Squeezed between two pieces of diamond, hydrogen has been transformed into a metallic form believed to exist inside giant planets like Jupiter, scientists reported on Thursday.

“You can see it becomes a lustrous, shiny material, which is what you expect for a metal,” said Isaac F. Silvera, a professor of physics at Harvard.

If some theoretical predictions turn out to be true, the new state of hydrogen could even be a solid metal that is metastable — remaining solid even after the crushing pressure is removed — and a superconductor, able to conduct electricity without resistance, Dr. Silvera said.

Dr. Silvera and Ranga P. Dias, a postdoctoral researcher, published the findings on Thursday in the journal Science.