Question: why is our atmosphere so unique?

Ian Cade answered on 17 Nov 2014:

The Earth’s atmosphere is unique when compared with the other 7 planets for the simple reason that the Earth is generally different from the other 7 planets in out solar system.

Essentially this is down to three variables:

The Earth is further from the sun than Mercury and Venus but closer than Mars and the outer planets

The Earth is more massive than Mercury and Mars but lighter than the outer planets (about the same as Venus).

The Earth has a fairly large magnetic field (in part due to its size – smaller planets have cooled to much to have a molten core to generate such a field. It also has the process of plate tectonics – meaning that the Earth has a fairly constant convection within its core and mantle needed to generate such a field. Unlike Venus which undergoes periods of extreme vulcanism then nothing for a long time… (though a little speculative on my part: this is likely to be due to the Earth being hit by something the size of Mars early on in its formation… and this collision formed the moon from the outer layers of the proto-Earth… meaning there are fewer ‘light’ rocks on the Earth than you might have predicted. This is important because these light rocks would gum up the process of plate-subduction essential for plate tectonics to work).

So…

Being as far as we are from the sun, the Earth is the right temperature to have liquid water and gaseous N2 and O2.

Having the mass it does, the Earth is able to hold onto the atmosphere it has by gravity. If the Earth was more massive the atmosphere would be thicker.

Having a magnetic field means the atmosphere is not stripped away by the solar wind as fast as it could be.

But outside our solar system the Earth’s atmosphere maybe fairly commonplace for any planet that lies a particular distance from its sun, with a particular mass and has a magnetic field.