If you're looking for manga similar to Soratobu Toshi Keikaku, you might like these titles.
Set in a post-apocalyptic world, Pez and Decosuke cross this now desolate landscape searching for signs of life. Along the way, they experience a number of moments that remind them of what it meant to be alive and why their lives are so precious.
It's 2036, and technological advancements in Hyper-Space Navigation and Gravity Manipulation have brought about an explosive acceleration in space exploration and the expansion of the human empire. With the scattering of the human race among the stars, birth rates have dropped and the demand for labor has skyrocketed. Thus, B-level and C-level robots were invented. They can complete simpler tasks, but as they lack the ability to do complex procedures, A-level robots took their place. However, A-level robots were scarcely available and expensive to most. One doctor solved this issue by creating humanoid animals with the ability to think, talk, and act like humans. This is the story of Bremen, a Japanese space captain, and her crew of lively humanoid animals.
In the year 19XX, a new celestial body has been discovered in the same orbit as Earth. Dubbed "Planet Deimon" after its discoverer, Doctor Deimon, the planet's orbit soon slows, coming into close proximity with Earth. After a severe storm, the planet becomes Earth's second satellite. Doctor Deimon's son, Rock, organizes an expedition to explore the new planet and discovers that there are two races of sentient beings living on Deimon. The "Epumu", a race of avian people with the ability to fly, and the "Ruboroom", a race of metamorphic clay men who are slaves to the "Epumu". After his first journey to Deimon, Rock adopts a baby "Epumu" chick named "Chiko" and raises him, learning more about Deimon culture and the differences between human and "Epumu" civilization. However, as the conflict between the people of Earth and the people of Deimon escalate, Rock becomes the ambassador for the two worlds and attempts to find a way to bring peace to everyone.