Bokura no Yoake

It's the year 2038, but more importantly, it's summer. A little boy named Yuma is obsessed with the impending return of the Arville Comet. Then, one day, he sneaks onto the roof of an apartment building and finds something even more amazing—something not from this world. Now Yuma and his friends, and his hovering autopet Nanako, must work together to uncover a secret that's slumbered in the Earth for years, as dawn breaks over a new era for him and for humanity...

(Source: Kodansha USA)