Back to my Cosmic supertype! In the past I've been doing various space-themed cards that also follow on Theros' connection of stars to enchantments. Cosmic is a supertype similar to Snow, in that it affects the mana produced by sources with it, and has effects that care about it.
There's only one Earth, so Legendary. I made the assumption that Planets would be a land type things would care about in later designs, so that's there. Now, I wanted to show Earth as out home, but the jumping-off point for humanity's ventures into the wider universe. The memory of it gives humans strength and resilience, so that's why it has the anthem. It also converts mana to cosmic mana for Humans, or just taps for cosmic mana. I added colourless as something it could make because... space.
I took a real photo of Earth from a distance, and quoted Carl Sagan because of course. What else could I do?
This design just came together quite easily. Wish that was the case all the time!