Mercury

The smallest planet, and the one closest to the Sun. A whole year on Mercury is shorter than one school term! And the little planet is slightly smaller than our own Moon.

What it is

Innermost planet, cratered, huge iron core, no atmosphere, extreme day/night swings.

Planet: Round, orbits a star, and has swept its orbit clear. Every kind of thing there is →

How to find Mercury

What you will really see

Worth knowing about Mercury

Mercury is about two-fifths the width of Earth. In weight, as light as a small fraction of Earth — about 1/20th. Less than half of Earth's distance from the Sun. Sunlight takes 3 minutes to reach Mercury — on average, because that distance changes as it goes round. Its year: about 3 months — one quarter of a year, like a whole summer season. Mercury spins all the way around every 59 Earth days — but it's also racing around the Sun so fast that the Sun doesn't actually rise again until about 176 Earth days later, almost two of Mercury's years.

What it is made of

Rocky, cratered like our Moon.

Its surface

Mercury's iron core reaches three-quarters of the way out from the centre to the surface — as if Earth's core stopped only just beneath our feet.

How we know: We've never drilled into Mercury — scientists tracked tiny wobbles in the MESSENGER spacecraft's orbit caused by Mercury's gravity, which reveals how its mass is distributed inside, combined with its overall density. Rung 3 of the cosmic ladder — how the solar system is arranged

Its weather

No atmosphere; swings from 430C day to -180C night.

The machines we sent

NASA's MESSENGER orbited it 2011-2015 and mapped the whole surface.

Seen next to things you know

How anyone knows this

Nobody flew a tape measure out there. Every number on this page was worked out from the ground:

• What shape is the solar system?
• How far away is the Sun?

What it belongs to

Then go outside

Clear tonight? The Planetarium will show you what is worth a look. Then just go out.