Why are are none of our neighbors gas giants?

This week, I got a question on social media about gas giants. Or rather, I got a comment from a blog reader who wanted answers to questions that were yet to be formulated. They had a big curiosity on gas giants but did know which answers to ask.

This post seeks to ask the good questions and give the corresponding answers. Consider it a Q&A for gas giants. Let’s begin.

What Is A Gas Giant?

There are two overall categories of planets. Both types are present in our solar system.

One category contains the rocky planets. These are, in our case, the 4 inner planets, Mercury, Venus, Earth and Mars. They all have in common that they are relatively small and they all have a rocky surface. That is how the category got its name. A rocky surface means it is solid and, could you survive the atmosphere, you would be able to stand on it with you own two feet and be held to the ground by gravity. This is what makes walking possible here on Earth.

The four inner planets in the solar system are rocky, and the four outer planets are gaseous. Therefore we named them the gas giants. Animation: Astronomicca.

The other category covers the gas giants. It includes the 4 outer planets, Jupiter, Saturn, Uranus and Neptune. These planets are way bigger than the rocky planets. Jupiter, being the biggest planet in the solar system, has a mass that corresponds to more than 300 Earths and its volume is 1300 times that of the Earth. So, it’s much heavier but much-much-much bigger in size! This eventually leads to a density that is around 1/4 of Earth’s and this is where it gets interesting.

Gas giants do not have a solid surface. Their surface is gaseous and moves like the wind. This is explained by the low density mentioned above. When there are fewer particles per volume it makes it more difficult to form a solid surface. If you try to stand on Jupiter’s surface, it would be like trying to stand on the surface of a cloud. You simply can’t.

This is a characteristics that is shared by all gas giants and that’s why we named them like we did.

Why Are The Gas Giants Located Further Away Than The Rocky Planets?

This question is so good, but the explanation can be tricky to understand.

The characteristics of the planets are practically decided by the distance to the host star, which in our case is the sun. The reason is not the distance itself, but more how the surrounding change the further we move away from the sun. The outer regions have a lower pressure (and lower temperature) than the inner regions, so water molecules would condense to ice in this cold region. The transition where this happens is called the frost line, and this line is found exactly between Mars and Jupiter, i.e. where the planets go from rocky to gaseous.

It is believed that the gas giants could accumulate their large mass because they captured all the ice molecules. Inside the frost line it was too warm for ice to form and hence these molecules were kept in a gas form allowing the planets to grow by accumulating rocky material that did not evaporate in the high temperature.

Do Gas Giants Have Moons?


Gas giants have plenty of moons. In fact, all planets in the solar system have moons, except the two inner planets, Mercury and Venus.

Jupiter has 79 moons, Saturn has 82 moons, Uranus has 27 moons (one is named Juliet, aww!) and Neptune comes in on a last place with 14 known moons.

Do Gas Giants Have A Similar Calendar To Us?

No, they don’t. And this is probably one of the most fun exercises to solve. How to create a calendar on a gas giant. I wrote about this in an earlier post, but I will recap the issue.

We all know what a day, week, month and year is. A day is the time it takes for Earth to spin around its own axis, and a year is the time it takes to orbit the sun. A month is roughly the time it takes for the moon to go through all cycles and a week is just some random measure someone invented to sell more calendars. Or something like that. Either way, it’s certainly not related to motion of celestial bodies, it likely comes from the Bible. Yikes. The B-word in a space blog. Moving right along.

What you may not think about so often is the assumption that goes in to these definitions in order to make a calendar that works everywhere on Earth. A very important requirement is that you need Earth to rotate at a steady frequency all over the planet, so that one day, 24 hours, is passing simultaneously in Chile and in Norway. We have different time zones, sure, but these do not affect how long it takes for a day to be completed, it merely affects the time we go to bed.

Since gas giants do not have a solid surface, they do not rotate at the same speed everywhere. One spin around Neptune’s own axis takes only 12 hours at the poles, but the same spin takes 18 hours at equator. This means people at the poles consider a day to be 12 hours and people at equator consider it to be 50% longer. An hour is still an hour, because this is defined universally, so how will you have people agreeing on when the day goes from Monday to Tuesday? You can read more about that problem here.

Creating a calendar for Neptune is definitely on my to-do!

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