1. Start with a 300x300 px image, white background. Then create a new layer, use your elliptical marquee tool create a circle and fill it with color.
2. With your selection still active make your foreground white, then select the gradient tool. Select the fade white to transparent. Then click hold and drag from bottom to top. You should have something similar to this.
3. Now you need to use the elliptical tool again and make as small oval shape at the top inside of the orb. Use the same gradient settings as before but this time drag from top to bottom. Now then go to Blending Options > Stroke add a 1 px stroke to the orb.
Here is my final result
