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Mercury’s small size is deceptive. Unlike the other rocky planets in our solar system, it has an immense iron core that makes up roughly 70 per cent of the planet’s volume. That’s massive compared to Earth, whose core makes up only 30 percent of its volume.

It’s also unexplained. Without another example like it in the solar system, astronomers have had a difficult time describing how such a wacky world formed.

That might be about to change. at the Aix-Marseille University in France, and his colleagues have discovered Mercury’s…