Here we go! Create new document, fill background layer with dark blue.

Open Layer Style -> Blending Options, select Gradient, change gradient blend mode to Overlay, and style to Radial. also scale up to 150%. (I also added black Inner Glow).

Create new layer, use selection tool or shape tool to create grey ellipse (this will be our earth). If you used shape, rasterize it.

Add some noise (menu Filter - Noise - Add Noise).

Tricky part now - we are going to simulate macro photo where only part of a photo is focused, and the rest is blurred.
- switch to quick mask mode (Q)
- select gradient tool. make sure that foreground color is black, and background is white
- on top of window, switch gradient type to Reflected
- create gradient similar to show here. Since we are in qucik mask mode, you should see gradient in red.

return to normal mode from quick mask mode (hit Q again). Your gradient should become the selection. Apply Gaussian Blur to selected area (Filter - Blur - Gaussian blur), widh raduis about 4-6 pixels. Now, only part of our "earth" is focused, and the rest is blurred.

change the layer's blend mode to Overlay. Now the earth is almost ready, but how about some grass growing on it?