As planets age the general rule is that they become darker and cooler - but Saturn is an exception. Why it looks so young for its age has been a space science topic since the late 1960s but a paper in Nature Geoscience says it has some answers.

Saturn is known to all by its distinctive rings but less well known is that it is one of the largest planets in our Solar System, second only in size to massive Jupiter. It is primarily made of hydrogen and helium and its excessive brightness has previously been attributed to helium rains, the result of helium failing to mix with Saturn's hydrogen rich atmosphere. 

Researchers from the University of Exeter and the Ecole Normale Supérieure de Lyon found that those layers of gas, generated by physical instability deep within the giant planet, prevent heat from escaping and have resulted in Saturn failing to cool down at the expected rate.

Saturn's tips for looking young and hot for millions of years

Layered convection, like that recently discovered in Saturn, has been observed in the Earth's oceans where warm, salty water lies beneath cool and less salty water. The denser, salty water prevents vertical currents forming between the different layers and so heat cannot be transported efficiently upwards.

Not how you usually picture Saturn, is it? She has many hidden ways to stay looking young and hot. Credit: NASA/JPL-Caltech/Space Science Institute. Link: PlanetBye on Science 2.0

Professor Gilles Chabrier from Physics&Astronomy at the University of Exeter said, "Scientists have been wondering for years if Saturn was using an additional source of energy to look so bright but instead our calculations show that Saturn appears young because it can't cool down. Instead of heat being transported throughout the planet by large scale (convective) motions, as previously thought, it must be partly transferred by diffusion across different layers of gas inside Saturn. These separate layers effectively insulate the planet and prevent heat from radiating out efficiently. This keeps Saturn warm and bright."

These findings suggest that the interior structure, composition and thermal evolution of giant planets in our Solar System, and beyond, may be much more complex than previously thought.