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How cold is Neptune? Which planet would you most like to visit?

Neptune is the furthest away of the eight planets in our solar system so it receives very little heat from the Sun so you would imagine it would have the coldest temperatures of any of our planets.

This picture of Neptune was produced by the Voyager 2 probe in 1989 as it sped away from the planet and into the outer solar system. Image Credit: NASA/JPL

Neptune’s gas clouds have an average temperature of 214 degrees below zero ( -214 ºC ) which is easily cold enough to turn the two major gas elements in the Earth’s atmosphere oxygen and nitrogen into a liquid and a solid, imagine oxygen rain and nitrogen snow on Earth.  The gases that remain in the atmosphere are mainly hydrogen and helium with a little bit of methane, which gives Neptune its vivid blue colour as seen in the Voyager 2 probe images (the only spacecraft ever to have flown close to Neptune as it is more than 30 times further from the Sun than the Earth is).

The coldest planet in our solar system on record goes to Uranus which is closer to the Sun and ‘only’ about 20 times further away from the Sun than the Earth is.  The lowest temperature recorded there was minus 224 degrees Celsius.  There are two main reasons scientists think this is the case; First, at these distances from the Sun there isn’t much heat to warm either of the planets, some heat comes from the planet's core, a bit like how the Earth’s core is hotter than its surface.  But the second is the methane in Neptune’s atmosphere, on Earth methane is known as a greenhouse gas, it is good at trapping heat like a nice jumper keeps you warm.  Uranus has methane in its atmosphere too, but not as much as Neptune, so Neptune can stay a little warmer than Uranus can.

Where would I most like to visit? That would probably be Venus.  It’s on the other end of the scale to the gas giants of Uranus and Neptune.  It is the hottest planet in our solar system, which again is strange because it is not the closest to the Sun, which is Mercury.  The reason once again is due to the atmosphere, it has thick clouds of nitrogen and carbon dioxide, another greenhouse gas, which means the surface temperature is 426 degrees Celsius, which is hot enough to melt the metal zinc that one-cent coins are made from in the USA.  So why would anyone want to go? Well before the run-away greenhouse effect started billions of years ago, Venus might have been quite similar to the Earth, maybe similar enough to have once supported life.  So, if I can visit at any time, I’d pick Venus about 4 billion years ago.