Why is Mars considered the outer edge of the "goldilocks zone"?

Question

In both books and documentaries I often see the Goldilocks Zone as described between Venus and Mars with the Earth "just right." This seems sort of ill thought out to me, because it assumes that Venus' atmosphere comes from its distance from the sun and the same with Mars. I'm not too familiar with current ideas about how Venus' atmosphere formed, but with Mars I'm fairly sure it's widely understood the reason the atmosphere is so small is because the gravity is so weak. Even some discussions about terra forming Mars mention that within a few million years, perhaps, the atmosphere would drift off into space.

So how can we be so certain that's the limits of the zones, especially with Mars? If Mars was as large as Earth/Venus and could hold on to a thicker atmosphere and thus retain more heat, then wouldn't that automatically push out the Goldilocks Zone? It seems like the edges have been places arbitrarily without consideration as to why they're the edges instead of "they just are." This comes up especially because even with its weak atmosphere, at the equator, it can reach up to 20 degrees which is well within the realm of liquid water.

Answer 1

The circumstellar habitalbe zone can be defined as the distance range around a star, where the mean temperature of a rotating planet would be between 0 and 100 centigrades, if radiation (heat) received from the star and thermal radiation emitted by the planet form an equilibrium.

But that's only a rule of thumb. It has been redefined, and is still disputed. Hence your criticism is justified to some degree. There may exist exceptions of habitable planets outside the presumed habitable zone; and there exist certainy many planets within the habitable zone which aren't habitable. But the probability for a planet similar to Earth to be habitable on its surface is thought to be highest in the habitable zone. When looking for habitable exoplanets, it's therefore more efficient to focus on the habitable zone.

Answer 2

We could err by being 'chauvinists' as Carl Sagan would have said, because we are reasoning taking in account the biology and chemistry of life of 1 planet only: a sample of 1...

Before 1995, we thought that alien solar systems would be similar to ours, with small rocky planets closer to the star and giant planets further out. Almost nobody thought about circumbinary planets, planets with periods under one day, scorched jupiters ridiculously close to their stars, ultra compact systems with four or more planets crowded in few tenths of Astronomical Units, free-floating cloudy dwarfs with precipitation of melted iron and hot sand (brown dwarfs? planets? planemos?) and the list goes on and on.

Enceladus, that is quite small for planetary standards and that are far from the most optimistic and inclusive Goldilocks boundary has a subsurface ocean, of water.