Uranus and Neptune could be stranger than we once thought. On the surface these two planets – roughly midway in size between Earth and Jupiter – seem unassuming, if different from one another. Uranus is a featureless, pale-azure planet, and Neptune a deep-blue one with white cloud bands and a dark storm system similar to Jupiter’s Great Red Spot. But at heart they may be much more alike, as well as unlike anything we would encounter on Earth. Studies are showing that in terms of chemistry, density, temperature and pressure, the interiors of these worlds have the complexity of a Shakespearean character, and even that they may have actual diamond rain.
Although NASA’s pioneering Voyager 2 spacecraft visited both planets in 1986 and 1989, sending back a wealth of images and data, no spacecraft has been to either world since. That may be due to the fact that Uranus and Neptune are the last two official planets, lying at the planetary edges of our Solar System 2.9 and 4.5 billion kilometres (1.8 and 2.8 billion miles) from the Sun respectively. Much of our information – and all of our up-close images of these worlds – come from Voyager 2, although both are studied by ground and space-based telescopes.
The giant planets formed in the outer Solar System where hydrogen and helium were more abundant. Clearly Uranus and Neptune aren’t small and rocky like the planets of the inner Solar System. But nor do they quite reach the status of ‘gas giant’
