Iwa was a planet in a single-star system.
Iwa had a barren surface, more rock than vegetation. In fact, when The Doctor’s TARDIS landed on the planet, its chameleon circuit chose to turn the vehicle’s exterior into a boulder out of deference to the craggy landscape. (Frayed)
Rain was relatively uncommon on the planet, and when it came, it did so extraordinarily briefly. As a result, the air remained fairly saturated and humid. The sky itself was blue, indicating similar chemical composition and light refraction to Earth. (Frayed)
Iwa was the location of a kind of penal colony which punished people for their potential crimes. It may be site of the first contact between the First Doctor and humans (Frayed), other than the Earth’s moon. (The Beginning) It was the location where Susan obtained the name “Susan”, and the Doctor was first called “The Doctor”. (Frayed)
At some point in its history, Iwa was placed under the protection of the Galactic Heritage. (The Tomorrow Windows)
BEHIND THE SCENES
Iwa means “rock” in Japanese.