[AnIdiotOnTheNet]

> planets can travel through space and have a protective shield - the atmosphere

Most worlds in our system don't have a significant atmosphere, most of those that do have too much of it. Like gravity, atmosphere also poses a problem for getting back to space.

> no man-made spaceship can match that for billions of years.

Not unmaintained, no, but if it was unmaintained that probably means everyone who lives there is dead anyway.