Mercury
Stop 3 of 12 · Inner Solar System
Mercury
The smallest planet and the closest to the Sun — a small, swift world of extremes.
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Discover what Mercury is like — the smallest planet, closest to the Sun — and why its days are so extreme.
After the Sun, the next stop on the journey is tiny and quick. If the planets were runners in a race, this one would be sprinting — it finishes a lap around the Sun in just 88 Earth days.
"Fastest runner?" Leela grins. "I like that one."
If Mercury is the closest to the Sun, it must be the hottest planet, right?
A very sensible guess! But here is a twist — there is another planet that keeps its heat even better. Watch for it later in the journey. For now, think: what happens to a pan left out in the desert at night?
Mercury is close to the Sun, but it is not the hottest planet. It has almost no air to trap heat, so the temperature swings wildly: hot enough to melt tin in the day, and cold enough to freeze carbon dioxide at night.
Also, Mercury spins very slowly — one of its days lasts about 59 Earth days. That gives the Sun plenty of time to bake one side.
A Small, Speedy World
Mercury is the smallest planet in the Solar System — only slightly larger than our Moon. It has no moons of its own, and no rings.
Yet it is remarkably dense. Beneath its rocky surface lies an enormous iron core that takes up most of the planet — a hint that Mercury may have formed in a much hotter neighborhood.
- Order from Sun
- First of the eight planets
- Approximate diameter
- 4,879 km
- Known moons
- 0
- Year length
- 88 Earth days
A year on Mercury is shorter than a Mercurian day — it orbits the Sun twice as fast as it spins once!
Day and Night, Extremes and All
Because Mercury has almost no atmosphere, there is nothing to spread heat around. The side facing the Sun can reach about 430 °C, while the side facing away can plunge to about -180 °C.
That is the biggest temperature swing of any planet in the Solar System.
What Mercury Teaches Us
Mercury may be the fastest planet, but it also teaches us about slowness. Because it spins so slowly, the Sun appears to move backward across its sky near certain times of its long day!
Even a small, seemingly simple world has surprises. If a planet with almost no atmosphere can have such dramatic behavior, imagine what the bigger, more complex worlds hold.
Mercury has almost no atmosphere. What would change for life on Earth if our atmosphere disappeared?
Quick Check
Why is Mercury NOT the hottest planet, even though it is closest to the Sun?
Mercury is the smallest planet and the fastest to orbit the Sun. With almost no atmosphere, its surface swings between extreme heat and extreme cold.
Reflect
What You Discovered
- The smallest planet and closest to the Sun
- A year on Mercury lasts just 88 Earth days
- Almost no atmosphere means wild temperature swings
- Mercury is NOT the hottest planet