SR388 is the home planet of the X-parasites and Metroids and where Metroid II and its remake took place. The Chozo were originally observing the planet and were horrified by the presence of a virulent life form only known as X that they created the ultimate warrior to destroy them, the Metroid.
SR388 is located in the SR system, it orbits its star along with an unknown ringed planet. The Galactic Federation Biological Science Labs (BSL) space station was built into a nearby asteroid.
SR388 was discovered long ago by the Chozo. They quietly observed the native life forms, refusing to interfere. However, the presence of the X Parasite forced them to intervene. They discovered the X could take over any life form and after killing it, take its form. They envisaged a nightmare scenario of friends and loved ones turning on one another, the X cruelly stealing forms to incite discord and violence.
They decided to create the ultimate life form to destroy the X, they dubbed this creature the ultimate warrior in their native language, the Metroid. The Chozo released the Metroids upon the planet, they soon drove the X to extinction, devouring them ravenously. However with the X completely wiped out the Metroids became equally virulent and turned on other life forms, killing without mercy. The Metroids soon became the dominant species on the planet.
Eventually, Samus is sent to SR388 to exterminate the Metroid threat. Most of the planet's fauna has been destroyed through the X Parasite and Metroid's hunger, leaving the planet fairly barren. Samus then, kills all the Metroids including their queen but, decides to spare an infant Metroid, which she takes to Ceres Space Colony.
The Galactic Federation later returns to the planet in an effort to revive the fauna from the planet and brings samples from the planet to the nearby Galactic Federation Biological Science Labs. This effort revives several species from SR-388 including the X Parasite