Here is the full list of all 25 Tasty Planet: Back for Seconds achievements.
Complete the future levels
Complete all levels
Eat 25,000 objects
Complete the Feudal Japan levels
Earn a bronze medal or higher on half of the levels
Complete the Ancient Rome levels
Eat 10,000 objects
Complete the Ancient Egypt levels
Eat 5,000 objects
Complete the dinosaur levels
Complete the laboratory levels
Earn a silver medal or higher on half of the levels
Earn gold medals on half of the levels
Earn a bronze medal or better on every level in the game
Earn a silver medal or better on every level in the game
Earn gold medals on all levels
Consume 2,000 cats
Eat 50,000 objects
Eat 100,000 objects
Eat 200,000 objects
Eat another grey goo
Complete all levels in 2-Player mode
Earn a bronze medal or better on every level in 2-Player mode
Earn a silver medal or better on every level in 2-Player mode
Earn a gold medal or better on every level in 2-Player mode