Please Save My Planet comes packed with some heavy drama, on the other hand Dear Mine is just a light shoujo with sugar and fluff. Both of those feature a couple with a noticeable age difference (mostly due to the fact that the guy is a grade school student). If after reading Please Save My Earth you realized that you were after something (much) lighter on drama, then Dear Mine might actually suit your taste.
Yuuhi Amamiya is a loner whose mistrust of people was instilled in him at a young age by his abusive grandfather. So when an annoying talking lizard tells the university student that he is one of twelve knights chosen to stop the gigantic "biscuit hammer" before it smashes the Earth to smithereens, Yuuhi has no inclination to rescue such an unkind world. Despite his apathy, the boy is targeted by a golem while going about his day-to-day life, and ends up teaming up with the other knights anyway. One of his new companions is Samidare Asahina, the "princess" and leader of the knights, who secretly is only trying to protect the Earth so she can destroy it herself! Entranced, Yuuhi swears eternal loyalty to this cheerful devil that promises to crush the world he despises.
Please Save My Earth and Hoshi no Samidare felt similar for some reason. It's difficult to explain, so I won't even try to do so. If you can trust my gut feel, then go and check out the one you haven't read yet. Heck, you might actually like it.
Both of those series are fairly long shoujo about a high school girl x elementry school boy romance. Both have very oldschool art style and fairly good story. Kumiko & Shingo has none of the sci-fi/fantasy elements that Boku no Chikyuu wo Mamotte features, but hopefully it won't prevent you from reading one if you read the other.
Airi has always dreamed of marrying her fated true love and living a normal, happy life – a dream disrupted forever when Aram, a prince from another world, emerges from her grandmother’s mirror. She takes the young prince home with her thinking he is a lost child, giving him a home cooked meal and a place to sleep. However, in the morning she discovers that, as the result of a magic spell, Aram transforms into a 17-year-old boy whenever it’s dark, and her kiss is the only way to change him back! If that wasn’t enough, Aram is in hiding from his older brother, the evil sorcerer Jeile, who has fallen in love with Airi and intends to make her his bride! Will Airi ever be able to find her happily-ever-after?
So, Merupuri also sports this grade school aged kid who is quite mature for his age and who also happens to be in love with a high school girl. The thing is, Merupuri ends up being a pretty much generic shoujo and the plot isn't its strong part. Anyway, if you found the drama to be too much in Please Save My Earth, then you might actually like Merupuri.