If Mercury ever applied for a job, its resume would be short on words and long on extremes. Small, fast, and built to survive conditions that would destroy almost anything else, it’s the closest planet to the Sun and, in a lot of ways, the most no-nonsense member of the Solar System. Here’s its CV, fact-checked against NASA data.

Basic profile
Mercury is a terrestrial planet, first in line from the Sun, with a diameter of 4,879 km, about 38% of Earth’s. Its mass is roughly 3.3 × 10²³ kg, only 5.5% of Earth’s, and its surface gravity works out to 3.7 m/s², a little over a third of what you’d feel standing at home. It has no moons and no rings, and what little atmosphere it has isn’t really an atmosphere at all, just an extremely thin exosphere made mostly of sodium, helium, hydrogen, and potassium.

Vital stats
Mercury formed about 4.56 billion years ago, around the same time as the rest of the Solar System. A single day, meaning one full rotation relative to the Sun, takes 58 days, 15 hours, and 30 minutes, while a full year (one orbit of the Sun) takes just 88 Earth days, so Mercury’s day is actually longer than its year. It orbits at an average distance of 57.9 million km from the Sun (0.39 AU), and moves fast while doing it: about 47.36 km/s on average, making it the quickest planet in the Solar System.

Orbit and position
Mercury also has the most eccentric orbit of any planet, swinging from a perihelion of 46.0 million km to an aphelion of 69.8 million km. That elliptical path, combined with its slow spin, is what produces one of Mercury’s strangest features: near perihelion, the Sun can briefly appear to stop, reverse, and resume its path across Mercury’s sky.

Atmosphere and weather
There’s effectively no weather on Mercury in the way we’d recognize it. Without a real atmosphere to trap or move heat around, the surface simply bakes on the sunlit side and freezes on the dark side. Daytime temperatures climb to around 430°C, while nighttime temperatures plunge to about -180°C, one of the largest day-night temperature swings of any body in the Solar System.

Experience and core competencies
What Mercury lacks in size, it makes up for in resilience. Its heavily cratered surface is the record of billions of years of impacts, and underneath that battered exterior sits a surprisingly large iron core, thought to make up a bigger share of the planet’s volume than the core of any other planet in the Solar System. That oversized core is likely the result of a violent early history, possibly including a giant impact that stripped away much of Mercury’s outer layers, leaving the dense metallic interior relatively more prominent.

Mercury vs. Earth, side by side
Next to Earth, Mercury is smaller in almost every measurable way, less mass, weaker gravity, thinner everything, except for two categories where it wins outright: it has the shortest year of any planet, and by far the widest temperature swing between day and night. It’s a planet built entirely around extremes, with very little room for anything in between.

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