Smart Earth is a planet very similar to Earth. It appeared in Captain Underpants and the Sensational Saga of Sir Stinks-A-Lot.
Smart Earth is a planet that orbited around the center star in Orion's belt. Smart Earth is almost identical to our planet except it is inhabited with geniuses. The reason everyone who lives on the planet is so smart is because it is made of a partially radioactive element called Zygo-Gogozizzle 24, which can be mixed with mayonnaise and dill-pickle relish to create both a tasty salad dressing and a clean-burning fuel source with enough power to light up a city. One of the noticeable side effects of being in close proximity or consuming Zygo-Gogozizzle 24 is becoming much smarter than necessary.
One day, one of the smartest scientists at Smart Harvard University (a parody of Harvard University) decided to do an experiment involving mixing Smart Diet Coke, Smart Mentos, and Smart Pop Rocks in her laboratory. Unfortunately, when the Mentos were mixed into the highly volatile concoction, Smart Earth exploded into huge chunks, resulting in the planet's extinction. Three pieces of the Hygo-Gogozizzle 24 landed in a pond on Badpun, a grape vineyard on Pinot, and a small Midwestern town in Piqua, Ohio on our planet. The chunk crashes through the roof of the Piqua Valley Home for the Reality-Challenged, where George and Harold's elementary school teachers were being kept. Mr. Meaner eats the piece of Hygo-Gogozizzle and becomes as intelligent as the other people on Smart Earth.
All the people who live on Smart Earth are highly intelligent due to the planet being made from a slightly radioactive chemical called Zygo-Gogozizzle 24, which makes anyone who is in close contact with it much smarter than they need to be. Even though Smart Earth is inhabited with geniuses, it is shares many similarities with our planet, including having McDonald's and smartphones (although on Smart Earth they are renamed "Smart McDonald's" and "smart smartphones").