This is so cool! Neptune is a giant ice planet surrounded by a dense atmosphere and a big part of its atmosphere is a gas called methane. Methane is a smelly and explosive gas made up of carbon and hydrogen atoms. Because of the extremely high temperatures and pressures on Neptune the methane gets broken down and the carbon gets separated from the hydrogen. The carbon atoms then get compressed by all the heat and pressure to form diamonds which rain down on the planets surface. This doesn’t just happen on Neptune – Uranus, Jupiter and Saturn probably also rain diamonds.