Is Mercury the hottest or coldest planet?

If you’re just starting to learn about the planets, then one of the most interesting to learn about is definitely Mercury. It is the smallest of the 8 planets that orbit around the Sun in our great solar system.

However, there are many questions that people have about Mercury, with people wondering whether it is the hottest planet in our solar system. I mean logically, it is the closest to the Sun, so it would be the hottest right? Well, no actually this isn’t true, and I’m going to explain why. So, let’s look at the hottest planet in our solar system.

Is Mercury the hottest or coldest planet?

The correct answer is that Mercury is the not hottest planet, nor is it the coldest planet. Mercury is the second hottest planet in our solar system, after the planet Venus.

This confuses some people., because as you probably know, Mercury is the closest planet to the Sun. So how comes it isn’t the hottest planet then? Well, the reason for this lies in the atmosphere that surrounds the planet Venus.

Mercury has little atmosphere

Because of Mercury’s small size, it does not have an atmosphere like Venus or our planet, Earth. And atmosphere is essentially several layers of gases that surround a planet, which causes the changing weather on our planet.

For example, the Earth’s atmosphere is made up primarily of Nitrogen, but also around 20% of oxygen too. This oxygen is what allows us to be able to breathe on our planet, and it’s the reason why we can’t go to any other moon or planet without oxygen support to breathe.

However, if we look at the planet Venus, it’s atmosphere is made up primarily of carbon dioxide, with little nitrogen and other traces of gases. This atmosphere is what causes Venus to be the hottest planet, and not Mercury.

Venus is retaining heat

When the Sun beats down on Mercury, this makes the surface of the planet extremely hot. However, the heat from the Sun quickly escapes from the planet, as it has no thick atmosphere of gases to keep the heat in.

But if we compare this to Venus, which has a very thick atmosphere of gases, it’s to total opposite. Yes, the original heat that gets to Venus isn’t as hot as that of Mercury (because it’s further away), but when it gets there, it stays there.

This is because the atmosphere that surrounds Venus is filled with carbon dioxide, making it very thick. When the heat tries to escape from the planet, it can’t due to this thick atmosphere. We call this the greenhouse effect.

Why Small Planets Have Less Atmosphere

So, why do smaller planets have less of an atmosphere than the larger planets in our solar system? Well the answer, like so many space related questions, is gravity.

The smaller the planet, the less gravitational pull that it has. This is why moons orbit our planets; they’re smaller, and they get pulled in by the planets gravitational effect. And in turn, this is why our planets orbit the Sun – the Sun’s mass is so large that our planets orbit it routinely.

So, the gravity doesn’t allow the gases surrounding the planet to escape, which is why Venus is the hottest planet, and hotter than Mercury!


So, hopefully this explain why Mercury isn’t the hottest planet in our solar system. However, it still is the second hottest and it’s not far behind Venus in terms of average temperatures. But it still remains far too hot for us to ever consider visiting the planet, so for now we just try to learn as much about it as we can!

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