The hottest planet in the solar system is Venus with an average temperature of 864 degrees Fahrenheit or 462 degrees Celsius. The coldest planet in the solar system is Neptune with an average temperature of -353 degrees Fahrenheit or -214 degrees Celsius.
If we looked at our solar system, it might seem easy to guess which planet could be the coldest and which one would be the hottest. After all, most of us learn to name the planets in order, beginning with the one closest to the sun and ending with the one farthest from it: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
The hottest planet in the solar system is:
- Venus (Even though it’s not the closest planet from the Sun)
The coldest planet in the solar system is:
- Neptune (This one is easy since it’s the farthest planet from the Sun)
So, it would seem to follow that the planet that is closest to the sun, Mercury, would be the hottest planet; and that which is farthest from the sun, Neptune, would be the coldest planet. But is this really so?
Defining Cold and Hot
The first thing that we need to define is what we consider “coldest” and “warmest.” Why? Because, as we know from our daily lives down here on planet Earth, temperatures vary greatly depending on geographical location and time of year. For example, average temperatures are colder in Alaska than they are in California. Also, there is a big difference between summer and winter temperatures in New York. We have all enough experience to know that. And, logically, if that is the case here on Earth, why would be the situation be different on the other planets in our Solar system?
Cold and hot are very relative terms. For someone who has endured a US Midwest winter, it might not seem possible that coldest temperatures could exist. And, for someone used to South Florida hot and humid weather, hotter temperatures might not seem likely at all.
And yet, planet Earth does not reach the coldest or hottest temperatures in the Solar system. Far from it!
“I can remember at the age of about six being fascinated by the planets and learning all about Mars and Venus and things.” — Richard Dawkins
It is true, however, that human beings (and many other species found on Earth) would not be able to survive the cold temperatures that would be experienced in all the other planets that are farther from the Sun or, indeed, the hotter temperatures in Mercury or Venus.
What Is the Coldest Planet?
If you expect the farthest planet from the Sun to be the coldest, you may be wondering about Pluto. Particularly, if you went to school in the 20th century you might think that Pluto is the farthest planet out in our Solar system.
But Pluto has been demoted to the category of a dwarf planet over ten years ago. After years of controversy, back in 2006, the International Astronomical Union (IAU) decided that Pluto should no longer be considered Pluto a planet.
When I was a little kid, we only knew about our planets. Since then, we’ve downgraded Pluto but have discovered that other solar systems and stars are common. So life is probably quite prevalent. – Buzz Aldrin
So, because there is no astronomical consensus that Pluto is no longer a planet, then you might logically assume that the coldest temperatures would be found in Neptune. After all, once you have removed poor Pluto from the equation, then Neptune becomes the farthest planet from the Sun.
But before we can address, we need to look again at what we would consider coldest.
Both Neptune and its neighbor, Uranus, are basically balls of rock, ice crystals (methane), ammonia, and water. But, which one of the two is actually the coldest planet?
Let’s look at their distance from the Sun. The first thing to note about distance is that it is not always the same. This is not just the case with Neptune and Uranus but with every other planet in the Solar system. As they orbit the Sun, planets are sometimes closer and sometimes farthest from the Sun. So, whenever we speak of planetary distance to the Sun, we can only speak of average distance with the understanding that distances vary and that this, among other things, would, of course, affect temperatures.
What this means in layman’s terms is that both planets are very far from the Sun (over a billion miles away no less!). But there is also a huge difference between Uranus’s distance from the Sun and that of Neptune’s. There is, in fact, an average distance of over a billion miles between both planets.
Because of that huge distance between the planets, there is a reduction of about 40% of solar light if we compare Neptune to Uranus.
It is true that, in fact, Neptune is, on average, colder than Uranus. The average temperature in Neptune is about -350º F (- 212.22º C) and that of Uranus is -325º F (-198.33º C). So, logic seems to prevail here, until we consider that, actually, that Uranus reaches coldest temperatures than Neptune.
So, even though the average temperature is lower in Neptune, Uranus can get colder. That means that Uranus and not Neptune is the planet in our Solar system with the coldest temperatures.
Why can Uranus reach colder temperatures when it receives considerably more solar light? Well, there are a few possible reasons for this. One of them would be Uranus’s active atmosphere, which could result in the planet’s loss of some of its heat. Neptune, however, would keep more heat, which would result in warmer (or, rather, less cold) temperatures.
What Is the Hottest Planet?
Now that we have looked at the coldest planets and have shown that average distance from the Sun is not the only thing to be factored in when trying to establish what planets are colder or warmer, you’d probably wouldn’t make assumptions about planets’ temperatures based solely on their proximity to the Sun.
Mercury is the closest planet to the Sun but, before we can say that that fact makes it the warmest planet in the Solar system, we’d need to look at its atmosphere. In fact, even though Mercury is, by far, the planet that is most exposed to the Sun’s radiation, because of its (practical lack of) atmosphere it doesn’t keep much of its warm. So, Mercury is not the hottest planet. In fact, it is a cold planet where average temperatures are over -100 º F (- 73.33 º C).
That position is reserved to Venus where its active volcanic activity and other factors make the average temperature 863 º F (462 º C).
So, we’ve seen that proximity to the Sun does not result in warmer temperatures. Atmospheric conditions are also key to understanding planetary temperatures.