i have my own adventure i want to go on. i want to create a real-time living "earth" like world. i need an engine or program(thats free) and very in depth with detail and can do this. i mean down to the animals roaming about, birds flying, wind, clouds, weather, etc etc...

now i know how hard it would to mimic the earth into a program, im not going THAT far, but im going to as much detail as i can without taking days on end for 1 little feature. i plan on going over the large aspects first and building from the top down.

and now, im looking for any software that anyone knows of that could do something like this? game engine maybe? maybe something else?

Something that contains as much detail as you describe does not (to the best of my knowledge) exist. Your best bet would be to perhaps mod an existing game or use an engine like Unity or UDK.

I will say that anything worth doing is hard, and if you really want to do anything with such an idea you would need to plan to spend many years on it. So in that case spending days implementing one feature should not be a big concern.

Let's take something like "birds flying". That sounds easy enough until we start to ask questions like:

1. What sorts of birds? Do you have different species or are they all the same? Are you going to recycle one model or have many different bird models?
2. How do you plan to model the bird AI? Flocking algorithms?
3. Are the birds going to be affected by changes in weather and/or temperature? (Going back to your desire to simulate weather).

So now with just one concept we have to get into modeling, rigging, texturing, AI, potentially physics simulation, and we have to make birds that may be affected by the weather simulations.

To me that sounds like months of work in itself. Are you going to study bird migration patterns and mating rituals?

Let's talk about clouds. How do you plan to generate the clouds? Are they going to be procedurally generated?

Do you plan on using voxels or polygons?

As you can see, digging into such a thing involves an explosion of questions.

yes i asked my self alot of the same questions and know how much detail it can go into, especially with having creatures on the world. i think just having the world created would be my first goal and just add animal life and AI all that later.

but i was thinking of using a game engine, im just not sure if they could handle something so massive

yes fairly like spore is my idea. and yes its a massive goal and i dont plan on finishing it all the way really but its something i want to work on. what was spore created with?

I believe the Spore engine was built on a modified version of EAGL, EA's in-house graphics engine. At least, this page indicates that is so. And I believe it used Havok for the physics.