Can you make these 2 answers shorter and more 7th grader like? All planets, including Earth, are almost spherical in shape due to the force of gravity.
Not all planets are irregular in shape, but many of them do have non-spherical shapes. This is primarily due to the forces that act on them during their formation and evolution.
All planets, including Earth, are round because of gravity.
Some planets are not round because of how they formed and changed over time.