After spending an eternity rocketing through the interstellar void, rogue planets can sometimes end their travels and find new solar systems to call home. But these adoptive parent stars aren't all that welcoming of their new planets.
The wildest ride in the galaxy is found on hypervelocity planets. These worlds got too close to the supermassive black hole at the center of our galaxy, and have been flung away at a twentieth of the speed of light.
Pluto is about forty times the distance from the Sun as Earth. But the Solar System is over 50000 times that length across, meaning it could be hiding some huge secrets. That's now looking like a small but real possibility.
We now estimate that every star in the galaxy has at least one planet, but that is leaving aside the potentially billions more planets that were ejected from their solar system and are now hurtling through the universe all alone.
This solar system just doesn't work. According to a new computer simulation, the planets could never have come together in their current configuration. The only explanation is that we once had a fifth gas giant...and it's still out there somewhere.