It took eight years, nine billion kilometres of travel through the inner solar system, and a gravity-assisted choreography involving Earth, Venus, and Mercury itself — but humanity has finally positioned its most sophisticated planetary probe in orbit around the solar system's most stubbornly mysterious world. As of this week, the BepiColombo mission is delivering its first sustained close-up of Mercury, and scientists say what comes next could fundamentally alter how we understand the formation of rocky planets, including our own. A Planet That Refuses to Make Sense Mercury should be simple. It is tiny — barely larger than Earth's Moon — and sits so close to the Sun that a year there lasts only 88 Earth days. It has no meaningful atmosphere, no moons, no rings. And yet every mission that has studied it has come away with more questions than answers. The most confounding of these concerns Mercury's core. By the best measurements scientists have, Mercury's iron core accounts for approximately 85 percent of its total radius. For comparison, Earth's core makes up about 55 percent of our planet's radius. This means Mercury is, in proportion, almost entirely metal. How that happened — whether it was a colossal ancient collision that stripped away much of the planet's rocky mantle, whether it formed unusually close to raw iron-rich material in the proto-solar nebula, or whether the Sun's early radiation blasted lighter material away — remains genuinely unresolved. BepiColombo's twin orbiters, the Mercury Planetary Orbiter built by ESA and the Mercury Magnetospheric Orbiter built by JAXA, carry instruments specifically designed to map Mercury's gravitational field with unprecedented precision. That data will give scientists their clearest picture yet of how mass is distributed deep inside the planet, and whether the core is solid, partially liquid, or something stranger still. The Ice Hidden in Hell One of the most counterintuitive facts about the planet closest to the Sun