When it comes to the members of our Solar System, there are multiple factors that decide their degree of hotness and coldness, ranging from atmospheric composition and structure, to their distances from the Sun. However, the hallmark that we’ll be keeping in mind is the amount of heat a planet can radiate and retain.
With that in mind, let us categorize our fellow planets, starting from the hottest to the coldest!
Venus is the hottest planet out of them all, with a staggering surface temperature of 475 deg C - hot enough to melt metals like lead! Now, you may be wondering - how on Earth (or… Venus?) can it be the hottest when it's the second farthest away from the Sun? The answer lies in Venus’ atmosphere, which is extremely dense and is made of carbon dioxide and huge clouds of sulphuric acid (yikes!). That, along with 1000s of active volcanoes, makes it to where the planet successfully captures and retains a large amount of heat that falls onto it.
Mercury is the second hottest planet, with a surface temperature of 427 deg C. Even though it is the closest planet to the Sun, it does not retain much of the heat it absorbs because it does not have an atmosphere.
Earth falls in third position, with a surface temperature of 15 deg C. It is also the third planet in the Solar System hierarchy, with a stable atmosphere consisting of nitrogen, oxygen, and trace amounts of other gases like carbon dioxide (the gas primarily responsible for the greenhouse effect.)
Mars is the fourth planet from the Sun, but because it is 250.55 million km away from the massive star to begin with, it’s surface temperature is about -63 deg C - way below the freezing point of water! It is a rocky, terrestrial planet, but also has an atmosphere that is very thin compared to that of the Earth. Thus Mars is not so good at retaining heat.
Just to put into perspective how cold Mars is, look at these ice clouds passing over the Martian sky!
Jupiter is the fifth planet from the Sun, with a temperature of -145 deg C. It’s atmosphere is filled with hydrogen and helium, much like the Sun, but it isn’t nearly as hot because of how far it is from the giant burning star!
Saturn, being the sixth planet in line, has a temperature of -185 deg C. This is no wonder when you note how far Saturn is from the Sun: a staggering 1.4869 billion km!
Uranus comes right after Saturn, with a temperature of -195 deg C. Not only is it extremely far from the Sun, it also has an atmosphere with swirling clouds made of methane ice crystals!
Coming in hot (well... cold) right after Uranus is Neptune, with a temperature of -201 deg C. It is extremely cold due to its proximity from the Sun; it is much too far to retain any considerable amount of heat.
Pluto, a dwarf planet, is the coldest planet of all, with a scary temperature between -226 and -240 deg C. It is so cold because of it being 39.5 astronomical units away from the Sun (in kilometers that is 5.9 billion km!). Moreover, it does have some degree of internal heating due to some radioactive elements present at their interiors, but Pluto is much too small for that heat to persist.
Just to put into perspective how small Pluto is: you could comfortably fit 154 Plutos inside Earth!