AU and the Circumstellar Habitable Zone

[Hanne]

Hi,
I'm an IB math student trying to begin my mathematics project and I just have a really basic question:

I'm trying to simulate some orbits within the Circumstellar Habitable Zone (the zone in which a planet can sustain liquid water) and I know that this zone exists from .725 AU to 3.0 AU. What I can't find, is if this is the "diameter" or "radius" of the orbit. I'm wondering, because if I extend the distance between my earth-like planet and the its to 2.0 AU, I cannot get it to maintain orbit on my orbit simulator. What am I doing wrong?

 

[e.bar.goum]

It's always radius.

How are you simulating the orbits? Very often, these simulators give unbound orbits because your step size is too large.

In any case, it doesn't matter if the planet is in the goldilocks zone or not, it still should maintain a stable orbit - think about it, neither Mercury nor Neptune are in the habitable zone, and they still have stable orbits.

 

[Drakkith]

What's wrong with the orbit? What kind of unstable are we talking about?