Rain, rain, go away,
Come again another day…
The clouds in the Earth’s atmosphere are made of tiny water droplets. When these droplets condense, they get bigger and heavier. Eventually they get too heavy to stay suspended in the air and it starts to rain! But planet Earth is not the only place where droplets fall from the sky. It rains on other celestial Worlds too! Only, instead of liquid water, it rains…well…other stuff!
Here are our favourite rainy planets and moons! We bet you would love to visit some of them!
Let’s start close to our home. You would never think that Venus, being the hottest planet in the Solar System, might also be a rainy place, but it is true! Upper Venus’ atmosphere is home to sulphuric clouds that produce sulphuric acid rains. Of course, the raindrops evaporate long before they reach the ground, rise again and so the cycle continues!
So, just to sum it up:
- the temperature on the surface of Venus is over 460 degrees
- the atmospheric pressure is 90 times that of the Earth
- there are acid rains up in the air
– what’s not to love about this planet, right?
Giant planets have a surprise for you too. Despite being made of gas and not having a solid surface, they boast some of the most precious (literally) rains there are.
The Giants of the Solar System
But square cut or pear shaped
These rocks don’t lose their shape
Diamonds are a girl’s best friend…
In the early 80s there was an article called The ice layer in Uranus and Neptune—diamonds in the sky?. Its author proposed that the conditions on the two outermost planets might be right for the methane in their atmospheres to break down (be pyrolyzed) to make diamonds. Recently we’ve got ways and means to test this theory. And it looks like the author was right. On the depths of 7000 kilometers beneath the surface of Uranus and Neptune, diamonds form out of methane and fall (or rather sink) towards the center. So it does rain diamonds on Uranus and Neptune. Well, at least somewhere deep down.
The other two gas giants, Jupiter and Saturn, do not have as much methane. But in 2013 several astronomers proposed that diamonds might still form and “rain” down in the gas giants atmospheres. We can only wish there was a way to observe the diamond rainfall directly!
Saturn’s moon Titan is the only moon known to have a substantial atmosphere. You might say it resembles the Earth in many ways.There are clouds, there are rivers, there are lakes and there is rain. Only it rains methane and ethane out there! Thanks to Titan’s low gravity the methane raindrops grow big and fall in slow motion onto the surface-made of frozen solid water ice. The methane raindrops are also extremely cold – definitely not the kind of rain you would want to be caught in.
Now let’s zoom out of the Solar System and look at some of the weird extrasolar Worlds and their exotic precipitation.
Molten iron rain? Welcome to WASP 76b, an “ultrahot” planet almost twice Jupiter’s size. This planet is tidally locked, meaning one side is always facing its star. The day-to-night temperature variation on WASP 76b is the most extreme we’ve seen so far: from over 2400 degrees Celsius on the day side to “just” 1500 degrees Celsius on the night side. It is so hot on the day side, that even iron evaporates. The 1800 km/h wind then carries the iron vapor over to the night side where it condenses and falls as rain.
We bet you are glad that WASP 76b is situated 640 light years away from us.
For more details check out the article Nightside condensation of iron in an ultrahot giant exoplanet published on March 11, 2020 in Nature.
The beautiful blue coloured HD 189733b was discovered back in 2005. This planet is slightly bigger than Jupiter and orbits its parent star once every 2.2 days. HD 189733b, just like most other planets orbiting that close, is tidally locked, with permanent day and night sides. Don’t let the familiar blue colour of this planet fool you. It’s no water, but silicate particles in the clouds. When they condense, like water droplets in Earth’s clouds, they become droplets of glass that rain down, or, rather, sideways, thanks to the 5400 miles per hour winds. Ouch!
Kepler 2b (aka HAT-P-7b)
This gas giant,16 times the size of the Earth,is situated over 1000 light years away. If we could have a closer look at Kepler 2b we would have discovered a bizarre world with powerful global winds and beautiful clouds made of corundum, the material of rubies and sapphires! Corundum clouds condense on the colder night side of the planet. Then the wind blows them over to the day side, where the clouds evaporate. So…technically the winds of rubies and sapphires blow on HAT-P-7b. What a weather!
For more details see the original publication Variability in the Atmosphere of the Hot Giant Planet HAT-P-7b.
The next planet might be of a particular interest to us as it is, unlike other planets we looked at so far, ROCKY!
CoRoT 7b is a very hot “super Earth” about 70% bigger than our planet. The planet, situated 489 light years away from us, was discovered in 2009 by the French “Convection, Rotation and planetary Transit Satellite” (hence the name). CoRoT-7b is 60 times closer to its star than the Earth to the Sun. Scientists think that the daytime temperature on its surface should be about 2000 C. That’s hot enough to melt rock! So, as you have probably already guessed, it rains rock on CoRoT-7b! But it is not what’s so interesting about this planet. Some astronomers think that when it was born, the planet was as big as Saturn and its orbit laid further out. With the time, CoRoT-7b drifted closer to its host star and its gaseous layers got blown away. So what’s left might be a so-called Chthonian Planet, i.e. the core of a former gas giant! But other astronomers disagree. They say CoRoT-7b was always a rocky planet as it is way too young for its atmosphere to have enough time to evaporate. We’ll see!
Now that you know all about alien rain, we bet you wonder Are there rainbows on other planets and moons in Space? You will find the answer to this and many other space questions in our Astronomy blog! As always, let us know if you have any questions or comments. Our mobile planetarium team would love to hear from you!