also

Keep in mind that one worry we have about sending probes to habitable worlds is we might accidently send life there via bacteria or other 'infectants'. In effect, the aliens would be us.

And

Our solar system is lean on earthen planets. Many other solar systems have a "superearth"

So

What if Planet 9 is full of folks who developed eons ago and moved their planet to a more convenient location.

However

On one of their earlier exploration missions to a developing rocky world devoid of life, they accidentally introduced bacterial life. When they realized it hundreds of millions of years later, the life had already spread to cover the oceans of this planet and was producing oxygen and starting to spread to the land.

This would put them in an interesting moral situation. What to do. Should they exterminate the life they infected on this planet; or, should they allow it to develop, and if so, what then?

They might decide to stand on guard and let this civilization develop naturally while hiding their own presence.

Maybe that civilization is us.