FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Quick Quiz on the Gas Giants

We’ve recently passed two of the great gas giants, Jupiter and Saturn, in our orbit around the sun. Both of these planets are still up in the early evening skies, Jupiter overhead, Saturn more to the west.

Maybe now would be a fitting time to take a quick little quiz about the outer planets - Jupiter, Saturn, Uranus, and Neptune. The statements below are all true or false, but remember, as usual, you will not be graded - but you must have fun!

Statement 1: The gas giants are made of gas.

Asking people what the four gas giants are made of is like asking who’s buried in Grant’s Tomb. “Gas giants” pretty much spells it out – they are giant planets made of gas, of course.

Kinda, sorta… not.

If by gas one means hydrogen and helium and methane then yes, the four jovians, as they are often called, are all pretty gassy. But (cue dramatic music) those elements and compounds aren’t always gasses!

The outer layers of the planets are gassy, to be sure, with their cloud patterns and storms like that Great Red Spot on Jupiter. But when you sink below the cloudy upper deck and in through an outer layer of gaseous hydrogen, the atmospheric pressure starts to kick in pretty heavily, and things change dramatically.

Going further down, the pressure becomes so great that the hydrogen gradually becomes a liquid. Go further down into Jupiter and Saturn where the pressure reaches about 2 million times what it is here in our atmosphere and the liquid hydrogen becomes the rare metallic hydrogen.

And way, way, way down deep is a rocky core, probably as big as Earth, experiencing pressures greater than 50 million times our own atmospheric pressure.

Uranus and Neptune aren’t big enough to have the liquid hydrogen but neither are they completely gassy.

Below their upper cloud decks is a thick layer of gaseous hydrogen. But down below that is an entire sphere of icy water and ammonia and methane. And way down deep they, too have a rocky core.

So, yes the “gas giants” have lots of gas, but they are no means the great gas bags in space many people see them as.

Answer: false

Statement 2: The fact that they are all gigantic and have lots of gas gives us hints into how they were created.

When one sees four inner planets, grouped together and all earthlike, surrounded by distant gigantic planets all bathed in hydrogen and other gases, us nerdy science types get all excited. There’s a pattern here and the planets are trying to tell us something.

What we believe these giant outer four are telling us is that they were formed about the same time, far away from a burning sun.

Far away means cooler, and cooler means different materials can be used as building blocks for planets; like gases and water ice, for example. Try building a planet of gas near a star and it won’t work. The gases get blown away by the star. That’s why the inner planets are all rocky bodies.

But the Gas Giants are telling us that out there hydrogen and helium and methane and even frozen water can be used to build planets. And the huge masses of Jupiter and Saturn indicate that there was a lot of “stuff” to vacuum up out there in the early days of our solar system, but not so much way out where Uranus and Neptune were created.

Answer: true

Statement 3: Of the gas giants, only Saturn has rings.

Saturn’s rings were the first to be seen when Galileo spotted them nearly 400 years ago. But it wasn’t until the last decades of the 20th century that the other jovian planets revealed their own little secrets.

It turns out that all four have ring systems. Of course Saturn’s is most famous and arguably the most beautiful. But Jupiter has a faint ring system of its own, made of dust so small that it can’t reflect light back towards us. That’s why we can’t see it from here. But spacecraft flying behind it have seen its dispersed light and imaged it.

Uranus has its own unique and beautiful ring system that now, with powerful telescopes, can be seen from here.

And distant Neptune has a bizarre ring system, barely there, and made of barely visible carbon compounds making it very difficult to see even by the Voyager spacecraft that have flew by years ago.

Answer: false