Well if the planets were terraformed then anything is possible. Just look how we've changed our planets temperature and percentages of air. Release enough oxygen nitrogen and helium into an atmosphere while changing the electromagnetic field around the planet could terraform the planets into what we see in Destiny. Notice the moon was changed by the hive at its core. This could have affects on gravity and atmosphere as well as electromagnetic fields. The vex terraformed Venus into a machine changing the planet density by using polymers and metals could affect gravity and atmosphere. The scary thing is we are already doing this to our own planet. Did you know that you could lower the average temperature of a city by painting the roofs white which would reflect instead of absorb heat like the tar and asphalt does now. It is perfectly logical to think an alien race of superior technology to ours could terraform a planet to their liking.