Asobi ni Iku yo!, Motto To LOVE-Ru Recommendations
They're both harem series with multiple types of girls liking the protagonist, who is very unaware of the situaion. Besides that, both of the main character girls are from space, big chested, wear tight space suits and have small helper(s). Both series have ALOT of fanservice as well as a somewhat similar art and story.
Sudden alien girlfriend appearance, and multiple girls like the main character