Mogar was a planet in the Perseus arm of the Mutter’s Spiral. It had no oxygen in its atmosphere and was rich in rare minerals, including vionesium.
Mogar was also the home planet of the Mogarians.
When Mogar was discovered by humanity, it was taken over for its mineral wealth. (Terror Of The Vervoids) The Earth Empire later terraformed it. (Christmas on a Rational Planet)
The Sixth Doctor claimed to have visited the planet. (Terror Of The Vervoids)