Using the 3D Sphere effect available in Photoshop, including Photoshop CS5, even a beginner can create a realistic planet. Here are five easy steps to make a planet in Photoshop like the one pictured here.
1. Create the Ocean
Launch Photoshop and create a new file by pressing “Ctrl-N” (“Command-N” on a Mac). Set the width and height at 800 pixels, with RGB Color and a transparent background.
Click the Foreground Color swatch in the Toolbox. Set the color at #2E4790 for the oceans.
Click the Background Color swatch and set the color at #386F40 for the land masses.
Select the Paint Bucket from the Toolbox and click the canvas to cover it with a blue ocean.
2. Create Continents
Select the Lasso from the Toolbox and drag it over the ocean to draw the outline of a continent as illustrated in the first picture.
Click the double arrow between the Foreground and Background Color swatches. Then click the Paint Bucket and click inside the continent. Draw one or two more continents. You can also add polar ice caps if you want by filling them with white.
Press “Ctrl-A” (“Command-A” on a Mac) to select all. Click Edit -> Free Transform. Drag the corners of the map so it fits in the left side of the canvas, then press “Enter.” Anything on the right, very top and very bottom of the canvas will not appear on the planet.
To preview what the map will look like, click the 3D menu, select New Shape from Layer, then click Sphere. The map is wrapped around itself to form a planet. Note that the colors are darker after this process, which we will adjust later.
Press “Ctrl-Z” to undo the sphere effect. Use the Selection Tool to move or resize the map as needed.
3. Create Clouds
With your map still flat, add a new layer by clicking the New Layer button at the bottom of the Layers Panel, or by clicking New under the the Layers menu at the top of the window.
Make sure the Foreground and Background Color swatches are white and blue. Click the Filter menu -> Render -> Clouds.
Click the Opacity menu on the Layers Panel and reduce the opacity of the clouds to about 70%. Click Layer -> Merge Visible.
Click the 3D menu ->New Shape from Layer -> Sphere.
Click Layer -> Rastersize -> 3D.
Click Image -> Adjustments ->Brightness/Contrast. Check the Preview box and drag the sliders to the right until they are at their optimum levels for your taste. 
4. Add Space and Stars
Click the New Layer button on the Layers Panel. Click the Foreground Color swatch in the Toolbox change it to black space (#000000) in the Color Picker.
Grab the Paint Bucket and click the canvas to change the new layer to black. Click Layer -> Arrange -> Move Backward to move the black space behind the planet.
Add stars by clicking the Pencil in the Toolbox. Change the Foreground Color to white and change the Pencil’s brush size to 1 pixel in the Options Bar at the top of the screen. Dab the tool across the black space. Add some larger stars by changing the brush size to 2 pixels.
5. Add a Sun
Click Filter -> Render -> Lens Flare. Type “122″ in the Brightness field and select the 105mm Prime lens. There is a + sign in the preview window to indicate where the flare will be centered. Move it towards the right corner, then click the OK button.
Select the Paintbrush in the Toolbox. Change the Foreground Color to black and paint over the secondary lens flare that appears in the bottom left of the image.
Select the layer containing the planet in the Layers Panel, then use the Selection Tool to move it so it creates a partial eclipse over the lens flare.
Sources:
Adobe Photoshop CS5: Render Filters
Adobe Photoshop CS5: Create 3D Objects
