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When plants lose water to evaporation in dry weather, their leaves become enriched in a heavy hydrogen isotope, deuterium.

The usual fuel for producing controlled fusion in reactors consists of a mix of the heavy hydrogen isotopes deuterium and tritium, which may unite to make helium.

It must also have been quite dry, as any water, which is naturally enriched in heavy hydrogen during its formation in interstellar space, would have raised the overall deuterium levels.

Nuclear fusion releases vast amounts of energy when heavy hydrogen atoms fuse together, but this requires a temperature of 150m C, 10 times hotter than the core of the sun.

Hydrogen’s cousins, deuterium and tritium, called heavy hydrogen, have been used to make hydrogen bombs.