Mercury

A world of extremes, wrapped around a huge iron heart

Mercury is barely larger than our Moon and, at a glance, looks much like it — grey, airless, and saturated with craters, including the vast Caloris impact basin. But it hides a surprise inside: an enormous iron core taking up perhaps 60% of the planet's volume, far more than any other planet, which makes Mercury unusually dense and gives it a weak magnetic field. Long wrinkle-ridges scar its crust where the whole planet shrank as that core cooled.

With almost no atmosphere to move heat around or hold it in, Mercury swings between the most brutal extremes in the Solar System: scorching to about 430 °C where the Sun beats down, and plunging to around −180 °C on the night side. Its slow spin (one rotation every 59 days) means a single day-night cycle lasts far longer than its 88-day year. And in the deep craters near its poles, where sunlight never reaches, it is cold enough to preserve water ice — confirmed by NASA's MESSENGER orbiter, an astonishing find on the planet nearest the Sun.

The planet Copernicus supposedly never saw

Mercury is the trickiest of the naked-eye planets, and catching it is a small triumph. Because it hugs the Sun, it never appears in a dark sky — you can only spot it low on the horizon during bright twilight, for a short window just after sunset or before sunrise, and only when its orbit swings it farthest from the Sun (an "elongation"). Legend has it that Copernicus, living in the misty north, never managed to see it at all.

To the naked eye Mercury looks like a modest, slightly pinkish "star" close to the horizon; an astronomy app will tell you when and where to look. A telescope shows a tiny disc that runs through phases like a miniature Moon, though its low altitude blurs any detail. Every so often Mercury crosses directly in front of the Sun — a transit — appearing as a small black dot, visible only with proper solar filters.

Not the hottest — and named for the swift messenger

The most common misconception is that Mercury, being closest to the Sun, must be the hottest planet. It isn't. With no atmosphere to trap warmth, its heat escapes to space the moment the Sun goes down, and its nights are frigid. Venus, farther out but smothered in a thick greenhouse atmosphere, is far hotter overall. A second surprise overturns intuition entirely: the planet closest to the Sun holds ice, tucked in craters the sunlight can never reach.

Mercury takes its name from the Roman Mercury, the swift-footed messenger of the gods — a fitting title for the fastest planet, which slips around the Sun quicker than any other. His Greek counterpart is Hermes, the winged herald and trickster who guided souls and carried word between the mortal and divine worlds.

Step into the Greek myths written across the night sky →