It might rain diamonds on more than 1900 exoplanets across the galaxy

The skies of icy planets across the cosmos may be full of diamonds. Compressed carbon compounds can turn into diamonds at less extreme temperatures than researchers thought were required, which may make diamond rain a common phenomenon inside ice giants.

In the past, laboratory experiments have led to confusion about the conditions under which diamonds could form inside ice giants such as Uranus and Neptune. There are two types of experiments investigating this: dynamic compression experiments, in which carbon compounds are subjected to a sudden shock, and static compression experiments, in which they are placed inside a chamber and compressed gradually. So far, dynamic compression experiments have required much higher temperatures and pressures to form diamonds.

Mungo Frost at the SLAC National Accelerator Laboratory in California and his colleagues performed a new set of experiments using static compression but dynamic heating, compressing polystyrene – the same polymer used to make Styrofoam – by squeezing it between two diamonds and then hitting it with pulses of X-ray light. They observed diamonds beginning to form from the polystyrene at temperatures of about 2200°C and pressures around 19 gigapascals, conditions similar to those in the shallow interiors of Uranus and Neptune.

These pressures are much lower than the pressures found to be necessary for diamond formation in earlier experiments using dynamic compression. The reaction took longer than dynamic compression experiments typically run, which might explain why such experiments haven’t picked up low-pressure diamond formation. “It disagreed with established results and wasn’t what we expected to see, but it fit in nicely and sort of tied everything together,” says Frost. “It turns out that was all down to different timescales.”

This could mean that diamond rain is possible on smaller planets than we previously thought. Of the 5600 or so confirmed exoplanets, the researchers calculated that more than 1900 could have diamond rain.

It also means that within the solar system, diamonds could form at shallower depths than we thought, which could change our understanding of the dynamics of the interiors of giant planets. This shallower formation could allow the diamond rain to pass through a layer of ice as it sinks towards the centres of these planets. This would, in turn, affect the icy worlds’ magnetic fields, which are complex and poorly understood.