They are all gas giants with ring systems
Personally, I find it fascinating to look at why they are all gas giants (I'm discounting pluto - sorry).
When the solar system formed from the protoplanetary disc (a disc of leftover material from forming the sun), the only matter that could condense (group together to form a solid rather than a gas) was heavy elements and compounds - like metals and rocky compounds. This is because heavy elements can condense at high temperatures.
However, heavy elements only form in very large, dying stars and make up just 2% of the solar system (most of it is hydrogen and helium) so these rocky planets stayed very small (in comparison to the gas giants). These planets cannot develop into gas giants as they are 1) too small to hold onto lighter gases and 2) too close to the sun for the gases to stay.
So, now we get to the gas giants.
They are past the frost line - which is the line after which volatile (easily turn to gas) compounds (like methane and ammonia) can condense to solid ice. Before this, they stay as a gas so planets cannot be formed from them. So we get lots of hydrogen compounds condensing and this forms the solid core of the gas giants. These cores can grow big enough to draw in light gases like hydrogen and helium. It also helps that it is much cooler further out in the solar system, so the gases aren't as hot and energetic and therefore much easier to keep a hold onto.
The ability to capture and keep a hold of these gases mean they develop into gas giants.
I don't know why but I think that's beautiful!
They also all have ring systems made up of little fragments of ice and dust.
Hope this helped - let me know if I can help in any other way:)