Mercury — the closest one
Innermost of the eight planets, hottest and coldest by turns, densest by composition, and — because ancient astronomers noticed it move faster across the sky than any other body — the planet named for the Roman messenger god. Its story is a set of extremes with a linguistic tail: centuries later the alchemists who named the liquid metal borrowed the same word.
The name
Two things share the name Mercury because both move quickly. In Roman mythology, Mercury (Mercurius) was the god of trade, communication, and messengers — the swift-footed intermediary between the mortal and the divine, equivalent to the Greek Hermes. Ancient astronomers, watching the innermost planet flick past against the star field faster than any other body they could see, gave it the messenger's name.
Centuries later, medieval alchemists — who paired the seven known metals with the seven classical planetary bodies — took the name a second time. Liquid metallic mercury, silver and pooling and impossibly quick to run and re-form, was assigned to the same god of speed. The planetary symbol ☿ — a stylised caduceus atop the god's winged helm — became the element's symbol too. It is one of the rare cases where the alchemical planetary naming convention survived intact into modern chemistry.
There is a footnote. In the eighteenth and nineteenth centuries, mercury nitrate was used to felt animal fur into hat material. Hatters chronically inhaled the fumes; the neurological damage from long exposure produced tremor, slurred speech, and cognitive impairment. The English language kept the expression.
A world of extremes
Mercury is small — 4,880 kilometres in diameter, closer to the size of Earth's Moon than to Earth. It orbits the Sun once every 88 Earth days on the most eccentric planetary orbit in the solar system, ranging from 46 million kilometres at perihelion to 70 million at aphelion.
What makes its calendar strange is the coupling of that fast, eccentric orbit to a slow rotation. Mercury is locked in a 3:2 spin-orbit resonance — for every two orbits it completes around the Sun, it turns on its axis exactly three times. A single Mercurian solar day, from local noon to the next local noon, therefore takes 176 Earth days: the length of two Mercurian years.
That slow spin under an unshielded Sun produces the most extreme diurnal temperature swing on any planetary surface in the system. The subsolar point reaches +430 °C. The unlit side — over the course of an 88-day night — plunges to −180 °C.
The polar ice
Mercury has one further extreme. Its axial tilt is essentially zero — 0.03 degrees — which means the deep floors of impact craters at both poles never see the Sun. They are permanently shadowed.
Trapped inside those craters, at temperatures below −170 °C, are substantial deposits of water ice — brought in by cometary and asteroidal impacts across billions of years and preserved by geometry alone. The MESSENGER spacecraft mapped them from orbit between 2011 and 2015 using neutron spectroscopy and Earth-based radar-bright polar returns.
On the hottest planet in the solar system, in craters that will never see daylight, water ice endures.
The iron heart
Mercury is the densest planet in the solar system when its bulk density is corrected for self-compression: 5.3 grams per cubic centimetre. That number requires an outsized iron core.
Modelling of MESSENGER gravity data places the core radius at approximately 2,020 kilometres — roughly 82 percent of the planetary radius, and about 57 percent of the planetary volume. For comparison, Earth's core is 17 percent of Earth's volume. Between the core and the surface, the silicate mantle and crust together account for only a few hundred kilometres of material.
Why Mercury ended up with so much iron and so little rocky mantle is unresolved. The leading hypotheses involve a giant impact early in the planet's history that stripped away much of the outer silicate layer, or preferential accretion of iron-rich material in the innermost part of the solar nebula. BepiColombo's radio-science and laser-altimetry payloads are designed to pin the answer down.
The magnetosphere
Mercury's molten outer core still churns, and generates a global dipolar magnetic field — Mercury is the only rocky planet other than Earth that has one. The field itself is weak, roughly 1 percent of Earth's, but it is enough to carve a compact magnetosphere between the planet and the incoming solar wind.
That magnetosphere is small, and Mercury is close to the Sun, which makes the whole system exceptionally dynamic. The ESA-JAXA BepiColombo mission — launched in 2018 and inbound to Mercury orbital insertion in November 2026 — has flown past Mercury six times on its long braking trajectory. During three of those flybys (MFB1 in October 2021, MFB2 in June 2022, and MFB3 in June 2023) the Miniature Ion Precipitation Analyzer (MIPA), part of the SERENA particle-sensor suite, sampled the magnetosphere directly.
MFB1 caught it in a quiet state — a steady low-latitude boundary layer, plasma-sheet ions at around 1 keV. MFB2 and MFB3 caught it active: when the interplanetary magnetic field turned southward, it reconnected with Mercury's field, and bursty bulk flows of plasma at up to 14 keV surged toward the surface along the reconnected field lines. The full picture of that magnetosphere — its structure, its response to solar-wind loading, its exchange with the exosphere and the crust — is what BepiColombo's two orbiters will spend the second half of the decade mapping continuously from orbit.
The record
Three spacecraft have visited Mercury. The first flew past three times fifty years ago; the second was the first orbiter, and it stayed for four years; the third is on its way now.
One planet named for how fast it moves across the sky. One element named for how fast it moves across a table. Same god.